Friday, 21 September 2012

Introduction

In the seventeenth century, the philosophy of space and time emerged as a central issue inepistemology and metaphysics. At its heart, Gottfried Leibniz, the German philosopher-mathematician, and Isaac Newton, the English physicist-mathematician, set out two opposing theories of what space is. Rather than being an entity that independently exists over and above other matter, Leibniz held that space is no more than the collection of spatial relations between objects in the world: "space is that which results from places taken together".[5] Unoccupied regions are those that could have objects in them, and thus spatial relations with other places. For Leibniz, then, space was an idealised abstractionfrom the relations between individual entities or their possible locations and therefore could not becontinuous but must be discrete.[6] Space could be thought of in a similar way to the relations between family members. Although people in the family are related to one another, the relations do not exist independently of the people.[7] Leibniz argued that space could not exist independently of objects in the world because that implies a difference between two universes exactly alike except for the location of the material world in each universe. But since there would be no observational way of telling these universes apart then, according to the identity of indiscernibles, there would be no real difference between them. According to the principle of sufficient reason, any theory of space that implied that there could be these two possible universes, must therefore be wrong.[8]

Columbia was destroyed at about 0900 EST on February 1, 2003 while re-entering the atmosphere after a 16-day scientific mission. The Columbia Accident Investigation Board determined that a hole was punctured in the leading edge on one of Columbia's wings, made of a carbon composite. The hole had formed when a piece of insulating foam from the external fuel tank peeled off during the launch 16 days earlier and struck the shuttle's wing. During the intense heat of re-entry, hot gases penetrated the interior of the wing, destroying the support structure and causing the rest of the shuttle to break apart. The nearly 84,000 pieces of collected debris of the vessel are stored in a 16th floor office suite in the Vehicle Assembly Building at the Kennedy Space Center. The collection was opened to the media once and has since been open only to researchers.[9][10] Unlike Space Shuttle Challenger, which had a replacement orbiter built, Columbia did not.

The seven crew members who died aboard this final mission were: Rick Husband, Commander; William C. McCool, Pilot; Michael P. Anderson, Payload Commander; David M. Brown, Mission Specialist 1; Kalpana Chawla, Mission Specialist 2; Laurel Clark, Mission Specialist 4; and Ilan Ramon, Payload Specialist 1.[11]
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Tribute

Although the debris field encompassed hundreds of miles across North East Texas and into Louisiana, the nose cone and remains of all seven crew members were found in the East Texas county of Sabine. To honor those who lost their lives aboard the shuttle and during the recovery, The Patricia Huffman Smith Museum; Remembering Columbia, has been opened in Hemphill, Texas. The museum tells the story of space exploration from the first mission of the Columbia to its last mission, STS-107. It also reveals the efforts of local citizens during the recovery of Space Shuttle Columbia, STS-107 and its Crew. An area is dedicated to each crewmember that was lost in the tragedy, including the Texas Forest Service employee and the helicopter pilot who lost their lives during the recovery effort. The families of the crew have contributed personal items belonging to their loved ones for permanent display. The museum houses many items and artifacts from NASA and its contractors, the families of STS-107, as well as from other individuals. The museum will feature two simulator interactive devices that emulate activities of the shuttle and orbiter. The classroom for the digital learning center will provide an excellent opportunity for the advancement of education for people of all ages.

The shuttle's final crew was honored in 2003 when the USGS's Board of Geographic Names approved the name Columbia Point for a 13,980-foot (4,260 m) mountain in Colorado's Sangre de Cristo Mountains, less than a half-mile from Challenger Point, a peak named after America's other lost shuttle. The Columbia Hills on Mars were also named in honor of the crew, and a host of other memorials were dedicated in various forms.

Fans of the original Star Trek television series were largely responsible for NASA naming the first Space Shuttle Enterprise. In the television series Star Trek: Enterprise both the first and second starships of the human-built NX-Class, registry numbers NX-01 & NX-02 respectively, were named in honor of pre-existing NASA space shuttles. The second vessel's name was first revealed in the season 3 episode "E²" to be Columbia, in honor of the space shuttle Columbia following its destruction on February 1, 2003. Uniforms on NX-02 Columbia bear a crew patch depicting 7 stars, in honor of the astronauts who died in the shuttle accident.

The Deep Purple song "Contact Lost" on their 2003 album Bananas was dedicated to, and written for, the astronauts whose lives were lost in the 2003 shuttle disaster. Astronaut Kalpana Chawla, one of the victims of the crash, took three CDs into space with her, two of which were Deep Purple albums (Machine Head and Purpendicular). Both CDs survived both the shuttle destruction and the 39-mile plunge.[12] Chawla also traded e-mails with the band while in space, making the tragedy even more personal for the group.[13]

The musical group Echo's Children included singer-songwriter Cat Faber's "Columbia" on their final album "From the Hazel Tree." [14]

The Long Winters 2005 Album "Ultimatum" features the song "The Commander Thinks Aloud", a tribute to the final Columbia crew. [15]

The Eric Johnson instrumental "Columbia" from his 2005 album Bloom was written as a commemoration and tribute to the lives that were lost. Johnson said "I wanted to make it more of a positive message, a salute, a celebration rather than just concentrating on a few moments of tragedy, but instead the bigger picture of these brave people’s lives."[16]

The graphic novel Orbiter by Warren Ellis and Colleen Doran was dedicated to the "lives, memories and legacies of the seven astronauts lost on space shuttle Columbia during mission STS-107."

The Columbia supercomputer at the NASA Advanced Supercomputing (NAS) Division located at Ames Research Center in California was named in honor of the crew lost in the 2003 disaster. Built as a joint effort between NASA and technical partners SGI and Intel in 2004, the supercomputer is used in scientific research of space, the Earth's climate, and aerodynamic design of space launch vehicles and aircraft.[17] The first part of the system, built in 2003, was dedicated to Columbia STS-107 astronaut and engineer Kalpana Chawla, who prior to joining the Space Shuttle program, worked at Ames Research Center.[1

                                                        Space shuttle disasters colombia

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See also
Wikipedia:Pediaphon, generating audio files of any Wikipedia article using speech synthesis
Category:Spoken articles
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External links
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A black hole is a region of spacetime where gravity prevents anything, including light, from escaping.[1] The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that marks the point of no return. It is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics.[2][3] Quantum mechanics predicts that black holes emit radiation like a black body with a finite temperature. This temperature is inversely proportional to the mass of the black hole, making it difficult to observe this radiation for black holes of stellar mass or greater.

Objects whose gravity field is too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was not fully appreciated for another four decades. Long considered a mathematical curiosity, it was during the 1960s that theoretical work showed black holes were a generic prediction of general relativity. The discovery of neutron stars sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.

Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle. After a black hole has formed it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form. There is general consensus that supermassive black holes exist in the centers of most galaxies.

Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with light and other electromagnetic radiation. Matter falling onto a black hole can form an accretion disk heated by friction, forming some of the brightest objects in the universe. If there are other stars orbiting a black hole, their orbit can be used to determine its mass and location. These data can be used to exclude possible alternatives (such as neutron stars). In this way, astronomers have identified numerous stellar black hole candidates in binary systems, and established that the core of our Milky Way galaxy contains a supermassive black hole of about 4.3 million solar masses.Contents [hide]

History

Simulation of gravitational lensing by a black hole, which distorts the image of a galaxy in the background (larger animation)

The idea of a body so massive that even light could not escape was first put forward by geologist John Michell in a letter written to Henry Cavendish in 1783 of the Royal Society:
If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae, with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity.
—John Michell[4]

In 1796, mathematician Pierre-Simon Laplace promoted the same idea in the first and second editions of his book Exposition du système du Monde (it was removed from later editions).[5][6] Such "dark stars" were largely ignored in the nineteenth century, since it was not understood how a massless wave such as light could be influenced by gravity.[7]
General relativity

In 1915, Albert Einstein developed his theory of general relativity, having earlier shown that gravity does influence light's motion. Only a few months later, Karl Schwarzschild found a solution to Einstein field equations, which describes the gravitational field of a point mass and a spherical mass.[8] A few months after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution for the point mass and wrote more extensively about its properties.[9][10] This solution had a peculiar behaviour at what is now called the Schwarzschild radius, where it became singular, meaning that some of the terms in the Einstein equations became infinite. The nature of this surface was not quite understood at the time. In 1924, Arthur Eddington showed that the singularity disappeared after a change of coordinates (see Eddington–Finkelstein coordinates), although it took until 1933 for Georges Lemaître to realize that this meant the singularity at the Schwarzschild radius was an unphysical coordinate singularity.[11]

In 1931, Subrahmanyan Chandrasekhar calculated, using special relativity, that a non-rotating body of electron-degenerate matter above a certain limiting mass (now called the Chandrasekhar limit at 1.4 solar masses) has no stable solutions.[12] His arguments were opposed by many of his contemporaries like Eddington and Lev Landau, who argued that some yet unknown mechanism would stop the collapse.[13] They were partly correct: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star,[14] which is itself stable because of the Pauli exclusion principle. But in 1939, Robert Oppenheimer and others predicted that neutron stars above approximately three solar masses (the Tolman–Oppenheimer–Volkoff limit) would collapse into black holes for the reasons presented by Chandrasekhar, and concluded that no law of physics was likely to intervene and stop at least some stars from collapsing to black holes.[15]

Oppenheimer and his co-authors interpreted the singularity at the boundary of the Schwarzschild radius as indicating that this was the boundary of a bubble in which time stopped. This is a valid point of view for external observers, but not for infalling observers. Because of this property, the collapsed stars were called "frozen stars,"[16] because an outside observer would see the surface of the star frozen in time at the instant where its collapse takes it inside the Schwarzschild radius.
Golden age
See also: Golden age of general relativity

In 1958, David Finkelstein identified the Schwarzschild surface as an event horizon, "a perfect unidirectional membrane: causal influences can cross it in only one direction".[17] This did not strictly contradict Oppenheimer's results, but extended them to include the point of view of infalling observers. Finkelstein's solution extended the Schwarzschild solution for the future of observers falling into a black hole. A complete extension had already been found by Martin Kruskal, who was urged to publish it.[18]

These results came at the beginning of the golden age of general relativity, which was marked by general relativity and black holes becoming mainstream subjects of research. This process was helped by the discovery of pulsars in 1967,[19][20] which, by 1969, were shown to be rapidly rotating neutron stars.[21] Until that time, neutron stars, like black holes, were regarded as just theoretical curiosities; but the discovery of pulsars showed their physical relevance and spurred a further interest in all types of compact objects that might be formed by gravitational collapse.

In this period more general black hole solutions were found. In 1963, Roy Kerr found the exact solution for a rotating black hole. Two years later, Ezra Newman found the axisymmetric solution for a black hole that is both rotating and electrically charged.[22] Through the work of Werner Israel,[23] Brandon Carter,[24][25] and David Robinson[26] the no-hair theorem emerged, stating that a stationary black hole solution is completely described by the three parameters of the Kerr–Newman metric; mass, angular momentum, and electric charge.[27]

At first, it was suspected that the strange features of the black hole solutions were pathological artifacts from the symmetry conditions imposed, and that the singularities would not appear in generic situations. This view was held in particular by Vladimir Belinsky, Isaak Khalatnikov, and Evgeny Lifshitz, who tried to prove that no singularities appear in generic solutions. However, in the late sixties Roger Penrose[28] and Stephen Hawking used global techniques to prove that singularities appear generically.[29]

Work by James Bardeen, Jacob Bekenstein, Carter, and Hawking in the early 1970s led to the formulation of black hole thermodynamics.[30] These laws describe the behaviour of a black hole in close analogy to the laws of thermodynamics by relating mass to energy, area to entropy, and surface gravity to temperature. The analogy was completed when Hawking, in 1974, showed that quantum field theory predicts that black holes should radiate like a black body with a temperature proportional to the surface gravity of the black hole.[31]

The term "black hole" was first publicly used by John Wheeler during a lecture in 1967. Although he is usually credited with coining the phrase, he always insisted that it was suggested to him by somebody else. The first recorded use of the term is in a 1964 letter by Anne Ewing to the American Association for the Advancement of Science.[32] After Wheeler's use of the term, it was quickly adopted in general use.
Properties and structure

The no-hair theorem states that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, charge, and angular momentum.[27] Any two black holes that share the same values for these properties, or parameters, are indistinguishable according to classical (i.e. non-quantum) mechanics.

These properties are special because they are visible from outside a black hole. For example, a charged black hole repels other like charges just like any other charged object. Similarly, the total mass inside a sphere containing a black hole can be found by using the gravitational analog of Gauss's law, the ADM mass, far away from the black hole.[33] Likewise, the angular momentum can be measured from far away using frame dragging by the gravitomagnetic field.

When an object falls into a black hole, any information about the shape of the object or distribution of charge on it is evenly distributed along the horizon of the black hole, and is lost to outside observers. The behavior of the horizon in this situation is a dissipative system that is closely analogous to that of a conductive stretchy membrane with friction and electrical resistance—the membrane paradigm.[34] This is different from other field theories like electromagnetism, which do not have any friction or resistivity at the microscopic level, because they are time-reversible. Because a black hole eventually achieves a stable state with only three parameters, there is no way to avoid losing information about the initial conditions: the gravitational and electric fields of a black hole give very little information about what went in. The information that is lost includes every quantity that cannot be measured far away from the black hole horizon, including approximately conserved quantum numbers such as the total baryon number and lepton number. This behavior is so puzzling that it has been called the black hole information loss paradox.[35][36]
Physical properties

The simplest black holes have mass but neither electric charge nor angular momentum. These black holes are often referred to as Schwarzschild black holes after Karl Schwarzschild who discovered this solution in 1916.[8] According to Birkhoff's theorem, it is the only vacuum solution that is spherically symmetric.[37] This means that there is no observable difference between the gravitational field of such a black hole and that of any other spherical object of the same mass. The popular notion of a black hole "sucking in everything" in its surroundings is therefore only correct near a black hole's horizon; far away, the external gravitational field is identical to that of any other body of the same mass.[38]

Solutions describing more general black holes also exist. Charged black holes are described by the Reissner–Nordström metric, while the Kerr metric describes a rotating black hole. The most general stationary black hole solution known is the Kerr–Newman metric, which describes a black hole with both charge and angular momentum.[39]

While the mass of a black hole can take any positive value, the charge and angular momentum are constrained by the mass. In Planck units, the total electric charge Q and the total angular momentum J are expected to satisfy


for a black hole of mass M. Black holes saturating this inequality are called extremal. Solutions of Einstein's equations that violate this inequality exist, but they do not possess an event horizon. These solutions have so-called naked singularities that can be observed from the outside, and hence are deemed unphysical. The cosmic censorship hypothesis rules out the formation of such singularities, when they are created through the gravitational collapse of realistic matter.[40] This is supported by numerical simulations.[41]

Due to the relatively large strength of the electromagnetic force, black holes forming from the collapse of stars are expected to retain the nearly neutral charge of the star. Rotation, however, is expected to be a common feature of compact objects. The black-hole candidate binary X-ray source GRS 1915+105[42] appears to have an angular momentum near the maximum allowed value.
Black hole classificationsClass Mass Size
Supermassive black hole ~105–109 MSun ~0.001–10 AU
Intermediate-mass black hole ~103 MSun ~103 km = REarth
Stellar black hole ~10 MSun ~30 km
Micro black hole up to ~MMoon up to ~0.1 mm


Black holes are commonly classified according to their mass, independent of angular momentum J or electric charge Q. The size of a black hole, as determined by the radius of the event horizon, or Schwarzschild radius, is roughly proportional to the mass M through


where rsh is the Schwarzschild radius and MSun is the mass of the Sun.[43] This relation is exact only for black holes with zero charge and angular momentum; for more general black holes it can differ up to a factor of 2.
Event horizon
Main article: Event horizon
Far away from the black hole a particle can move in any direction, as illustrated by the set of arrows. It is only restricted by the speed of light.

Closer to the black hole spacetime starts to deform. There are more paths going towards the black hole than paths moving away.[Note 1]

Inside of the event horizon all paths bring the particle closer to the center of the black hole. It is no longer possible for the particle to escape.


The defining feature of a black hole is the appearance of an event horizon—a boundary in spacetime through which matter and light can only pass inward towards the mass of the black hole. Nothing, not even light, can escape from inside the event horizon. The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if such an event occurred.[45]

As predicted by general relativity, the presence of a mass deforms spacetime in such a way that the paths taken by particles bend towards the mass.[46] At the event horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hole.

To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole.[47] Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it.[48] At the same time, all processes on this object slow down causing emitted light to appear redder and dimmer, an effect known as gravitational redshift.[49] Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.

On the other hand, an observer falling into a black hole does not notice any of these effects as he crosses the event horizon. According to his own clock, he crosses the event horizon after a finite time, although he is unable to determine exactly when he crosses it, as it is impossible to determine the location of the event horizon from local observations.[50]

The shape of the event horizon of a black hole is always approximately spherical.[Note 2][53] For non-rotating (static) black holes the geometry is precisely spherical, while for rotating black holes the sphere is somewhat oblate.
Singularity
Main article: Gravitational singularity

At the center of a black hole as described by general relativity lies a gravitational singularity, a region where the spacetime curvature becomes infinite.[54] For a non-rotating black hole, this region takes the shape of a single point and for a rotating black hole, it is smeared out to form a ring singularity lying in the plane of rotation.[55] In both cases, the singular region has zero volume. It can also be shown that the singular region contains all the mass of the black hole solution.[56] The singular region can thus be thought of as having infinite density.

Observers falling into a Schwarzschild black hole (i.e. non-rotating and no charges) cannot avoid being carried into the singularity, once they cross the event horizon. They can prolong the experience by accelerating away to slow their descent, but only up to a point; after attaining a certain ideal velocity, it is best to free fall the rest of the way.[57] When they reach the singularity, they are crushed to infinite density and their mass is added to the total of the black hole. Before that happens, they will have been torn apart by the growing tidal forces in a process sometimes referred to as spaghettification or the "noodle effect".[58]

In the case of a charged (Reissner–Nordström) or rotating (Kerr) black hole, it is possible to avoid the singularity. Extending these solutions as far as possible reveals the hypothetical possibility of exiting the black hole into a different spacetime with the black hole acting as a wormhole.[59] The possibility of traveling to another universe is however only theoretical, since any perturbation will destroy this possibility.[60] It also appears to be possible to follow closed timelike curves (going back to one's own past) around the Kerr singularity, which lead to problems with causality like the grandfather paradox.[61] It is expected that none of these peculiar effects would survive in a proper quantum mechanical treatment of rotating and charged black holes.[62]

The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory.[63] This breakdown, however, is expected; it occurs in a situation where quantum mechanical effects should describe these actions due to the extremely high density and therefore particle interactions. To date, it has not been possible to combine quantum and gravitational effects into a single theory. It is generally expected that a theory of quantum gravity will feature black holes without singularities.[64][65]
Photon sphere
Main article: Photon sphere

The photon sphere is a spherical boundary of zero thickness such that photons moving along tangents to the sphere will be trapped in a circular orbit. For non-rotating black holes, the photon sphere has a radius 1.5 times the Schwarzschild radius. The orbits are dynamically unstable, hence any small perturbation (such as a particle of infalling matter) will grow over time, either setting it on an outward trajectory escaping the black hole or on an inward spiral eventually crossing the event horizon.[66]

While light can still escape from inside the photon sphere, any light that crosses the photon sphere on an inbound trajectory will be captured by the black hole. Hence any light reaching an outside observer from inside the photon sphere must have been emitted by objects inside the photon sphere but still outside of the event horizon.[66]

Other compact objects, such as neutron stars, can also have photon spheres.[67] This follows from the fact that the gravitational field of an object does not depend on its actual size, hence any object that is smaller than 1.5 times the Schwarzschild radius corresponding to its mass will indeed have a photon sphere.
Ergosphere
Main article: Ergosphere

The ergosphere is an oblate spheroid region outside of the event horizon, where objects cannot remain stationary.

Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still, called the ergosphere. This is the result of a process known as frame-dragging; general relativity predicts that any rotating mass will tend to slightly "drag" along the spacetime immediately surrounding it. Any object near the rotating mass will tend to start moving in the direction of rotation. For a rotating black hole, this effect becomes so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still.[68]

The ergosphere of a black hole is bounded by the (outer) event horizon on the inside and an oblate spheroid, which coincides with the event horizon at the poles and is noticeably wider around the equator. The outer boundary is sometimes called the ergosurface.

Objects and radiation can escape normally from the ergosphere. Through the Penrose process, objects can emerge from the ergosphere with more energy than they entered. This energy is taken from the rotational energy of the black hole causing it to slow down.[69]
Formation and evolution

Considering the exotic nature of black holes, it may be natural to question if such bizarre objects could exist in nature or to suggest that they are merely pathological solutions to Einstein's equations. Einstein himself wrongly thought that black holes would not form, because he held that the angular momentum of collapsing particles would stabilize their motion at some radius.[70] This led the general relativity community to dismiss all results to the contrary for many years. However, a minority of relativists continued to contend that black holes were physical objects,[71] and by the end of the 1960s, they had persuaded the majority of researchers in the field that there is no obstacle to forming an event horizon.

Once an event horizon forms, Penrose proved that a singularity will form somewhere inside it.[28] Shortly afterwards, Hawking showed that many cosmological solutions describing the Big Bang have singularities without scalar fields or other exotic matter (see Penrose-Hawking singularity theorems). The Kerr solution, the no-hair theorem and the laws of black hole thermodynamics showed that the physical properties of black holes were simple and comprehensible, making them respectable subjects for research.[72] The primary formation process for black holes is expected to be the gravitational collapse of heavy objects such as stars, but there are also more exotic processes that can lead to the production of black holes.
Gravitational collapse
Main article: Gravitational collapse

Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. For stars this usually occurs either because a star has too little "fuel" left to maintain its temperature through stellar nucleosynthesis, or because a star that would have been stable receives extra matter in a way that does not raise its core temperature. In either case the star's temperature is no longer high enough to prevent it from collapsing under its own weight.[73] The ideal gas law explains the connection between pressure, temperature, and volume.

The collapse may be stopped by the degeneracy pressure of the star's constituents, condensing the matter in an exotic denser state. The result is one of the various types of compact star. The type of compact star formed depends on the mass of the remnant—the matter left over after the outer layers have been blown away, such from a supernova explosion or by pulsations leading to a planetary nebula. Note that this mass can be substantially less than the original star—remnants exceeding 5 solar masses are produced by stars that were over 20 solar masses before the collapse.[73]

If the mass of the remnant exceeds about 3–4 solar masses (the Tolman–Oppenheimer–Volkoff limit[15])—either because the original star was very heavy or because the remnant collected additional mass through accretion of matter—even the degeneracy pressure of neutrons is insufficient to stop the collapse. No known mechanism (except possibly quark degeneracy pressure, see quark star) is powerful enough to stop the implosion and the object will inevitably collapse to form a black hole.[73]

The gravitational collapse of heavy stars is assumed to be responsible for the formation of stellar mass black holes. Star formation in the early universe may have resulted in very massive stars, which upon their collapse would have produced black holes of up to 103 solar masses. These black holes could be the seeds of the supermassive black holes found in the centers of most galaxies.[74]

While most of the energy released during gravitational collapse is emitted very quickly, an outside observer does not actually see the end of this process. Even though the collapse takes a finite amount of time from the reference frame of infalling matter, a distant observer sees the infalling material slow and halt just above the event horizon, due to gravitational time dilation. Light from the collapsing material takes longer and longer to reach the observer, with the light emitted just before the event horizon forms is delayed an infinite amount of time. Thus the external observer never sees the formation of the event horizon; instead, the collapsing material seems to become dimmer and increasingly red-shifted, eventually fading away.[75]
Primordial black holes in the Big Bang

Gravitational collapse requires great density. In the current epoch of the universe these high densities are only found in stars, but in the early universe shortly after the big bang densities were much greater, possibly allowing for the creation of black holes. The high density alone is not enough to allow the formation of black holes since a uniform mass distribution will not allow the mass to bunch up. In order for primordial black holes to form in such a dense medium, there must be initial density perturbations that can then grow under their own gravity. Different models for the early universe vary widely in their predictions of the size of these perturbations. Various models predict the creation of black holes, ranging from a Planck mass to hundreds of thousands of solar masses.[76] Primordial black holes could thus account for the creation of any type of black hole.
High-energy collisions

A simulated event in the CMS detector, a collision in which a micro black hole may be created.

Gravitational collapse is not the only process that could create black holes. In principle, black holes could be formed in high-energy collisions that achieve sufficient density. As of 2002, no such events have been detected, either directly or indirectly as a deficiency of the mass balance in particle accelerator experiments.[77] This suggests that there must be a lower limit for the mass of black holes. Theoretically, this boundary is expected to lie around the Planck mass (mP = √ħc/G ≈ 1.2×1019 GeV/c2 ≈ 2.2×10−8 kg), where quantum effects are expected to invalidate the predictions of general relativity.[78] This would put the creation of black holes firmly out of reach of any high energy process occurring on or near the Earth. However, certain developments in quantum gravity suggest that the Planck mass could be much lower: some braneworld scenarios for example put the boundary as low as 1 TeV/c2.[79] This would make it conceivable for micro black holes to be created in the high energy collisions occurring when cosmic rays hit the Earth's atmosphere, or possibly in the new Large Hadron Collider at CERN. Yet these theories are very speculative, and the creation of black holes in these processes is deemed unlikely by many specialists.[80] Even if micro black holes should be formed in these collisions, it is expected that they would evaporate in about 10−25 seconds, posing no threat to the Earth.[81]
Growth

Once a black hole has formed, it can continue to grow by absorbing additional matter. Any black hole will continually absorb gas and interstellar dust from its direct surroundings and omnipresent cosmic background radiation. This is the primary process through which supermassive black holes seem to have grown.[74] A similar process has been suggested for the formation of intermediate-mass black holes in globular clusters.[82]

Another possibility is for a black hole to merge with other objects such as stars or even other black holes. This is thought to have been important especially for the early development of supermassive black holes, which could have formed from the coagulation of many smaller objects.[74] The process has also been proposed as the origin of some intermediate-mass black holes.[83][84]
Evaporation
Main article: Hawking radiation

In 1974, Hawking showed that black holes are not entirely black but emit small amounts of thermal radiation;[31] an effect that has become known as Hawking radiation. By applying quantum field theory to a static black hole background, he determined that a black hole should emit particles in a perfect black body spectrum. Since Hawking's publication, many others have verified the result through various approaches.[85] If Hawking's theory of black hole radiation is correct, then black holes are expected to shrink and evaporate over time because they lose mass by the emission of photons and other particles.[31] The temperature of this thermal spectrum (Hawking temperature) is proportional to the surface gravity of the black hole, which, for a Schwarzschild black hole, is inversely proportional to the mass. Hence, large black holes emit less radiation than small black holes.[86]

A stellar black hole of one solar mass has a Hawking temperature of about 100 nanokelvins. This is far less than the 2.7 K temperature of the cosmic microwave background radiation. Stellar mass or larger black holes receive more mass from the cosmic microwave background than they emit through Hawking radiation and thus will grow instead of shrink. To have a Hawking temperature larger than 2.7 K (and be able to evaporate), a black hole needs to have less mass than the Moon. Such a black hole would have a diameter of less than a tenth of a millimeter.[87]

If a black hole is very small the radiation effects are expected to become very strong. Even a black hole that is heavy compared to a human would evaporate in an instant. A black hole the weight of a car would have a diameter of about 10−24 m and take a nanosecond to evaporate, during which time it would briefly have a luminosity more than 200 times that of the sun. Lower mass black holes are expected to evaporate even faster; for example, a black hole of mass 1 TeV/c2 would take less than 10−88 seconds to evaporate completely. For such a small black hole, quantum gravitation effects are expected to play an important role and could even—although current developments in quantum gravity do not indicate so[88]—hypothetically make such a small black hole stable.[89]
Observational evidence

By their very nature, black holes do not directly emit any signals other than the hypothetical Hawking radiation; since the Hawking radiation for an astrophysical black hole is predicted to be very weak, this makes it impossible to directly detect astrophysical black holes from the Earth. A possible exception to the Hawking radiation being weak is the last stage of the evaporation of light (primordial) black holes; searches for such flashes in the past has proven unsuccessful and provides stringent limits on the possibility of existence of light primordial black holes.[90] NASA's Fermi Gamma-ray Space Telescope launched in 2008 will continue the search for these flashes.[91]

Astrophysicists searching for black holes thus have to rely on indirect observations. A black hole's existence can sometimes be inferred by observing its gravitational interactions with its surroundings. A project run by MIT's Haystack Observatory is attempting to observe the event horizon of a black hole directly. Initial results are encouraging.[92]
Accretion of matter
See also: Accretion disc


A computer simulation of a star being consumed by a black hole. The blue dot indicates the location of the black hole.

Due to conservation of angular momentum, gas falling into the gravitational well created by a massive object will typically form a disc-like structure around the object. Friction within the disc causes angular momentum to be transported outward, allowing matter to fall further inward, releasing potential energy and increasing the temperature of the gas.[93] In the case of compact objects such as white dwarfs, neutron stars, and black holes, the gas in the inner regions becomes so hot that it will emit vast amounts of radiation (mainly X-rays), which may be detected by telescopes. This process of accretion is one of the most efficient energy-producing processes known; up to 40% of the rest mass of the accreted material can be emitted in radiation.[93] (In nuclear fusion only about 0.7% of the rest mass will be emitted as energy.) In many cases, accretion discs are accompanied by relativistic jets emitted along the poles, which carry away much of the energy. The mechanism for the creation of these jets is currently not well understood.

As such many of the universe's more energetic phenomena have been attributed to the accretion of matter on black holes. In particular, active galactic nuclei and quasars are believed to be the accretion discs of supermassive black holes.[94] Similarly, X-ray binaries are generally accepted to be binary star systems in which one of the two stars is a compact object accreting matter from its companion.[94] It has also been suggested that some ultraluminous X-ray sources may be the accretion disks of intermediate-mass black holes.[95]
X-ray binaries
See also: X-ray binary

X-ray binaries are binary star systems that are luminous in the X-ray part of the spectrum. These X-ray emissions are generally thought to be caused by one of the component stars being a compact object accreting matter from the other (regular) star. The presence of an ordinary star in such a system provides a unique opportunity for studying the central object and determining if it might be a black hole.

Artist impression of a binary system with an accretion disk around a black hole being fed by material from the companion star.

If such a system emits signals that can be directly traced back to the compact object, it cannot be a black hole. The absence of such a signal does, however, not exclude the possibility that the compact object is a neutron star. By studying the companion star it is often possible to obtain the orbital parameters of the system and obtain an estimate for the mass of the compact object. If this is much larger than the Tolman–Oppenheimer–Volkoff limit (that is, the maximum mass a neutron star can have before collapsing) then the object cannot be a neutron star and is generally expected to be a black hole.[94]


This animation compares the X-ray 'heartbeats' of GRS 1915 and IGR J17091, two black holes that ingest gas from companion stars.

The first strong candidate for a black hole, Cygnus X-1, was discovered in this way by Charles Thomas Bolton,[96] Louise Webster and Paul Murdin[97] in 1972.[98][99] Some doubt, however, remained due to the uncertainties resultant from the companion star being much heavier than the candidate black hole.[94] Currently, better candidates for black holes are found in a class of X-ray binaries called soft X-ray transients.[94] In this class of system the companion star is relatively low mass allowing for more accurate estimates in the black hole mass. Moreover, these systems are only active in X-ray for several months once every 10–50 years. During the period of low X-ray emission (called quiescence), the accretion disc is extremely faint allowing for detailed observation of the companion star during this period. One of the best such candidates is V404 Cyg.
Quiescence and advection-dominated accretion flow

The faintness of the accretion disc during quiescence is suspected to be caused by the flow entering a mode called an advection-dominated accretion flow (ADAF). In this mode, almost all the energy generated by friction in the disc is swept along with the flow instead of radiated away. If this model is correct, then it forms strong qualitative evidence for the presence of an event horizon.[100] Because, if the object at the center of the disc had a solid surface, it would emit large amounts of radiation as the highly energetic gas hits the surface, an effect that is observed for neutron stars in a similar state.[93]
Quasi-periodic oscillations
Main article: Quasi-periodic oscillations

The X-ray emission from accretion disks sometimes flickers at certain frequencies. These signals are called quasi-periodic oscillations and are thought to be caused by material moving along the inner edge of the accretion disk (the innermost stable circular orbit). As such their frequency is linked to the mass of the compact object. They can thus be used as an alternative way to determine the mass of potential black holes.[101]
Galactic nuclei
See also: Active galactic nucleus

Astronomers use the term "active galaxy" to describe galaxies with unusual characteristics, such as unusual spectral line emission and very strong radio emission. Theoretical and observational studies have shown that the activity in these active galactic nuclei (AGN) may be explained by the presence of supermassive black holes. The models of these AGN consist of a central black hole that may be millions or billions of times more massive than the Sun; a disk of gas and dust called an accretion disk; and two jets that are perpendicular to the accretion disk.[102][103]

Although supermassive black holes are expected to be found in most AGN, only some galaxies' nuclei have been more carefully studied in attempts to both identify and measure the actual masses of the central supermassive black hole candidates. Some of the most notable galaxies with supermassive black hole candidates include the Andromeda Galaxy, M32, M87, NGC 3115, NGC 3377, NGC 4258, and the Sombrero Galaxy.[104]

It is now widely accepted that the center of (nearly) every galaxy (not just active ones) contains a supermassive black hole.[105] The close observational correlation between the mass of this hole and the velocity dispersion of the host galaxy's bulge, known as the M-sigma relation, strongly suggests a connection between the formation of the black hole and the galaxy itself. [106]

Simulation of gas cloud after close approach to the black hole at the centre of the Milky Way.[107]

Currently, the best evidence for a supermassive black hole comes from studying the proper motion of stars near the center of our own Milky Way.[108] Since 1995 astronomers have tracked the motion of 90 stars in a region called Sagittarius A*. By fitting their motion to Keplerian orbits they were able to infer in 1998 that 2.6 million solar masses must be contained in a volume with a radius of 0.02 lightyears.[109] Since then one of the stars—called S2—has completed a full orbit. From the orbital data they were able to place better constraints on the mass and size of the object causing the orbital motion of stars in the Sagittarius A* region, finding that there is a spherical mass of 4.3 million solar masses contained within a radius of less than 0.002 lightyears.[108] While this is more than 3000 times the Schwarzschild radius corresponding to that mass, it is at least consistent with the central object being a supermassive black hole, and no "realistic cluster [of stars] is physically tenable."[109]
Effects of strong gravity

Another way that the black hole nature of an object may be tested in the future is through observation of effects caused by strong gravity in their vicinity. One such effect is gravitational lensing: The deformation of spacetime around a massive object causes light rays to be deflected much like light passing through an optic lens. Observations have been made of weak gravitational lensing, in which light rays are deflected by only a few arcseconds. However, it has never been directly observed for a black hole.[110] One possibility for observing gravitational lensing by a black hole would be to observe stars in orbit around the black hole. There are several candidates for such an observation in orbit around Sagittarius A*.[110]

Another option would be the direct observation of gravitational waves produced by an object falling into a black hole, for example a compact object falling into a supermassive black hole through an Extreme mass ratio inspiral. Matching the observed waveform to the predictions of general relativity would allow precision measurements of the mass and angular momentum of the central object, while at the same time testing general relativity.[111] These types of events are a primary target for the proposed Laser Interferometer Space Antenna.
Alternatives

The evidence for stellar black holes strongly relies on the existence of an upper limit for the mass of a neutron star. The size of this limit heavily depends on the assumptions made about the properties of dense matter. New exotic phases of matter could push up this bound.[94] A phase of free quarks at high density might allow the existence of dense quark stars,[112] and some supersymmetric models predict the existence of Q stars.[113] Some extensions of the standard model posit the existence of preons as fundamental building blocks of quarks and leptons, which could hypothetically form preon stars.[114] These hypothetical models could potentially explain a number of observations of stellar black hole candidates. However, it can be shown from general arguments in general relativity that any such object will have a maximum mass.[94]

Since the average density of a black hole inside its Schwarzschild radius is inversely proportional to the square of its mass, supermassive black holes are much less dense than stellar black holes (the average density of a 108 solar mass black hole is comparable to that of water).[94] Consequently, the physics of matter forming a supermassive black hole is much better understood and the possible alternative explanations for supermassive black hole observations are much more mundane. For example, a supermassive black hole could be modelled by a large cluster of very dark objects. However, typically such alternatives are not stable enough to explain the supermassive black hole candidates.[94]

The evidence for stellar and supermassive black holes implies that in order for black holes not to form, general relativity must fail as a theory of gravity, perhaps due to the onset of quantum mechanical corrections. A much anticipated feature of a theory of quantum gravity is that it will not feature singularities or event horizons (and thus no black holes).[115] In recent years[when?], much attention has been drawn by the fuzzball model in string theory. Based on calculations in specific situations in string theory, the proposal suggest that generically the individual states of a black hole solution do not have an event horizon or singularity, but that for a classical/semi-classical observer the statistical average of such states does appear just like an ordinary black hole in general relativity.[116]
Open questions
Entropy and thermodynamics
Further information: Black hole thermodynamics

The formula for the Bekenstein–Hawking entropy (S) of a black hole, which depends on the area of the black hole (A). The constants are the speed of light (c), the Boltzmann constant (k), Newton's constant (G), and the reduced Planck constant (ħ).

In 1971, Hawking showed under general conditions[Note 3] that the total area of the event horizons of any collection of classical black holes can never decrease, even if they collide and merge.[117] This result, now known as the second law of black hole mechanics, is remarkably similar to the second law of thermodynamics, which states that the total entropy of a system can never decrease. As with classical objects at absolute zero temperature, it was assumed that black holes had zero entropy. If this were the case, the second law of thermodynamics would be violated by entropy-laden matter entering a black hole, resulting in a decrease of the total entropy of the universe. Therefore, Bekenstein proposed that a black hole should have an entropy, and that it should be proportional to its horizon area.[118]

The link with the laws of thermodynamics was further strengthened by Hawking's discovery that quantum field theory predicts that a black hole radiates blackbody radiation at a constant temperature. This seemingly causes a violation of the second law of black hole mechanics, since the radiation will carry away energy from the black hole causing it to shrink. The radiation, however also carries away entropy, and it can be proven under general assumptions that the sum of the entropy of the matter surrounding a black hole and one quarter of the area of the horizon as measured in Planck units is in fact always increasing. This allows the formulation of the first law of black hole mechanics as an analogue of the first law of thermodynamics, with the mass acting as energy, the surface gravity as temperature and the area as entropy.[118]

One puzzling feature is that the entropy of a black hole scales with its area rather than with its volume, since entropy is normally an extensive quantity that scales linearly with the volume of the system. This odd property led Gerard 't Hooft and Leonard Susskind to propose the holographic principle, which suggests that anything that happens in a volume of spacetime can be described by data on the boundary of that volume.[119]

Although general relativity can be used to perform a semi-classical calculation of black hole entropy, this situation is theoretically unsatisfying. In statistical mechanics, entropy is understood as counting the number of microscopic configurations of a system that have the same macroscopic qualities (such as mass, charge, pressure, etc.). Without a satisfactory theory of quantum gravity, one cannot perform such a computation for black holes. Some progress has been made in various approaches to quantum gravity. In 1995, Andrew Strominger and Cumrun Vafa showed that counting the microstates of a specific supersymmetric black hole in string theory reproduced the Bekenstein–Hawking entropy.[120] Since then, similar results have been reported for different black holes both in string theory and in other approaches to quantum gravity like loop quantum gravity.[121]
Information loss paradox
Main article: Black hole information paradox
List of unsolved problems in physicsIs physical information lost in black holes?


Because a black hole has only a few internal parameters most of the information about the matter that went into forming the black hole is lost. It does not matter if it is formed from television sets or chairs, in the end the black hole only remembers the total mass, charge, and angular momentum. As long as black holes were thought to persist forever this information loss is not that problematic, as the information can be thought of as existing inside the black hole, unaccessible from the outside. However, black holes slowly evaporate by emitting Hawking radiation. This radiation does not appear to carry any detailed information about the stuff that formed the black hole, meaning that this information appears to be gone forever.[122]

For a long time, the question whether information is truly lost in black holes (the black hole information paradox) has divided the theoretical physics community (see Thorne–Hawking–Preskill bet). In quantum mechanics, loss of information corresponds to the violation of vital property called unitarity, which has to do with the conservation of probability. It has been argued that loss of unitarity would also imply violation of conservation of energy.[123] Over recent years evidence has been building that indeed information and unitarity are preserved in a full quantum gravitational treatment of the problem.[124]
See also Star portal

Black brane
Black hole complementarity
Black holes in fiction
Black string
Black hole complementarity
Black holes in fiction
Black string
BTZ black hole
Dumb hole
Kugelblitz (astrophysics)
List of black holes
Susskind-Hawking battle
Black hole complementarity
Black holes in fiction
Black string
BTZ black hole
Dumb hole
Kugelblitz (astrophysics)
List of black holes
Susskind-Hawking battle
Timeline of black hole physics
White hole
Wormhole
Notes
^ The set of possible paths, or more accurately the future light cone containing all possible world lines (in this diagram represented by the yellow/blue grid), is tilted in this way in Eddington–Finkelstein coordinates (the diagram is a "cartoon" version of an Eddington–Finkelstein coordinate diagram), but in other coordinates the light cones are not tilted in this way, for example in Schwarzschild coordinates they simply narrow without tilting as one approaches the event horizon, and in Kruskal–Szekeres coordinates the light cones don't change shape or orientation at all.[44]
^ This is true only for 4-dimensional spacetimes. In higher dimensions more complicated horizon topologies like a black ring are possible.[51][52]
^ In particular, he assumed that all matter satisfies the weak energy condition.
References
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^ Davies, P. C. W. (1978). "Thermodynamics of Black Holes". Reports on Progress in Physics 41 (8): 1313–1355. Bibcode 1978RPPh...41.1313D. doi:10.1088/0034-4885/41/8/004.
^ Michell, J. (1784). "On the Means of Discovering the Distance, Magnitude, &c. of the Fixed Stars, in Consequence of the Diminution of the Velocity of Their Light, in Case Such a Diminution Should be Found to Take Place in any of Them, and Such Other Data Should be Procured from Observations, as Would be Farther Necessary for That Purpose". Philosophical Transactions of the Royal Society 74 (0): 35–57. Bibcode 1784RSPT...74...35M. doi:10.1098/rstl.1784.0008. JSTOR 106576.
^ Gillispie, C. C. (2000). Pierre-Simon Laplace, 1749–1827: a life in exact science. Princeton paperbacks. Princeton University Press. p. 175. ISBN 0-691-05027-9.
^ Israel, W. (1989). "Dark stars: the evolution of an idea". In Hawking, S. W.; Israel, W.. 300 Years of Gravitation. Cambridge University Press. ISBN 978-0-521-37976-2.
^ Thorne 1994, pp. 123–124
^ a b Schwarzschild, K. (1916). "Über das Gravitationsfeld eines Massenpunktes nach der Einsteinschen Theorie". Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften 7: 189–196. and Schwarzschild, K. (1916). "Über das Gravitationsfeld eines Kugel aus inkompressibler Flüssigkeit nach der Einsteinschen Theorie". Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften 18: 424–434.
^ Droste, J. (1917). "On the field of a single centre in Einstein's theory of gravitation, and the motion of a particle in that field". Proceedings Royal Academy Amsterdam (KNAW) 19 (1): 197–215.
^ Kox, A.J. (1992). "General Relativity in the Netherlands:1915-1920". In Eisenstaedt, J.; Kox, A.J.. Studies in the history of general relativity. Birkhäuser. p. 41. ISBN 978-0-8176-3479-7.
^ 't Hooft, G. (2009). Introduction to the Theory of Black Holes. Institute for Theoretical Physics / Spinoza Institute. pp. 47–48.
^ Venkataraman, G. (1992). Chandrasekhar and his limit. Universities Press. p. 89. ISBN 81-7371-035-X.
^ Detweiler, S. (1981). "Resource letter BH-1: Black holes". American Journal of Physics 49 (5): 394–400. Bibcode 1981AmJPh..49..394D. doi:10.1119/1.12686.
^ Harpaz, A. (1994). Stellar evolution. A K Peters. p. 105. ISBN 1-56881-012-1.
^ a b Oppenheimer, J. R.; Volkoff, G. M. (1939). "On Massive Neutron Cores". Physical Review 55 (4): 374–381. Bibcode 1939PhRv...55..374O. doi:10.1103/PhysRev.55.374.
^ Ruffini, R.; Wheeler, J. A. (1971). "Introducing the black hole". Physics Today (1): 30–41.
^ Finkelstein, D. (1958). "Past-Future Asymmetry of the Gravitational Field of a Point Particle". Physical Review 110 (4): 965–967. Bibcode 1958PhRv..110..965F. doi:10.1103/PhysRev.110.965.
^ Kruskal, M. (1960). "Maximal Extension of Schwarzschild Metric". Physical Review 119 (5): 1743. Bibcode 1960PhRv..119.1743K. doi:10.1103/PhysRev.119.1743.
^ Hewish, A. et al. (1968), "Observation of a Rapidly Pulsating Radio Source", Nature 217 (5130): 709–713, Bibcode 1968Natur.217..709H, doi:10.1038/217709a0
^ Pilkington, J. D. H. et al. (1968), "Observations of some further Pulsed Radio Sources", Nature 218 (5137): 126–129, Bibcode 1968Natur.218..126P, doi:10.1038/218126a0
^ Hewish, A. (1970). "Pulsars". Annual Review of Astronomy and Astrophysics 8 (1): 265–296. Bibcode 1970ARA&A...8..265H. doi:10.1146/annurev.aa.08.090170.001405.
^ Newman, E. T. et al. (1965), "Metric of a Rotating, Charged Mass", Journal of Mathematical Physics 6 (6): 918, Bibcode 1965JMP.....6..918N, doi:10.1063/1.1704351
^ Israel, W. (1967). "Event Horizons in Static Vacuum Space-Times". Physical Review 164 (5): 1776. Bibcode 1967PhRv..164.1776I. doi:10.1103/PhysRev.164.1776.
^ Carter, B. (1971). "Axisymmetric Black Hole Has Only Two Degrees of Freedom". Physical Review Letters 26 (6): 331. Bibcode 1971PhRvL..26..331C. doi:10.1103/PhysRevLett.26.331.
^ Carter, B. (1977). "The vacuum black hole uniqueness theorem and its conceivable generalisations". Proceedings of the 1st Marcel Grossmann meeting on general relativity. pp. 243–254.
^ Robinson, D. (1975). "Uniqueness of the Kerr Black Hole". Physical Review Letters 34 (14): 905. Bibcode 1975PhRvL..34..905R. doi:10.1103/PhysRevLett.34.905.
^ a b Heusler, M. (1998). "Stationary Black Holes: Uniqueness and Beyond". Living Reviews in Relativity 1 (6). Retrieved 2011-02-08.
^ a b Penrose, R. (1965). "Gravitational Collapse and Space-Time Singularities". Physical Review Letters 14 (3): 57. Bibcode 1965PhRvL..14...57P. doi:10.1103/PhysRevLett.14.57.
^ Ford, L. H. (2003). "The Classical Singularity Theorems and Their Quantum Loopholes". International Journal of Theoretical Physics 42 (6): 1219. doi:10.1023/A:1025754515197.
^ Bardeen, J. M.; Carter, B.; Hawking, S. W. (1973). "The four laws of black hole mechanics". Communications in Mathematical Physics 31 (2): 161–170. Bibcode 1973CMaPh..31..161B. doi:10.1007/BF01645742. MR MR0334798. Zbl 1125.83309.
^ a b c Hawking, S. W. (1974). "Black hole explosions?". Nature 248 (5443): 30–31. Bibcode 1974Natur.248...30H. doi:10.1038/248030a0.
^ Quinion, M. (26 April 2008). "Black Hole". World Wide Words. Retrieved 2008-06-17.
^ Carroll 2004, p. 253
^ Thorne, K. S.; Price, R. H. (1986). Black holes: the membrane paradigm. Yale University Press. ISBN 978-0-300-03770-8.
^ Anderson, Warren G. (1996). "The Black Hole Information Loss Problem". Usenet Physics FAQ. Retrieved 2009-03-24.
^ Preskill, J. (1994-10-21). "Black holes and information: A crisis in quantum physics". Caltech Theory Seminar.
^ Hawking & Ellis 1973, Appendix B
^ Seeds, Michael A.; Backman, Dana E. (2007), Perspectives on Astronomy, Cengage Learning, p. 167, ISBN 0-495-11352-2
^ Shapiro, S. L.; Teukolsky, S. A. (1983). Black holes, white dwarfs, and neutron stars: the physics of compact objects. John Wiley and Sons. p. 357. ISBN 0-471-87316-0.
^ Wald, R. M. (1997). "Gravitational Collapse and Cosmic Censorship". arXiv:gr-qc/9710068 [gr-qc].
^ Berger, B. K. (2002). "Numerical Approaches to Spacetime Singularities". Living Reviews in Relativity 5. Retrieved 2007-08-04.
^ McClintock, J. E.; Shafee, R.; Narayan, R.; Remillard, R. A.; Davis, S. W.; Li, L.-X. (2006). "The Spin of the Near-Extreme Kerr Black Hole GRS 1915+105". Astrophysical Journal 652 (1): 518–539. arXiv:astro-ph/0606076. Bibcode 2006ApJ...652..518M. doi:10.1086/508457.
^ Wald 1984, pp. 124–125
^ Thorne, Misner & Wheeler 1973, p. 848
^ Wheeler 2007, p. 179
^ Carroll 2004, Ch. 5.4 and 7.3
^ Carroll 2004, p. 217
^ Carroll 2004, p. 218
^ "Inside a black hole". Knowing the universe and its secrets. Retrieved 2009-03-26.
^ Carroll 2004, p. 222
^ Emparan, R.; Reall, H. S. (2008). "Black Holes in Higher Dimensions". Living Reviews in Relativity 11 (6). arXiv:0801.3471. Bibcode 2008LRR....11....6E. Retrieved 2011-02-10.
^ Obers, N. A. (2009). Papantonopoulos, Eleftherios. ed. "Black Holes in Higher-Dimensional Gravity". Lecture Notes in Physics 769: 211–258. arXiv:0802.0519. doi:10.1007/978-3-540-88460-6.
^ hawking & ellis 1973, Ch. 9.3
^ Carroll 2004, p. 205
^ Carroll 2004, pp. 264–265
^ Carroll 2004, p. 252
^ Lewis, G. F.; Kwan, J. (2007). "No Way Back: Maximizing Survival Time Below the Schwarzschild Event Horizon". Publications of the Astronomical Society of Australia 24 (2): 46–52. arXiv:0705.1029. Bibcode 2007PASA...24...46L. doi:10.1071/AS07012.
^ Wheeler 2007, p. 182
^ Carroll 2004, pp. 257–259 and 265–266
^ Droz, S.; Israel, W.; Morsink, S. M. (1996). "Black holes: the inside story". Physics World 9 (1): 34–37. Bibcode 1996PhyW....9...34D.
^ Carroll 2004, p. 266
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^ Wald 1984, p. 212
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^ a b Nitta, Daisuke; Chiba, Takeshi; Sugiyama, Naoshi (September 2011), "Shadows of colliding black holes", Physical Review D 84 (6), arXiv:1106.242, Bibcode 2011PhRvD..84f3008N, doi:10.1103/PhysRevD.84.063008
^ Nemiroff, R. J. (1993). "Visual distortions near a neutron star and black hole". American Journal of Physics 61 (7): 619. arXiv:astro-ph/9312003. Bibcode 1993AmJPh..61..619N. doi:10.1119/1.17224.
^ Carroll 2004, Ch. 6.6
^ Carroll 2004, Ch. 6.7
^ Einstein, A. (1939). "On A Stationary System With Spherical Symmetry Consisting of Many Gravitating Masses". Annals of Mathematics 40 (4): 922–936. doi:10.2307/1968902.
^ Kerr, R. P. (2009). "The Kerr and Kerr-Schild metrics". In Wiltshire, D. L.; Visser, M.; Scott, S. M.. The Kerr Spacetime. Cambridge University Press. arXiv:0706.1109. ISBN 978-0-521-88512-6.
^ Hawking, S. W.; Penrose, R. (January 1970). "The Singularities of Gravitational Collapse and Cosmology". Proceedings of the Royal Society A 314 (1519): 529–548. Bibcode 1970RSPSA.314..529H. doi:10.1098/rspa.1970.0021. JSTOR 2416467.
^ a b c Carroll 2004, Section 5.8
^ Schutz, Bernard F. (2003). Gravity from the ground up. Cambridge University Press. p. 110. ISBN 0-521-45506-5.
^ Davies, P. C. W. (1978). "Thermodynamics of Black Holes". Reports on Progress in Physics 41 (8): 1313–1355. Bibcode 1978RPPh...41.1313D. doi:10.1088/0034-4885/41/8/004.
^ Michell, J. (1784). "On the Means of Discovering the Distance, Magnitude, &c. of the Fixed Stars, in Consequence of the Diminution of the Velocity of Their Light, in Case Such a Diminution Should be Found to Take Place in any of Them, and Such Other Data Should be Procured from Observations, as Would be Farther Necessary for That Purpose". Philosophical Transactions of the Royal Society 74 (0): 35–57. Bibcode 1784RSPT...74...35M. doi:10.1098/rstl.1784.0008. JSTOR 106576.
^ Gillispie, C. C. (2000). Pierre-Simon Laplace, 1749–1827: a life in exact science. Princeton paperbacks. Princeton University Press. p. 175. ISBN 0-691-05027-9.
^ Israel, W. (1989). "Dark stars: the evolution of an idea". In Hawking, S. W.; Israel, W.. 300 Years of Gravitation. Cambridge University Press. ISBN 978-0-521-37976-2.
^ Thorne 1994, pp. 123–124
^ a b Schwarzschild, K. (1916). "Über das Gravitationsfeld eines Massenpunktes nach der Einsteinschen Theorie". Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften 7: 189–196. and Schwarzschild, K. (1916). "Über das Gravitationsfeld eines Kugel aus inkompressibler Flüssigkeit nach der Einsteinschen Theorie". Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften 18: 424–434.
^ Droste, J. (1917). "On the field of a single centre in Einstein's theory of gravitation, and the motion of a particle in that field". Proceedings Royal Academy Amsterdam (KNAW) 19 (1): 197–215.
^ Kox, A.J. (1992). "General Relativity in the Netherlands:1915-1920". In Eisenstaedt, J.; Kox, A.J.. Studies in the history of general relativity. Birkhäuser. p. 41. ISBN 978-0-8176-3479-7.
^ 't Hooft, G. (2009). Introduction to the Theory of Black Holes. Institute for Theoretical Physics / Spinoza Institute. pp. 47–48.
^ Venkataraman, G. (1992). Chandrasekhar and his limit. Universities Press. p. 89. ISBN 81-7371-035-X.
^ Detweiler, S. (1981). "Resource letter BH-1: Black holes". American Journal of Physics 49 (5): 394–400. Bibcode 1981AmJPh..49..394D. doi:10.1119/1.12686.
^ Harpaz, A. (1994). Stellar evolution. A K Peters. p. 105. ISBN 1-56881-012-1.
^ a b Oppenheimer, J. R.; Volkoff, G. M. (1939). "On Massive Neutron Cores". Physical Review 55 (4): 374–381. Bibcode 1939PhRv...55..374O. doi:10.1103/PhysRev.55.374.
^ Ruffini, R.; Wheeler, J. A. (1971). "Introducing the black hole". Physics Today (1): 30–41.
^ Finkelstein, D. (1958). "Past-Future Asymmetry of the Gravitational Field of a Point Particle". Physical Review 110 (4): 965–967. Bibcode 1958PhRv..110..965F. doi:10.1103/PhysRev.110.965.
^ Kruskal, M. (1960). "Maximal Extension of Schwarzschild Metric". Physical Review 119 (5): 1743. Bibcode 1960PhRv..119.1743K. doi:10.1103/PhysRev.119.1743.
^ Hewish, A. et al. (1968), "Observation of a Rapidly Pulsating Radio Source", Nature 217 (5130): 709–713, Bibcode 1968Natur.217..709H, doi:10.1038/217709a0
^ Pilkington, J. D. H. et al. (1968), "Observations of some further Pulsed Radio Sources", Nature 218 (5137): 126–129, Bibcode 1968Natur.218..126P, doi:10.1038/218126a0
^ Hewish, A. (1970). "Pulsars". Annual Review of Astronomy and Astrophysics 8 (1): 265–296. Bibcode 1970ARA&A...8..265H. doi:10.1146/annurev.aa.08.090170.001405.
^ Newman, E. T. et al. (1965), "Metric of a Rotating, Charged Mass", Journal of Mathematical Physics 6 (6): 918, Bibcode 1965JMP.....6..918N, doi:10.1063/1.1704351
^ Israel, W. (1967). "Event Horizons in Static Vacuum Space-Times". Physical Review 164 (5): 1776. Bibcode 1967PhRv..164.1776I. doi:10.1103/PhysRev.164.1776.
^ Carter, B. (1971). "Axisymmetric Black Hole Has Only Two Degrees of Freedom". Physical Review Letters 26 (6): 331. Bibcode 1971PhRvL..26..331C. doi:10.1103/PhysRevLett.26.331.
^ Carter, B. (1977). "The vacuum black hole uniqueness theorem and its conceivable generalisations". Proceedings of the 1st Marcel Grossmann meeting on general relativity. pp. 243–254.
^ Robinson, D. (1975). "Uniqueness of the Kerr Black Hole". Physical Review Letters 34 (14): 905. Bibcode 1975PhRvL..34..905R. doi:10.1103/PhysRevLett.34.905.
^ a b Heusler, M. (1998). "Stationary Black Holes: Uniqueness and Beyond". Living Reviews in Relativity 1 (6). Retrieved 2011-02-08.
^ a b Penrose, R. (1965). "Gravitational Collapse and Space-Time Singularities". Physical Review Letters 14 (3): 57. Bibcode 1965PhRvL..14...57P. doi:10.1103/PhysRevLett.14.57.
^ Ford, L. H. (2003). "The Classical Singularity Theorems and Their Quantum Loopholes". International Journal of Theoretical Physics 42 (6): 1219. doi:10.1023/A:1025754515197.
^ Bardeen, J. M.; Carter, B.; Hawking, S. W. (1973). "The four laws of black hole mechanics". Communications in Mathematical Physics 31 (2): 161–170. Bibcode 1973CMaPh..31..161B. doi:10.1007/BF01645742. MR MR0334798. Zbl 1125.83309.
^ a b c Hawking, S. W. (1974). "Black hole explosions?". Nature 248 (5443): 30–31. Bibcode 1974Natur.248...30H. doi:10.1038/248030a0.
^ Quinion, M. (26 April 2008). "Black Hole". World Wide Words. Retrieved 2008-06-17.
^ Carroll 2004, p. 253
^ Thorne, K. S.; Price, R. H. (1986). Black holes: the membrane paradigm. Yale University Press. ISBN 978-0-300-03770-8.
^ Anderson, Warren G. (1996). "The Black Hole Information Loss Problem". Usenet Physics FAQ. Retrieved 2009-03-24.
^ Preskill, J. (1994-10-21). "Black holes and information: A crisis in quantum physics". Caltech Theory Seminar.
^ Hawking & Ellis 1973, Appendix B
^ Seeds, Michael A.; Backman, Dana E. (2007), Perspectives on Astronomy, Cengage Learning, p. 167, ISBN 0-495-11352-2
^ Shapiro, S. L.; Teukolsky, S. A. (1983). Black holes, white dwarfs, and neutron stars: the physics of compact objects. John Wiley and Sons. p. 357. ISBN 0-471-87316-0.
^ Wald, R. M. (1997). "Gravitational Collapse and Cosmic Censorship". arXiv:gr-qc/9710068 [gr-qc].
^ Berger, B. K. (2002). "Numerical Approaches to Spacetime Singularities". Living Reviews in Relativity 5. Retrieved 2007-08-04.
^ McClintock, J. E.; Shafee, R.; Narayan, R.; Remillard, R. A.; Davis, S. W.; Li, L.-X. (2006). "The Spin of the Near-Extreme Kerr Black Hole GRS 1915+105". Astrophysical Journal 652 (1): 518–539. arXiv:astro-ph/0606076. Bibcode 2006ApJ...652..518M. doi:10.1086/508457.
^ Wald 1984, pp. 124–125
^ Thorne, Misner & Wheeler 1973, p. 848
^ Wheeler 2007, p. 179
^ Carroll 2004, Ch. 5.4 and 7.3
^ Carroll 2004, p. 217
^ Carroll 2004, p. 218
^ "Inside a black hole". Knowing the universe and its secrets. Retrieved 2009-03-26.
^ Carroll 2004, p. 222
^ Emparan, R.; Reall, H. S. (2008). "Black Holes in Higher Dimensions". Living Reviews in Relativity 11 (6). arXiv:0801.3471. Bibcode 2008LRR....11....6E. Retrieved 2011-02-10.
^ Obers, N. A. (2009). Papantonopoulos, Eleftherios. ed. "Black Holes in Higher-Dimensional Gravity". Lecture Notes in Physics 769: 211–258. arXiv:0802.0519. doi:10.1007/978-3-540-88460-6.
^ hawking & ellis 1973, Ch. 9.3
^ Carroll 2004, p. 205
^ Carroll 2004, pp. 264–265
^ Carroll 2004, p. 252
^ Lewis, G. F.; Kwan, J. (2007). "No Way Back: Maximizing Survival Time Below the Schwarzschild Event Horizon". Publications of the Astronomical Society of Australia 24 (2): 46–52. arXiv:0705.1029. Bibcode 2007PASA...24...46L. doi:10.1071/AS07012.
^ Wheeler 2007, p. 182
^ Carroll 2004, pp. 257–259 and 265–266
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^ Carroll 2004, p. 266
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^ Carroll 2004, Ch. 6.6
^ Carroll 2004, Ch. 6.7
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^ Hawking, S. W.; Penrose, R. (January 1970). "The Singularities of Gravitational Collapse and Cosmology". Proceedings of the Royal Society A 314 (1519): 529–548. Bibcode 1970RSPSA.314..529H. doi:10.1098/rspa.1970.0021. JSTOR 2416467.
^ a b c Carroll 2004, Section 5.8
^ a b c Rees, M. J.; Volonteri, M. (2007). "Massive black holes: formation and evolution". In Karas, V.; Matt, G.. Black Holes from Stars to Galaxies—Across the Range of Masses. Cambridge University Press. pp. 51–58. arXiv:astro-ph/0701512. ISBN 978-0-521-86347-6.
^ Penrose, R. (2002). "Gravitational Collapse: The Role of General Relativity". General Relativity and Gravitation 34 (7): 1141. Bibcode 2002GReGr..34.1141P. doi:10.1023/A:1016578408204.
^ Carr, B. J. (2005). "Primordial Black Holes: Do They Exist and Are They Useful?". In Suzuki, H.; Yokoyama, J.; Suto, Y. et al.. Inflating Horizon of Particle Astrophysics and Cosmology. Universal Academy Press. arXiv:astro-ph/0511743. ISBN 4-946443-94-0.
^ Giddings, S. B.; Thomas, S. (2002). "High energy colliders as black hole factories: The end of short distance physics". Physical Review D 65 (5): 056010. arXiv:hep-ph/0106219. Bibcode 2002PhRvD..65e6010G. doi:10.1103/PhysRevD.65.056010.
^ Harada, T. (2006). "Is there a black hole minimum mass?". Physical Review D 74 (8): 084004. arXiv:gr-qc/0609055. Bibcode 2006PhRvD..74h4004H. doi:10.1103/PhysRevD.74.084004.
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^ LHC Safety Assessment Group. "Review of the Safety of LHC Collisions". CERN.
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^ Vesperini, E.; McMillan, S. L. W.; D'Ercole, A. et al. (2010). "Intermediate-Mass Black Holes in Early Globular Clusters". The Astrophysical Journal Letters 713 (1): L41–L44. arXiv:1003.3470. Bibcode 2010ApJ...713L..41V. doi:10.1088/2041-8205/713/1/L41.
^ Zwart, S. F. P.; Baumgardt, H.; Hut, P. et al. (2004). "Formation of massive black holes through runaway collisions in dense young star clusters". Nature 428 (6984): 724. arXiv:astro-ph/0402622. Bibcode 2004Natur.428..724P. doi:10.1038/nature02448. PMID 15085124.
^ O’leary, R. M.; Rasio, F. A.; Fregeau, J. M. et al. (2006). "Binary Mergers and Growth of Black Holes in Dense Star Clusters". The Astrophysical Journal 637 (2): 937. arXiv:astro-ph/0508224. Bibcode 2006ApJ...637..937O. doi:10.1086/498446.
^ Page, D. N. (2005). "Hawking radiation and black hole thermodynamics". New Journal of Physics 7: 203. arXiv:hep-th/0409024. Bibcode 2005NJPh....7..203P. doi:10.1088/1367-2630/7/1/203.
^ Carroll 2004, Ch. 9.6
^ "Evaporating black holes?". Einstein online. Max Planck Institute for Gravitational Physics. 2010. Retrieved 2010-12-12.
^ Giddings, S. B.; Mangano, M. L. (2008). "Astrophysical implications of hypothetical stable TeV-scale black holes". Physical Review D 78 (3): 035009. arXiv:0806.3381. Bibcode 2008PhRvD..78c5009G. doi:10.1103/PhysRevD.78.035009.
^ Peskin, M. E. (2008). "The end of the world at the Large Hadron Collider?". Physics 1: 14. Bibcode 2008PhyOJ...1...14P. doi:10.1103/Physics.1.14.
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^ Naeye, R.. "Testing Fundamental Physics". NASA. Retrieved 2008-09-16.
^ "Event Horizon Telescope". MIT Haystack Observatory. Retrieved 6 April 2012.
^ a b c McClintock, J. E.; Remillard, R. A. (2006). "Black Hole Binaries". In Lewin, W.; van der Klis, M.. Compact Stellar X-ray Sources. Cambridge University Press. arXiv:astro-ph/0306213. ISBN 0-521-82659-4. section 4.1.5.
^ a b c d e f g h i Celotti, A.; Miller, J. C.; Sciama, D. W. (1999). "Astrophysical evidence for the existence of black holes". Classical and Quantum Gravity 16 (12A): A3–A21. arXiv:astro-ph/9912186. doi:10.1088/0264-9381/16/12A/301.
^ Winter, L. M.; Mushotzky, R. F.; Reynolds, C. S. (2006). "XMM‐Newton Archival Study of the Ultraluminous X‐Ray Population in Nearby Galaxies". The Astrophysical Journal 649 (2): 730. arXiv:astro-ph/0512480. Bibcode 2006ApJ...649..730W. doi:10.1086/506579.
^ Bolton, C. T. (1972). "Identification of Cygnus X-1 with HDE 226868". Nature 235 (5336): 271–273. Bibcode 1972Natur.235..271B. doi:10.1038/235271b0.
^ Webster, B. L.; Murdin, P. (1972). "Cygnus X-1—a Spectroscopic Binary with a Heavy Companion ?". Nature 235 (5332): 37–38. Bibcode 1972Natur.235...37W. doi:10.1038/235037a0.
^ Rolston, B. (10 November 1997). "The First Black Hole". The bulletin. University of Toronto. Archived from the original on 2008-05-02. Retrieved 2008-03-11.
^ Shipman, H. L. (1 January 1975). "The implausible history of triple star models for Cygnus X-1 Evidence for a black hole". Astrophysical Letters 16 (1): 9–12. Bibcode 1975ApL....16....9S. doi:10.1016/S0304-8853(99)00384-4.
^ Narayan, R.; McClintock, J. (2008). "Advection-dominated accretion and the black hole event horizon". New Astronomy Reviews 51 (10–12): 733. arXiv:0803.0322. Bibcode 2008NewAR..51..733N. doi:10.1016/j.newar.2008.03.002.
^ "NASA scientists identify smallest known black hole" (Press release). Goddard Space Flight Center. 2008-04-01. Retrieved 2009-03-14.
^ Krolik, J. H. (1999). Active Galactic Nuclei. Princeton University Press. Ch. 1.2. ISBN 0-691-01151-6.
^ Sparke, L. S.; Gallagher, J. S. (2000). Galaxies in the Universe: An Introduction. Cambridge University Press. Ch. 9.1. ISBN [[Special:BookSources/0-521-59704-4|0-521-59704-4]].
^ Kormendy, J.; Richstone, D. (1995). "Inward Bound—The Search For Supermassive Black Holes In Galactic Nuclei". Annual Reviews of Astronomy and Astrophysics 33 (1): 581–624. Bibcode 1995ARA&A..33..581K. doi:10.1146/annurev.aa.33.090195.003053.
^ King, A. (2003). "Black Holes, Galaxy Formation, and the MBH-σ Relation". The Astrophysical Journal Letters 596 (1): 27–29. arXiv:astro-ph/0308342. Bibcode 2003ApJ...596L..27K. doi:10.1086/379143.
^ Ferrarese, L.; Merritt, D. (2000). "A Fundamental Relation Between Supermassive Black Holes and their Host Galaxies". The Astrophysical Journal Letters 539 (1): 9–12. arXiv:astro-ph/0006053. Bibcode 2000ApJ...539L...9F. doi:10.1086/312838.
^ "A Black Hole's Dinner is Fast Approaching". ESO Press Release. Retrieved 6 February 2012.
^ a b Gillessen, S.; Eisenhauer, F.; Trippe, S. et al. (2009). "Monitoring Stellar Orbits around the Massive Black Hole in the Galactic Center". The Astrophysical Journal 692 (2): 1075. arXiv:0810.4674. Bibcode 2009ApJ...692.1075G. doi:10.1088/0004-637X/692/2/1075.
^ a b Ghez, A. M.; Klein, B. L.; Morris, M. et al. (1998). "High Proper‐Motion Stars in the Vicinity of Sagittarius A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy". The Astrophysical Journal 509 (2): 678. arXiv:astro-ph/9807210. Bibcode 1998ApJ...509..678G. doi:10.1086/306528.
^ a b Bozza, V. (2010). "Gravitational Lensing by Black Holes". General Relativity and Gravitation (42): 2269–2300. arXiv:0911.2187. Bibcode 2010GReGr..42.2269B. doi:10.1007/s10714-010-0988-2.
^ Barack, L.; Cutler, C. (2004). "LISA capture sources: Approximate waveforms, signal-to-noise ratios, and parameter estimation accuracy". Physical Review D (69): 082005. arXiv:gr-qc/0310125. Bibcode 2004PhRvD..69h2005B. doi:10.1103/PhysRevD.69.082005.
^ Kovacs, Z.; Cheng, K. S.; Harko, T. (2009). "Can stellar mass black holes be quark stars?". Monthly Notices of the Royal Astronomical Society 400 (3): 1632–1642. arXiv:0908.2672. Bibcode 2009MNRAS.400.1632K. doi:10.1111/j.1365-2966.2009.15571.x.
^ Kusenko, A. (2006). "Properties and signatures of supersymmetric Q-balls". arXiv:hep-ph/0612159 [hep-ph].
^ Hansson, J.; Sandin, F. (2005). "Preon stars: a new class of cosmic compact objects". Physics Letters B 616 (1–2): 1. arXiv:astro-ph/0410417. Bibcode 2005PhLB..616....1H. doi:10.1016/j.physletb.2005.04.034.
^ Kiefer, C. (2006). "Quantum gravity: general introduction and recent developments". Annalen der Physik 15 (1–2): 129. arXiv:gr-qc/0508120. Bibcode 2006AnP...518..129K. doi:10.1002/andp.200510175.
^ Skenderis, K.; Taylor, M. (2008). "The fuzzball proposal for black holes". Physics Reports 467 (4–5): 117. arXiv:0804.0552. Bibcode 2008PhR...467..117S. doi:10.1016/j.physrep.2008.08.001.
^ Hawking, S. W. (1971). "Gravitational Radiation from Colliding Black Holes". Physical Review Letters 26 (21): 1344–1346. Bibcode 1971PhRvL..26.1344H. doi:10.1103/PhysRevLett.26.1344.
^ a b Wald, R. M. (2001). "The Thermodynamics of Black Holes". Living Reviews in Relativity 4 (6). arXiv:gr-qc/9912119. Bibcode 1999gr.qc....12119W. Retrieved 2011-02-10.
^ 't Hooft, G. (2001). "The Holographic Principle". In Zichichi, A.. Basics and highlights in fundamental physics. Subnuclear series. 37. World Scientific. arXiv:hep-th/0003004. ISBN 978-981-02-4536-8.
^ Strominger, A.; Vafa, C. (1996). "Microscopic origin of the Bekenstein-Hawking entropy". Physics Letters B 379 (1–4): 99. arXiv:hep-th/9601029. Bibcode 1996PhLB..379...99S. doi:10.1016/0370-2693(96)00345-0.
^ Carlip, S. (2009). "Black Hole Thermodynamics and Statistical Mechanics". Lecture Notes in Physics 769: 89. arXiv:0807.4520. doi:10.1007/978-3-540-88460-6_3.
^ Hawking, S. W.. "Does God Play Dice?". www.hawking.org.uk. Retrieved 2009-03-14.
^ Giddings, S. B. (1995). "The black hole information paradox". Particles, Strings and Cosmology. Johns Hopkins Workshop on Current Problems in Particle Theory 19 and the PASCOS Interdisciplinary Symposium 5. arXiv:hep-th/9508151.
^ Mathur, S. D. (2011). "The information paradox: conflicts and resolutions". XXV International Symposium on Lepton Photon Interactions at High Energies. arXiv:1201.2079.
Further reading
Popular reading
Ferguson, Kitty (1991). Black Holes in Space-Time. Watts Franklin. ISBN 0-531-12524-6.
Hawking, Stephen (1988). A Brief History of Time. Bantam Books, Inc. ISBN 0-553-38016-8.
Hawking, Stephen; Penrose, Roger (1996). The Nature of Space and Time. Princeton University Press. ISBN [[Special:BookSources/0-691-03791-2|0-691-03791-2]].
Melia, Fulvio (2003). The Black Hole at the Center of Our Galaxy. Princeton U Press. ISBN 978-0-691-09505-9.
Melia, Fulvio (2003). The Edge of Infinity. Supermassive Black Holes in the Universe. Cambridge U Press. ISBN 978-0-521-81405-8.
Pickover, Clifford (1998). Black Holes: A Traveler's Guide. Wiley, John & Sons, Inc. ISBN 0-471-19704-1.
Stern, B. (2008). "Blackhole"., poem.
Thorne, Kip S. (1994). Black Holes and Time Warps. Norton, W. W. & Company, Inc. ISBN 0-393-31276-3.
Wheeler, J. Craig (2007). Cosmic Catastrophes (2nd ed.). Cambridge University Press. ISBN 0-521-85714-7.
University textbooks and monographs
Carroll, Sean M. (2004). Spacetime and Geometry. Addison Wesley. ISBN 0-8053-8732-3., the lecture notes on which the book was based are available for free from Sean Carroll's website.
Carter, B. (1973). "Black hole equilibrium states". In DeWitt, B.S.; DeWitt, C.. Black Holes.
Chandrasekhar, Subrahmanyan (1999). Mathematical Theory of Black Holes. Oxford University Press. ISBN 0-19-850370-9.
Frolov, V.P.; Novikov, I.D. (1998). Black hole physics.
Hawking, S.W.; Ellis, G.F.R. (1973). Large Scale Structure of space time. Cambridge University Press. ISBN 0-521-09906-4.
Melia, Fulvio (2007). The Galactic Supermassive Black Hole. Princeton U Press. ISBN 978-0-691-13129-0.
Taylor, Edwin F.; Wheeler, John Archibald (2000). Exploring Black Holes. Addison Wesley Longman. ISBN 0-201-38423-X.
Thorne, Kip S.; Misner, Charles; Wheeler, John (1973). Gravitation. W. H. Freeman and Company. ISBN 0-7167-0344-0.
Wald, Robert M. (1984). General Relativity. University of Chicago Press. ISBN 978-0-226-87033-5.
Wald, Robert M. (1992). Space, Time, and Gravity: The Theory of the Big Bang and Black Holes. University of Chicago Press. ISBN 0-226-87029-4.
Review papers
Gallo, Elena; Marolf, Donald (2009). "Resource Letter BH-2: Black Holes". American Journal of Physics 77 (4): 294. arXiv:0806.2316. Bibcode 2009AmJPh..77..294G. doi:10.1119/1.3056569.
Hughes, Scott A. (2005). "Trust but verify: The case for astrophysical black holes". arXiv:hep-ph/0511217 [hep-ph]. Lecture notes from 2005 SLAC Summer Institute.
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Videos
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v · t · e
Black holes

Types Schwarzschild · Rotating · Charged · Virtual

Size Micro · Extremal (Electron) · Stellar · Intermediate-mass · Supermassive · Quasar (Active galactic nucleus · Blazar)

Formation Stellar evolution · Gravitational collapse · Neutron star (Related links) · Compact star (Quark · Exotic) · Tolman–Oppenheimer–Volkoff limit · White dwarf (Related links) · Supernova (Related links) · Hypernova · Gamma-ray burst

Properties Thermodynamics · Schwarzschild radius · M-sigma relation · Event horizon · Quasi-periodic oscillation · Photon sphere · Ergosphere · Hawking radiation · Penrose process · Bondi accretion · Spaghettification · Gravitational lens

Models Gravitational singularity (Penrose–Hawking singularity theorems) · Primordial black hole · Gravastar · Dark star · Dark energy star · Black star · Magnetospheric eternally collapsing object · Fuzzball · White hole · Naked singularity · Ring singularity · Immirzi parameter · Membrane paradigm · Kugelblitz · Wormhole · Quasi-star

Issues No-hair theorem · Information paradox · Cosmic censorship · Alternative models · Holographic principle · Black hole complementarity

Metrics Schwarzschild · Kerr · Reissner–Nordström · Kerr–Newman

Related List of black holes · Timeline of black hole physics · Rossi X-ray Timing Explorer · Hypercompact stellar system

                                                  The Milky Way is the galaxy that contains the Earth.[11][nb 1] This name derives from its appearance as a dim "milky" glowing band arching across the night sky, in which the naked eye cannot distinguish individual stars. The term "Milky Way" is a translation of the Classical Latin via lactea, from the Hellenistic Greek γαλαξίας κύκλος (pr. galaxías kýklos, "milky circle").[12][13][14] The Milky Way appears like a band because it is a disk-shaped structure being viewed from inside. The fact that this faint band of light is made up of stars was proven in 1610 when Galileo Galilei used his telescope to resolve it into individual stars. In the 1920s, observations by astronomer Edwin Hubble showed that the Milky Way is just one of many galaxies.

The Milky Way is a barred spiral galaxy 100,000–120,000 light-years in diameter containing 200–400 billion stars. It may contain at least as many planets, with an estimated 10 billion of those orbiting in the habitable zone of their parent stars.[15][not in citation given] The Solar System is located within the disk, around two thirds of the way out from the Galactic Center, on the inner edge of a spiral-shaped concentration of gas and dust called the Orion–Cygnus Arm. The stars in the inner ≈10,000 light-years are organized in a bulge and one or more bars. The very center is marked by an intense radio source named Sagittarius A* which is likely to be a supermassive black hole. The Galaxy rotates differentially, faster towards the center and slower towards the outer edge. The rotational period is about 200 million years at the position of the Sun.[8] The Galaxy as a whole is moving at a velocity of 552 to 630 km per second, depending on the relative frame of reference. It is estimated to be about 13.2 billion years old, nearly as old as the Universe. Surrounded by several smaller satellite galaxies, the Milky Way is part of the Local Group of galaxies, which forms a subcomponent of the Virgo Supercluster.Contents [hide]
1 Appearance
2 Size and composition
3 Structure
3.1 Galactic Center
3.2 Spiral arms
3.3 Halo
3.4 Gamma-ray bubbles
3.5 Sun's location and neighborhood
3.6 Galactic rotation
4 Formation
4.1 Age
5 Environment
6 Velocity
7 Etymology and mythology
8 Astronomical history
9 See also
10 Notes
11 References
12 Further reading
13 External links

[edit]
Appearance

A view of the Milky Way towards the Constellation Sagittarius (including the Galactic Center) as seen from a non-light polluted area (the Black Rock Desert, Nevada).

When observing the night sky, the term "Milky Way" is limited to the hazy band of white light some 30 degrees wide arcing across the sky[16] (although all of the stars that can be seen with the naked eye are part of the Milky Way Galaxy). The light in this band originates from un-resolved stars and other material that lie within the Galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, correspond to areas where light from distant stars is blocked by interstellar dust.

The Milky Way has a relatively low surface brightness. Its visibility can be greatly reduced by background light such as light pollution or stray light from the moon. It is readily visible when the limiting magnitude is +5.1 or better, while showing a great deal of detail at +6.1.[17] This makes the Milky Way difficult to see from any brightly-lit urban or suburban location but very prominent when viewed from a rural area when the moon is below the horizon.[nb 2]

The Milky Way passes through parts of roughly 30 constellations. The center of the Galaxy lies in the direction of the constellation Sagittarius; it is here that the Milky Way is brightest. From Sagittarius, the hazy band of white light appears to pass westward to the Galactic anticenter in Auriga. The band then continues westward the rest of the way around the sky back to Sagittarius. The fact that the band divides the night sky into two roughly equal hemispheres indicates that the Solar System lies close to the Galactic plane.[citation needed]

The Galactic plane is inclined by about 60 degrees to the ecliptic (the plane of the Earth's orbit). Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth's equatorial plane and the plane of the ecliptic relative to the Galactic plane. The north Galactic pole is situated at right ascension 12h 49m, declination +27.4° (B1950) near beta Comae Berenices, and the south Galactic pole is near alpha Sculptoris. Because of this high inclination, depending on the time of night and the year, the arc of Milky Way can appear relatively low or relatively high in the sky. For observers from about 65 degrees north to 65 degrees south on the Earth's surface the Milky Way passes directly overhead twice a day.

A fish-eye mosaic of the Milky Way arching at a high inclination across the night sky, shot from a dark sky location in Chile.
[edit]
Size and composition

Schematic illustration showing the galaxy in profile.

The stellar disk of the Milky Way Galaxy is approximately 100,000 light-years (30 kiloparsecs) in diameter, and is, on average, about 1,000 ly (0.3 kpc) thick.[2] As a guide to the relative physical scale of the Milky Way, if it were reduced to 100 meters (110 yd) in diameter, the Solar System, including the hypothesized Oort cloud, would be no more than 1 millimeter (0.039 in) in width. The nearest star, Proxima Centauri, would be 4.2 mm (0.17 in) distant.[nb 3] Alternatively visualized, if the Solar System out to Pluto were the size of a US quarter (1 inch or 25mm in diameter) the Milky way would be a disk approximately 2,000 kilometers (1,200 miles) in diameter, having roughly one-third the area of the United States.[18]

The Milky Way contains at least 100 billion stars[19] and may have up to 400 billion stars.[20][21] The exact figure depends on the number of very low-mass, or dwarf stars, which are hard to detect, especially at distances of more than 300 ly (90 pc) from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (1012) stars.[22] Filling the space between the stars is a disk of gas and dust called the interstellar medium. This disk has at least a comparable extent in radius to the stars,[23] while the thickness of the gas layer ranges from hundreds of light years for the colder gas to thousands of light years for warmer gas.[24][25] Both gravitational microlensing and planetary transit observations indicate that there may be at least as many planets bound to stars as there are stars in the Milky Way,[15][26] while microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars.[27][28] Earth-sized planets may be more numerous than gas giants.[15]

The disk of stars in the Milky Way does not have a sharp edge beyond which there are no stars. Rather, the concentration of stars drops smoothly with distance from the center of the Galaxy. Beyond a radius of roughly 40,000 ly (12 kpc), the number of stars per cubic parsec drops much faster with radius,[29] for reasons that are not understood. Surrounding the Galactic disk is a spherical Galactic Halo of stars and globular clusters that extends further outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose closest approach to the Galactic center is about 180,000 ly (55 kpc).[30] At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds. Hence, such objects would likely be ejected from the vicinity of the Milky Way. The integrated absolute visual magnitude of the Milky Way is estimated to be −20.9.[31]

360-degree panorama view of the Milky Way Galaxy (an assembled mosaic of photographs).

Estimates for the mass of the Milky Way vary, depending upon the method and data used. At the low end of the estimate range, the mass of the Milky Way is 5.8×1011 solar masses (M☉), somewhat smaller than the Andromeda Galaxy.[32][33][34] Measurements using the Very Long Baseline Array in 2009 found velocities as large as 254 km/s for stars at the outer edge of the Milky Way, higher than the previously accepted value of 220 km/s.[35] As the orbital velocity depends on the total mass inside the orbital radius, this suggests that the Milky Way is more massive, roughly equaling the mass of Andromeda Galaxy at 7×1011 M☉ within 50 kiloparsecs (160,000 ly) of its center.[36] A 2010 measurement of the radial velocity of halo stars finds the mass enclosed within 80 kiloparsecs is 7×1011 M☉.[37] Most of the mass of the Galaxy appears to be matter of unknown form which interacts with other matter through gravitational but not electromagnetic forces; this is dubbed dark matter. A dark matter halo is spread out relatively uniformly to a distance beyond one hundred kiloparsecs from the Galactic Center. Mathematical models of the Milky Way suggests that the total mass of the entire Galaxy lies in the range 1-1.5×1012 M☉.[6]
[edit]
Structure

Artist's conception of the spiral structure of the Milky Way with two major stellar arms and a bar.[38]

A false-color infrared image of the core of the Milky Way Galaxy taken by NASA's Spitzer Space Telescope. Older cool stars are blue, dust features lit up by large hot stars are shown in a reddish hue, and the bright white spot in the middle marks the site of Sagittarius A*, the super-massive black hole at the center of the Galaxy.

The Galaxy consists of a bar-shaped core region surrounded by a disk of gas, dust and stars. The gas, dust and stars are organized in roughly logarithmic spiral arm structures (see Spiral arms below). The mass distribution within the Galaxy closely resembles the SBc Hubble classification, which is a spiral galaxy with relatively loosely wound arms.[1] Astronomers first began to suspect that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in the 1990s.[39] Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005[40] that showed the Galaxy's central bar to be larger than previously suspected.
[edit]
Galactic Center
Main article: Galactic Center

The Sun is 8.0–8.7 kpc (26,000–28,000 ly) from the Galactic Center. This value is estimated based upon geometric-based methods or using selected astronomical objects that serve as standard candles, with different techniques yielding different values within this approximate range.[7][41][42][43][44] In the inner few kpc (≈10,000 light-years) is a dense concentration of mostly old stars in a roughly spheroidal shape called the bulge.[45]

The Galactic Center is marked by an intense radio source named Sagittarius A*. The motion of material around the center indicates that Sagittarius A* harbors a massive, compact object.[46] This concentration of mass is best explained as a supermassive black hole[nb 4][7][41] with an estimated mass of 4.1–4.5 million times the mass of the Sun.[41] Observations indicate that there are supermassive black holes located near the center of most normal galaxies.[47][48]

The nature of the Galaxy's bar is actively debated, with estimates for its half-length and orientation spanning from 1–5 kpc (3,300–16,000 ly) (short or a long bar) and 10–50 degrees relative to the line of sight from Earth to the Galactic Center.[43][44][49] Certain authors advocate that the Galaxy features two distinct bars, one nestled within the other.[50] The bar is delineated by red clump stars. However, RR Lyr variables do not trace a prominent Galactic bar.[44][51][52] The bar may be surrounded by a ring called the "5-kpc ring" that contains a large fraction of the molecular hydrogen present in the Galaxy, as well as most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of our own Galaxy.[53]
[edit]
Spiral arms

Beyond the gravitational influence of the Galactic bars, astronomers generally organize the interstellar medium and stars in the disk of the Milky Way in four spiral arms.[54] All of these arms contain more interstellar gas and dust than the Galactic average as well as a high concentration of star formation, traced by H II regions[55][56] and molecular clouds.[57] Counts of stars in near infrared light indicate that two arms contain approximately 30% more red giant stars than would be expected in the absence of a spiral arm, while two do not contain more red giant stars than regions outside of arms.[58][59]

Maps of the Milky Way's spiral structure are notoriously uncertain and exhibit striking differences.[38][54][56][60][61][62][63][64] Some 150 years after Alexander (1852)[65] first suggested that the Milky Way was a spiral, there is currently no consensus on the nature of the Galaxy's spiral arms. Perfect logarithmic spiral patterns ineptly describe features near the Sun,[56][63] namely since galaxies commonly exhibit arms that branch, merge, twist unexpectedly, and feature a degree of irregularity.[44][63][64] The possible scenario of the Sun within a spur / Local arm[56] emphasizes that point and indicates that such features are likely not unique, and exist elsewhere in the Galaxy.[63]

As in most spiral galaxies, each spiral arm can be described as a logarithmic spiral. Estimates of the pitch angle of the arms range from ≈7° to ≈25°.[58][66] Until recently, there were thought to be four major spiral arms which all start near the Galaxy's center. These are named as follows, with the positions of the arms shown in the image at right:

Observed and extrapolated structure of the spiral arms. The gray lines radiating from the Sun's position (upper center) list the three-letter abbreviations of the corresponding constellations.Color Arm(s)
cyan 3-kpc and Perseus Arm
purple Norma and Outer arm (Along with extension discovered in 2004[67])
green Scutum–Centaurus Arm
pink Carina–Sagittarius Arm
There are at least two smaller arms or spurs, including:
orange Orion–Cygnus Arm (which contains the Sun and Solar System)


Two spiral arms, the Scutum–Centaurus arm and the Carina–Sagittarius arm, have tangent points inside the Sun's orbit around the center of the Milky Way. If these arms contain an overdensity of stars compared to the average density of stars in the Galactic disk, it would be detectable by counting the stars near the tangent point. Two surveys of near-infrared light, which is sensitive primarily to red giant stars and not affected by dust extinction, detected the predicted overabundance in the Scutum–Centaurus arm but not in the Carina–Sagittarius arm.[58][59] In 2008, Robert Benjamin of the University of Wisconsin–Whitewater used this observation to suggest that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum–Centaurus arm. The rest of the arms contain excess gas but not excess stars.[38]

Outside of the major spiral arms is the Monoceros Ring (or Outer Ring), proposed by astronomers Brian Yanny and Heidi Jo Newberg, a ring of gas and stars torn from other galaxies billions of years ago.

Another interesting aspect is the so-called "wind-up problem" of the spiral arms. If the inner parts of the arms rotate faster than the outer part, then the galaxy will wind up so much that the spiral structure will be thinned out. But this is not what is observed in spiral galaxies; instead, astronomers propose that the spiral pattern is a density wave emanating from the Galactic Center. This can be likened to a moving traffic jam on a highway—the cars are all moving, but there is always a region of slow-moving cars. This model also agrees with enhanced star formation in or near spiral arms; the compressional waves increase the density of molecular hydrogen and protostars form as a result.
[edit]
Halo

The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters, of which 90% lie within 100,000 light-years (30 kpc) of the Galactic Center,[68] suggesting a stellar halo diameter of 200,000 light-years. However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years away from the Galactic Center. About 40% of the galaxy's clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation.[69] The globular clusters can follow rosette orbits about the Galaxy, in contrast to the elliptical orbit of a planet around a star.[70]

While the disk contains gas and dust which obscure the view in some wavelengths, the spheroid component does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but not in the halo. Open clusters also occur primarily in the disk.

Discoveries in the early 21st century have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disk of the Andromeda Galaxy (M31) extends much further than previously thought,[71] the possibility of the disk of the Milky Way Galaxy extending further is apparent, and this is supported by evidence from the 2004 discovery of the Outer Arm extension of the Cygnus Arm.[67][72] With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy, it was found that a ring of galactic debris from its interaction with the Milky Way encircles the Galactic disk.

On January 9, 2006, Mario Jurić and others of Princeton University announced that the Sloan Digital Sky Survey of the northern sky found a huge and diffuse structure (spread out across an area around 5,000 times the size of a full moon) within the Milky Way that does not seem to fit within current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the Galaxy. The proposed likely interpretation is that a dwarf galaxy is merging with the Milky Way. This galaxy is tentatively named the Virgo Stellar Stream and is found in the direction of Virgo about 30,000 light-years (9 kpc) away.[73]

Illustration of the two gigantic X-ray/gamma-ray bubbles (blue-violet) of the Milky Way (center).
[edit]
Gamma-ray bubbles

On November 9, 2010, Doug Finkbeiner of the Harvard–Smithsonian Center for Astrophysics announced that he had detected two gigantic spherical bubbles of high energy emission that are erupting to the north and the south of the Milky Way core, using data of the Fermi Gamma-ray Space Telescope. The diameter of each of the bubbles is about 25,000 light-years (7.7 kpc); they stretch up to Grus and to Virgo on the night-sky of the southern hemisphere. Their origin remains unclear, so far.[74][75]
[edit]
Sun's location and neighborhood

Diagram of the Sun location in the Milky Way Galaxy. The angles represent longitudes in the galactic coordinate system.

Diagram of the stars in the Solar neighborhood.

The Sun (and therefore the Earth and the Solar System) may be found close to the inner rim of the Galaxy's Orion Arm, in the Local Fluff inside the Local Bubble, and in the Gould Belt, at a distance of 8.33 ± 0.35 kiloparsecs (27,200 ± 1,100 ly) from the Galactic Center.[7][41][76] The Sun is currently 5–30 parsecs (16–98 ly) from the central plane of the Galactic disk.[77] The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years (2.0 kpc).[78] The Sun, and thus the Solar System, is found in the Galactic habitable zone.

There are about 208 stars brighter than absolute magnitude 8.5 within a sphere with a radius of 15 parsecs (49 ly) from the Sun, giving a density of 0.0147 such stars per cubic parsec, or 0.000424 per cubic light-year (from List of nearest bright stars). On the other hand, there are 64 known stars (of any magnitude, not counting 4 brown dwarfs) within 5 parsecs (16 ly) of the Sun, giving a density of 0.122 stars per cubic parsec, or 0.00352 per cubic light-year (from List of nearest stars), illustrating the fact that most stars are less bright than absolute magnitude 8.5.[citation needed][original research?]

The Apex of the Sun's Way, or the solar apex, is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun's Galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center. The Sun's orbit around the Galaxy is expected to be roughly elliptical with the addition of perturbations due to the Galactic spiral arms and non-uniform mass distributions. In addition, the Sun oscillates up and down relative to the Galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. These oscillations were until recently thought to coincide with mass extinction periods on Earth.[79] However, a reanalysis of the effects of the Sun's transit through the spiral structure based on CO data has failed to find these correlations.[80]

It takes the Solar System about 225–250 million years to complete one orbit around the Galaxy (a Galactic year),[81] so the Sun is thought to have completed 18–20 orbits during its lifetime and 1/1250 of a revolution since the origin of humans. The orbital speed of the Solar System about the center of the Galaxy is approximately 220 km/s or 0.073% of the speed of light. At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU (astronomical unit).[82]

Galaxy rotation curve for the Milky Way. Vertical axis is speed of rotation about the Galactic Center. Horizontal axis is distance from the Galactic Center in kpcs. The Sun is marked with a yellow ball. The observed curve of speed of rotation is blue. The predicted curve based upon stellar mass and gas in the Milky Way is red. Scatter in observations roughly indicated by gray bars. The difference is due to dark matter.[83][84][85]
[edit]
Galactic rotation

The stars and gas in the Galaxy rotate about its center differentially, meaning that the rotation period varies with location. As is typical for spiral galaxies, the distribution of mass in the Milky Way Galaxy is such that the orbital speed of most stars in the Galaxy does not depend strongly on their distance from the center. Away from the central bulge or outer rim, the typical stellar orbital speed is between 210 and 240 km/s.[86] Hence the orbital period of the typical star is directly proportional only to the length of the path traveled. This is unlike the situation within the Solar System, where two-body gravitational dynamics dominate and different orbits have significantly different velocities associated with them. The rotation curve (shown in the figure) describes this rotation.

If the Galaxy contained only the mass observed in stars, gas, and other baryonic (ordinary) matter, the rotation speed would decrease with distance from the center. However, the observed curve is relatively flat, indicating that there is additional mass that cannot be detected directly with electromagnetic radiation. This inconsistency is attributed to dark matter.[84] Alternatively, a minority of astronomers propose that a modification of the law of gravity may explain the observed rotation curve.[87] The constant rotation speed of most of the Galaxy means that objects further from the Galactic center take longer to orbit the center than objects closer in.
[edit]
Formation This section requires expansion. (February 2012)

Main article: Galaxy formation and evolution

The Milky Way began as one or several small overdensities in the mass distribution in the Universe shortly after the Big Bang. Some of these overdensities were the seeds of globular clusters in which the oldest remaining stars in what is now the Milky Way formed. These stars and clusters now comprise the stellar halo of the Galaxy. Within a few billion years of the birth of the first stars, the mass of the Milky Way was large enough so that it was spinning relatively quickly. Due to conservation of angular momentum, this led the gaseous interstellar medium to collapse from a roughly spheroidal shape to a disk. Therefore, later generations of stars formed in this spiral disk. Most younger stars, including the Sun, are observed to be in the disk.[88][89]

Since the first stars began to form, the Milky Way has grown through both galaxy mergers (particularly early in the Galaxy's growth) and accretion of gas directly from the Galactic halo.[89] The Milky Way is currently accreting material from its two nearest satellite galaxies, the Large and Small Magellanic Clouds, through the Magellanic Stream. Direct accretion of gas is observed in high velocity clouds like the Smith Cloud.[90][91] However, properties of the Milky Way such as stellar mass, angular momentum, and metallicity in its outermost regions suggest it has suffered no mergers with large galaxies in the last 10 billion years. This lack of recent major mergers is unusual among similar spiral galaxies; its neighbour the Andromeda Galaxy appears to have a more typical history shaped by more recent mergers with relatively large galaxies.[92][93]

According to recent studies, the Milky Way as well as Andromeda lie in what in the galaxy color-magnitude diagram is known as the green valley, a region populated by galaxies in transition from the blue cloud (galaxies actively forming new stars) to the red sequence (galaxies that lack star formation) that are slowing its star formation activity as they run out of hydrogen to born new stars. In simulated galaxies with similar properties, star formation will typically have been extinguished within about five billion years from now, even accounting for the expected, short-term increase in the rate of star formation due to the collision between both galaxies.[94] In fact, measurements of other galaxies similar to our own suggest it's among the reddest and brightest spiral galaxies that are still forming new stars and it's just slightly bluer than the bluest red sequence galaxies.[95]
[edit]
Age

The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long-lived radioactive elements such as thorium-232 and uranium-238, then comparing the results to estimates of their original abundance, a technique called nucleocosmochronology. These yield values of about 14.0 ± 2.4 billion years (Ga) for CS 31082-001 and 13.8 ± 4 billion years for BD+17° 3248. Once a white dwarf star is formed, it begins to undergo radiative cooling and the surface temperature steadily drops. By measuring the temperatures of the coolest of these white dwarfs and comparing them to their expected initial temperature, an age estimate can be made. With this technique, the age of the globular cluster M4 was estimated as 12.7 ± 0.7 billion years. Globular clusters are among the oldest objects in the Milky Way Galaxy, which thus set a lower limit on the Galaxy age. Age estimates of the oldest of these clusters gives a best fit estimate of 12.6 billion years, and a 95% confidence upper limit of 16 billion years.[96]

In 2007, a star in the Galactic halo, HE 1523-0901, was estimated to be about 13.2 billion years old, ≈0.5 billion years less than the age of the universe. As the oldest known object in the Milky Way at that time, this measurement placed a lower limit on the age of the Milky Way.[5] This estimate was determined using the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure the relative strengths of spectral lines caused by the presence of Thorium and other elements created by the R-process. The line strengths yield abundances of different elemental isotopes, from which an estimate of the age of the star can be derived using nucleocosmochronology.[5]

The age of stars in the Galactic thin disk has also been estimated using nucleocosmochronology. Measurements of thin disk stars yield an estimate that the thin disk formed between 8.8 ± 1.7 billion years ago. These measurements suggest there was a hiatus of almost 5 billion years between the formation of the Galactic halo and the thin disk.[97]
[edit]
Environment

Diagram of the galaxies in the Local Group relative to the Milky Way.

The position of the Local Group within the Virgo Supercluster.
Main articles: Local Group and Andromeda–Milky Way collision

The Milky Way and the Andromeda Galaxy are a binary system of giant spiral galaxies belonging to a group of 50 closely bound galaxies known as the Local Group, itself being part of the Virgo Supercluster.

Two smaller galaxies and a number of dwarf galaxies in the Local Group orbit the Milky Way. The largest of these is the Large Magellanic Cloud with a diameter of 20,000 light-years. It has a close companion, the Small Magellanic Cloud. The Magellanic Stream is a peculiar streamer of neutral hydrogen gas connecting these two small galaxies. The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Milky Way. Some of the dwarf galaxies orbiting the Milky Way are Canis Major Dwarf (the closest), Sagittarius Dwarf Elliptical Galaxy, Ursa Minor Dwarf, Sculptor Dwarf, Sextans Dwarf, Fornax Dwarf, and Leo I Dwarf. The smallest Milky Way dwarf galaxies are only 500 light-years in diameter. These include Carina Dwarf, Draco Dwarf, and Leo II Dwarf. There may still be undetected dwarf galaxies, which are dynamically bound to the Milky Way, as well as some that have already been absorbed by the Milky Way, such as Omega Centauri. Observations through the Zone of Avoidance are frequently detecting new distant and nearby galaxies. Some galaxies consisting mostly of gas and dust may also have evaded detection so far.

In January 2006, researchers reported that the heretofore unexplained warp in the disk of the Milky Way has now been mapped and found to be a ripple or vibration set up by the Large and Small Magellanic Clouds as they circle the Galaxy, causing vibrations at certain frequencies when they pass through its edges.[98] Previously, these two galaxies, at around 2% of the mass of the Milky Way, were considered too small to influence the Milky Way. However, by taking into account dark matter, the movement of these two galaxies creates a wake that influences the larger Milky Way. Taking dark matter into account results in an approximately twentyfold increase in mass for the galaxy. This calculation is according to a computer model made by Martin Weinberg of the University of Massachusetts Amherst. In this model, the dark matter is spreading out from the Galactic disk with the known gas layer. As a result, the model predicts that the gravitational effect of the Magellanic Clouds is amplified as they pass through the Galaxy.

Current measurements suggest the Andromeda Galaxy is approaching us at 100 to 140 kilometers per second. The Milky Way may collide with it in 3 to 4 billion years, depending on the importance of unknown lateral components to the galaxies' relative motion. If they collide, individual stars within the galaxies would not collide, but instead the two galaxies will merge to form a single elliptical galaxy over the course of about a billion years.[99]
[edit]
Velocity

In the general sense, the absolute velocity of any object through space is not a meaningful question according to Einstein's special theory of relativity, which declares that there is no "preferred" inertial frame of reference in space with which to compare the object's motion. (Motion must always be specified with respect to another object.) This must be kept in mind when discussing the Galaxy's motion.

Astronomers believe the Milky Way is moving at approximately 630 km per second relative to the average velocity of galaxies taken over a large enough volume so that the expansion of the Universe dominates over local, random motions: the local co-moving frame of reference that moves with the Hubble flow.[100][further explanation needed] The Milky Way is moving in the general direction of the Great Attractor and other galaxy clusters, including the Shapley supercluster, behind it.[101] The Local Group (a cluster of gravitationally bound galaxies containing, among others, the Milky Way and the Andromeda Galaxy) is part of a supercluster called the Local Supercluster, centered near the Virgo Cluster: although they are moving away from each other at 967 km/s as part of the Hubble flow, this velocity is less than would be expected given the 16.8 million pc distance due to the gravitational attraction between the Local Group and the Virgo Cluster.[102]

Another reference frame is provided by the cosmic microwave background (CMB). The Milky Way is moving at 552 ± 6 km/s[10] with respect to the photons of the CMB, toward 10.5 right ascension, −24° declination (J2000 epoch, near the center of Hydra). This motion is observed by satellites such as the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP) as a dipole contribution to the CMB, as photons in equilibrium in the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction.[10]
[edit]
Etymology and mythology
Main articles: List of names for the Milky Way and Milky Way (mythology)

In western culture the name "Milky Way" is derived from its appearance as a dim un-resolved "milky" glowing band arching across the night sky. The term is a translation of the Classical Latin via lactea, in turn derived from the Hellenistic Greek γαλαξίας, short for γαλαξίας κύκλος (pr. galaktikos kyklos, "milky circle"). The Ancient Greek γαλαξίας (galaxias), from root γαλακτ-, γάλα (milk) + -ίας (forming adjectives), is also the root of "galaxy", the name for our, and later all such, collections of stars.[12][103][104][105] The Milky Way "milk circle" was just one of 11 circles the Greeks identified in the sky, others being the zodiac, the meridian, the horizon, the equator, the tropics of Cancer and Capricorn, Arctic and Antarctic circles, and two colure circles passing through both poles.[106]

There are many creation myths around the world which explain the origin of the Milky Way and give it its name. In Greek myth, the Milky Way was caused by milk spilt by Hera when suckling Heracles.[107] It is also described as the road to mount Olympus, and the path of ruin made by the chariot of the Sun god Helios.[108]

In Sanskrit and several other Indo-Aryan languages, the Milky Way is called Akash Ganga (आकाशगंगा, Ganges of the heavens); it is held to be sacred in the Hindu Puranas (scriptures), and the Ganges and the Milky Way are considered to be terrestrial and celestial analogs.[109][110] Kshira (क्षीर, milk) is an alternative name for the Milky Way in Hindu texts in Sanskrit.[111]
[edit]
Astronomical history
See also: Galaxy#Observation history

The shape of the Milky Way as deduced from star counts by William Herschel in 1785; the Solar System was assumed near center

As Aristotle (384–322 BC) informs us in Meteorologica (DK 59 A80), the Greek philosophers Anaxagoras (ca. 500–428 BC) and Democritus (450–370 BC) proposed the Milky Way might consist of distant stars. However, Aristotle himself believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions."[112] The Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 A.D.) criticized this view, arguing that if the Milky Way were sublunary it should appear different at different times and places on the Earth, and that it should have parallax, which it does not. In his view, the Milky Way was celestial. This idea would be influential later in the Islamic world.[113]

According to Mohaini Mohamed, the Arabian astronomer, Alhazen (965–1037 AD), refuted Aristotle's view by making the first attempt at observing and measuring the Milky Way's parallax.[114][verification needed] He determined that the Milky Way has no parallax and concluded that it must be remote from the Earth, not part of Earth's atmosphere.[115]

The Persian astronomer Abū Rayhān al-Bīrūnī (973–1048) proposed that the Milky Way is "a collection of countless fragments of the nature of nebulous stars".[116] The Andalusian astronomer Avempace (d. 1138) proposed the Milky Way to be made up of many stars but appears to be a continuous image due to the effect of refraction in the Earth's atmosphere, citing his observation of a conjunction of Jupiter and Mars in 1106 or 1107 as evidence.[112] Ibn Qayyim Al-Jawziyya (1292–1350) proposed the Milky Way Galaxy to be "a myriad of tiny stars packed together in the sphere of the fixed stars" and that these stars are larger than planets.[117]

According to Jamil Ragep, the Persian astronomer Naṣīr al-Dīn al-Ṭūsī (1201,1274) in his Tadhkira writes: "The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly-clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. because of this, it was likened to milk in color."[118]

Actual proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it was composed of a huge number of faint stars.[119] In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright, speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by gravitational forces akin to the Solar System but on much larger scales. The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the nebulae visible in the night sky might be separate "galaxies" themselves, similar to our own. Kant referred to both our Galaxy and the "extragalactic nebulae" as "island universes", a term still current up to the 1930s.[120]

The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the visible sky. He produced a diagram of the shape of the Galaxy with the Solar System close to the center.

In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral-shaped nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.[121]

Photograph of the "Great Andromeda Nebula" from 1899, later identified as the Andromeda Galaxy

In 1917, Heber Curtis had observed the nova S Andromedae within the "Great Andromeda Nebula" (Messier object M31). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our Galaxy. As a result he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the "island universes" hypothesis, which held that the spiral nebulae were actually independent galaxies.[122] In 1920 the Great Debate took place between Harlow Shapley and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula was an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.[123]

The matter was conclusively settled by Edwin Hubble in the early 1920s using the Mount Wilson observatory 100 inch (2.5 m) Hooker telescope. With the light-gathering power of this new telescope he was able to produce astronomical photographs that resolved the outer parts of some spiral nebulae as collections of individual stars. He was also able to identify some Cepheid variables that he could use as a benchmark to estimate the distance to the nebulae: proving they were far too distant to be part of the Milky Way.[124] In 1936, Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence.[125]
[edit]
See also Astronomy portal
Star portal
Space portal

Baade's Window
Galactic coordinate system
MilkyWay@Home, a distributed computing project that attempts to generate highly accurate three-dimensional dynamic models of stellar streams in the immediate vicinity of our Milky Way Galaxy.
Oort constants
[edit]
Notes
^ Some sources hold that, strictly speaking, the term Milky Way should refer exclusively to the band of light that the galaxy forms in the night sky, while the galaxy should receive the full name Milky Way Galaxy, or alternatively the Galaxy. However, it is unclear how widespread this convention is, and the term Milky Way is routinely used in either context. See:
Freedman, Roger A.; Kaufmann, William J. (2007). Universe. WH Freeman & Co.. p. 605. ISBN 0-7167-8584-6.
"Galaxies — Milky Way Galaxy". Encyclopædia Britannica. 19. Encyclopædia Britannica, Inc.. 1998. pp. 618.
"Galaxies — Milky Way Galaxy". Encyclopædia Britannica. 19. Encyclopædia Britannica, Inc.. 1998. pp. 618.
Pasachoff, Jay M. (1994). Astronomy: From the Earth to the Universe. Harcourt School. p. 500. ISBN 0-03-001667-3.
^ See also Bortle Dark-Sky Scale
^ The scale is 1 mm equals 1 ly.
^ For a photo see: "Sagittarius A*: Milky Way monster stars in cosmic reality show". Chandra X-ray Observatory. Harvard-Smithsonian Center for Astrophysics. January 6, 2003. Retrieved 2012-05-20.
[edit]
References
^ a b Gerhard, O. (2002). "Mass distribution in our Galaxy". Space Science Reviews 100 (1/4): 129–138. arXiv:astro-ph/0203110. Bibcode 2002astro.ph..3110G. doi:10.1023/A:1015818111633. edit
^ a b c Christian, Eric; Safi-Harb, Samar. "How large is the Milky Way?". NASA: Ask an Astrophysicist. Retrieved 2007-11-28.
^ "NASA – Galaxy". NASA and World Book. Nasa.gov. November 29, 2007. Retrieved 2010-08-10.[dead link]
^ Staff (December 16, 2008). "How Many Stars are in the Milky Way?". Universe Today. Retrieved 2010-08-10.
^ a b c Frebel, A. et al. (2007). "Discovery of HE 1523-0901, a strongly r-process-enhanced metal-poor star with detected uranium". The Astrophysical Journal 660 (2): L117. arXiv:astro-ph/0703414. Bibcode 2007ApJ...660L.117F. doi:10.1086/518122. edit
^ a b McMillan, P. J. (July 2011). "Mass models of the Milky Way". Monthly Notices of the Royal Astronomical Society 414 (3): 2446–2457. Bibcode 2011MNRAS.414.2446M. doi:10.1111/j.1365-2966.2011.18564.x. edit
^ a b c d Gillessen, S. et al. (2009). "Monitoring stellar orbits around the massive black hole in the Galactic Center". Astrophysical Journal 692 (2): 1075–1109. arXiv:0810.4674. Bibcode 2009ApJ...692.1075G. doi:10.1088/0004-637X/692/2/1075. edit
^ a b Gunter Faure, Teresa M. Mensing, Introduction to Planetary Science: The Geological Perspective, page 45
^ a b Bissantz, N.; Englmaier, P.; Gerhard, O. (2003). "Gas dynamics in the Milky Way: second pattern speed and large-scale morphology". Monthly Notices of the Royal Astronomical Society 340 (3): 949. arXiv:astro-ph/0212516. Bibcode 2003MNRAS.340..949B. doi:10.1046/j.1365-8711.2003.06358.x. edit
^ a b c Kogut, A. et al. (1993). "Dipole anisotropy in the COBE differential microwave radiometers first-year sky maps". The Astrophysical Journal 419: 1. arXiv:astro-ph/9312056. Bibcode 1993ApJ...419....1K. doi:10.1086/173453. edit
^ "Collins Elementary English Dictionary – Complete and Unabridged 1991-2003 - Milky Way". The American Heritage Science Dictionary. thefreedictionary.com. 2005. Retrieved 2012-05-20.
^ a b Harper, Douglas. "galaxy". Online Etymology Dictionary. Retrieved 2012-05-20.
^ Jankowski, Connie (2010). Pioneers of Light and Sound. Compass Point Books. p. 6. ISBN 0-7565-4306-1.
^ Schiller, Jon (2010). Big Bang & Black Holes. CreateSpace. p. 163. ISBN 1-4528-6552-3.
^ a b c Cassan, A. et al. (January 11, 2012). "One or more bound planets per Milky Way star from microlensing observations". Nature 481 (7380): 167–169. Bibcode 2012Natur.481..167C. doi:10.1038/nature10684. PMID 22237108. edit
^ Pasachoff, Jay M. (1994). Astronomy: From the Earth to the Universe. Harcourt School. p. 500. ISBN 0-03-001667-3.
^ Steinicke, Wolfgang; Jakiel, Richard (2007). Galaxies and how to observe them. Astronomers' observing guides. Springer. p. 94. ISBN 1-85233-752-4.
^ http://www.nasa.gov/audience/foreducators/5-8/features/F_How_Big_is_Our_Universe.html
^ Villard, Ray (January 11, 2012). "The Milky Way Contains at Least 100 Billion Planets According to Survey". HubbleSite.org. Retrieved 2012-01-11.
^ Frommert, H.; Kronberg, C. (August 25, 2005). "The Milky Way Galaxy". SEDS. Retrieved 2007-05-09.
^ Wethington, Nicholos. "How Many Stars are in the Milky Way?". Retrieved 2010-04-09.
^ Young, Kelly (June 6, 2006). "Andromeda Galaxy hosts a trillion stars". NewScientist. Retrieved 2006-06-08.
^ Levine, E. S.; Blitz, L.; Heiles, C. (2006). "The spiral structure of the outer Milky Way in hydrogen". Science 312 (5781): 1773–1777. arXiv:astro-ph/0605728. Bibcode 2006Sci...312.1773L. doi:10.1126/science.1128455. PMID 16741076. edit
^ Dickey, J. M.; Lockman, F. J. (1990). "H I in the Galaxy". Annual Review of Astronomy and Astrophysics 28: 215. Bibcode 1990ARA&A..28..215D. doi:10.1146/annurev.aa.28.090190.001243. edit
^ Savage, B. D.; Wakker, B. P. (2009). "The extension of the transition temperature plasma into the lower galactic halo". The Astrophysical Journal 702 (2): 1472. Bibcode 2009ApJ...702.1472S. doi:10.1088/0004-637X/702/2/1472. edit
^ Borenstein, Seth (February 19, 2011). "Cosmic census finds crowd of planets in our galaxy". The Washington Post. Associated Press. Archived from the original on 2011-02-21.
^ Sumi, T. et al. (2011). "Unbound or distant planetary mass population detected by gravitational microlensing". Nature 473 (7347): 349–352. arXiv:1105.3544. Bibcode 2011Natur.473..349S. doi:10.1038/nature10092. PMID 21593867. edit
^ "Free-Floating Planets May be More Common Than Stars". Pasadena, CA: NASA's Jet Propulsion Laboratory. February 18, 2011. Archived from the original on 2011-05-25. "The team estimates there are about twice as many of them as stars."
^ Sale, S. E. et al. (February 2010). "The structure of the outer Galactic disc as revealed by IPHAS early a stars". Monthly Notices of the Royal Astronomical Society 402 (2): 713–723. Bibcode 2010MNRAS.402..713S. doi:10.1111/j.1365-2966.2009.15746.x. edit
^ Connors, Tim W.; Kawata, Daisuke; Gibson, Brad K. (2006). "N-body simulations of the Magellanic stream". Monthly Notices of the Royal Astronomical Society 371 (1): 108–120. arXiv:astro-ph/0508390. Bibcode 2006MNRAS.371..108C. doi:10.1111/j.1365-2966.2006.10659.x. edit
^ Coffey, Jerry. "Absolute Magnitude". Retrieved 2010-04-0.
^ Karachentsev, I. D.; Kashibadze, O. G. (2006). "Masses of the local group and of the M81 group estimated from distortions in the local velocity field". Astrophysics 49 (1): 3–18. Bibcode 2006Ap.....49....3K. doi:10.1007/s10511-006-0002-6. edit
^ Vayntrub, Alina (2000). "Mass of the Milky Way". The Physics Factbook. Retrieved 2007-05-09.
^ Battaglia, G. et al. (2005). "The radial velocity dispersion profile of the Galactic halo: Constraining the density profile of the dark halo of the Milky Way". Monthly Notices of the Royal Astronomical Society: 433–442. arXiv:astro-ph/0506102. Bibcode 2005MNRAS.364..433B. doi:10.1111/j.1365-2966.2005.09367.x. edit
^ Finley, Dave; Aguilar, David (January 5, 2009). "Milky Way a Swifter Spinner, More Massive, New Measurements Show". National Radio Astronomy Observatory. Retrieved 2009-01-20.
^ Reid, M. J. et al. (2009). "Trigonometric parallaxes of massive star-forming regions. VI. Galactic structure, fundamental parameters, and noncircular motions". The Astrophysical Journal 700: 137–148. arXiv:0902.3913. Bibcode 2009ApJ...700..137R. doi:10.1088/0004-637X/700/1/137. edit
^ Gnedin, O. Y. et al. (2010). "The mass profile of the Galaxy to 80 kpc". The Astrophysical Journal 720: L108. arXiv:1005.2619. Bibcode 2010ApJ...720L.108G. doi:10.1088/2041-8205/720/1/L108. edit
^ a b c Benjamin, R. A. (2008). "The Spiral Structure of the Galaxy: Something Old, Something New...". In Beuther, H.; Linz, H.; Henning, T. (ed.). Massive Star Formation: Observations Confront Theory. 387. Astronomical Society of the Pacific Conference Series. pp. 375. Bibcode 2008ASPC..387..375B.
See also Bryner, Jeanna (June 3, 2008). "New Images: Milky Way Loses Two Arms". Space.com. Retrieved 2008-06-04.
^ Chen, W.; Gehrels, N.; Diehl, R.; Hartmann, D. (1996). "On the spiral arm interpretation of COMPTEL ^26^Al map features". Space Science Reviews 120: 315–316. Bibcode 1996A&AS..120C.315C.
^ McKee, Maggie (August 16, 2005). "Bar at Milky Way's heart revealed". New Scientist. Retrieved 2009-06-17.
^ a b c d Ghez, A. M. et al. (December 2008). "Measuring distance and properties of the Milky Way's central supermassive black hole with stellar orbits". The Astrophysical Journal 689 (2): 1044–1062. Bibcode 2008ApJ...689.1044G. doi:10.1086/592738. edit
^ Reid, M. J. et al. (November 2009). "A trigonometric parallax of Sgr B2". The Astrophysical Journal 705 (2): 1548–1553. Bibcode 2009ApJ...705.1548R. doi:10.1088/0004-637X/705/2/1548. edit
^ a b Vanhollebeke, E.; Groenewegen, M. A. T.; Girardi, L. (April 2009). "Stellar populations in the Galactic bulge. Modelling the Galactic bulge with TRILEGAL". Astronomy and Astrophysics 498: 95–107. Bibcode 2009A&A...498...95V. doi:10.1051/0004-6361/20078472. edit
^ a b c d Majaess, D. (March 2010). "Concerning the Distance to the Center of the Milky Way and Its Structure". Acta Astronomica 60 (1): 55. arXiv:1002.2743. Bibcode 2010AcA....60...55M.
^ Grant, J.; Lin, B. (2000). "The Stars of the Milky Way". Fairfax Public Access Corporation. Retrieved 2007-05-09.
^ Jones, Mark H.; Lambourne, Robert J.; Adams, David John (2004). An Introduction to Galaxies and Cosmology. Cambridge University Press. pp. 50–51. ISBN 0-521-54623-0.
^ Blandford, R. D. (1999). "Origin and Evolution of Massive Black Holes in Galactic Nuclei". Galaxy Dynamics, proceedings of a conference held at Rutgers University, 8–12 August 1998, ASP Conference Series vol. 182. Bibcode 1999ASPC..182...87B.
^ Frolov, Valeri P.; Zelnikov, Andrei (2011). Introduction to Black Hole Physics. Oxford University Press. pp. 11, 36. ISBN 0199692297.
^ Cabrera-Lavers, A. et al. (December 2008). "The long Galactic bar as seen by UKIDSS Galactic plane survey". Astronomy and Astrophysics 491 (3): 781–787. Bibcode 2008A&A...491..781C. doi:10.1051/0004-6361:200810720. edit
^ Nishiyama, S. et al. (2005). "A distinct structure inside the Galactic bar". The Astrophysical Journal 621 (2): L105. arXiv:astro-ph/0502058. Bibcode 2005ApJ...621L.105N. doi:10.1086/429291. edit
^ Alcock, C. et al. (1998). "The RR Lyrae population of the Galactic Bulge from the MACHO database: mean colors and magnitudes". The Astrophysical Journal 492 (2): 190. arXiv:astro-ph/0502058. Bibcode 2005ApJ...621L.105N. doi:10.1086/305017. edit
^ Kunder, A.; Chaboyer, B. (2008). "Metallicity analysis of Macho Galactic Bulge RR0 Lyrae stars from their light curves". The Astronomical Journal 136 (6): 2441. Bibcode 2008AJ....136.2441K. doi:10.1088/0004-6256/136/6/2441. edit
^ Staff (September 12, 2005). "Introduction: Galactic Ring Survey". Boston University. Retrieved 2007-05-10.
^ a b Churchwell, E. et al. (2009). "The Spitzer/GLIMPSE surveys: a new view of the Milky Way". Publications of the Astronomical Society of the Pacific 121 (877): 213. Bibcode 2009PASP..121..213C. doi:10.1086/597811. edit
^ Taylor, J. H.; Cordes, J. M. (1993). "Pulsar distances and the galactic distribution of free electrons". The Astrophysical Journal 411: 674. doi:10.1086/172870. edit
^ a b c d Russeil, D. (2003). "Star-forming complexes and the spiral structure of our Galaxy". Astronomy and Astrophysics 397: 133–146. Bibcode 2003A&A...397..133R. doi:10.1051/0004-6361:20021504. edit
^ Dame, T. M.; Hartmann, D.; Thaddeus, P. (2001). "The Milky Way in Molecular Clouds: A New Complete CO Survey". The Astrophysical Journal 547 (2): 792. doi:10.1086/318388. edit
^ a b c Drimmel, R. (2000). "Evidence for a two-armed spiral in the Milky Way". Astronomy & Astrophysics 358: L13–L16. arXiv:astro-ph/0005241. Bibcode 2000A&A...358L..13D.
^ a b Benjamin, R. A. et al. (2005). "First GLIMPSE results on the stellar structure of the Galaxy". The Astrophysical Journal 630 (2): L149–L152. arXiv:astro-ph/0508325. Bibcode 2005ApJ...630L.149B. doi:10.1086/491785. edit
^ Nakanishi, Hiroyuki; Sofue, Yoshiaki (2003). "Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: I. The H I Disk". Publications of the Astronomical Society of Japan 55: 191. arXiv:astro-ph/0304338. Bibcode 2003PASJ...55..191N.
^ Vallée, J. P. (2008). "New velocimetry and revised cartography of the spiral arms in the Milky Way—a consistent symbiosis". The Astronomical Journal 135 (4): 1301. Bibcode 2008AJ....135.1301V. doi:10.1088/0004-6256/135/4/1301. edit
^ Hou, L. G.; Han, J. L.; Shi, W. B. (2009). "The spiral structure of our Milky Way Galaxy". Astronomy and Astrophysics 499 (2): 473.           conseellations

In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms (which themselves are generally referred to in non-technical language as "constellations"), which are patterns formed by prominent stars within apparent proximity to one another on Earth's night sky.


There are also numerous historical constellations not recognized by the IAU, or constellations recognized in regional traditions of astronomy or astrology, such as Chinese, Hindu and Australian Aboriginal.Contents [hide]
1 Terminology
2 History
2.1 Ancient Near East
2.2 Graeco-Roman
2.3 Classical Chinese constellations
2.4 Early Modern era
3 IAU constellations
4 Asterisms
5 Dark cloud constellations
6 See also
7 Notes
8 Further reading
8.1 Mythology, lore, history, and archaeoastronomy
8.2 Atlases and celestial maps
8.3 Catalogs
9 External links

[edit]
Terminology

The Late Latin term constellātiō can be translated as "set with stars". The term was first used in astrology, of asterisms that supposedly exerted influence, attested in Ammianus (4th century). In English the term was used from the 14th century, also in astrology, of conjunctions of planets. The modern astronomical sense of "area of the celestial sphere around a specific asterism" dates to the mid 16th century.

Colloquial usage does not distinguish the senses of "asterism" and "area surrounding an asterism". The modern system of constellations used in astronomy focuses primarily on constellations as grid-like segments of the celestial sphere rather than as patterns, while the term for a star-pattern is asterism. For example, the asterism known as the Big Dipper corresponds to the seven brightest stars of the larger IAU constellation of Ursa Major.

The term circumpolar constellation is used for any constellation that, from a particular latitude on Earth, never sets below the horizon. From the north pole, all constellations north of the celestial equator are circumpolar constellations. In the northern latitudes, the informal term equatorial constellation has sometimes used for constellations that lie to the south of the circumpolar constellations.[1] Depending on the definition, equatorial constellations can include those that lie entirely between declinations 45° north and 45° south,[2] or those that pass overhead between the tropics of Cancer and Capricorn. They generally include all constellations that intersect the celestial equator.
[edit]
History
Further information: Former constellations and Star Names: Their Lore and Meaning

The current list of 88 constellations recognised by the International Astronomical Union since 1922 is based on the 48 listed by Ptolemy in his Almagest in the 2nd century.[3][4] Ptolemy's catalogue is informed by Eudoxus of Cnidus, a Greek astronomer of the 4th century BC who introduced earlier Babylonian astronomy to the Hellenistic culture. Of the 48 constellations listed by Ptolemy, thirty can be shown to have a much longer history, reaching back into at least the Late Bronze Age. This concerns the zodiacal constellations in particular.
[edit]
Ancient Near East
See also: Babylonian star catalogues and MUL.APIN

The oldest catalogues of stars and constellations are from Old Babylonian astronomy, beginning in the Middle Bronze Age. The numerous Sumerian names in these catalogues suggest that they build on older, but otherwise unattested, Sumerian traditions of the Early Bronze Age. The classical Zodiac is a product of a revision of the Old Babylonian system in later Neo-Babylonian astronomy 6th century BC. Knowledge of the Neo-Babylonian zodiac is also reflected in the Hebrew Bible. E. W. Bullinger interpreted the creatures appearing in the books of Ezekiel (and thence in Revelation) as the middle signs of the four quarters of the Zodiac,[5][6] with the Lion as Leo, the Bull is Taurus, the Man representing Aquarius and the Eagle standing in for Scorpio.[7] The biblical Book of Job (dating to the 6th to 4th century BC) also makes reference to a number of constellations, including עיש `Ayish "bier", כסיל Kĕciyl "fool" and כימה Kiymah "heap" (Job 9:9, 38:31-32), rendered as "Arcturus, Orion and Pleiades" by the KJV, but `Ayish "the bier" actually corresponding to Ursa Major.[8] The term Mazzaroth מַזָּרֹות, a hapax legomenon in Job 38:32, may be the Hebrew word for the zodiacal constellations.

The Greeks adopted the Babylonian system in the 4th century BC. A total of twenty Ptolemaic constellations are directly continued from the Ancient Near East. Another ten have the same stars but different names.[9]
[edit]
Graeco-Roman

[citation needed]

There is only limited information on indigenous Greek constellations. Some evidence is found in Hesiod.[clarification needed] Greek astronomy essentially adopted the older Babylonian system in the Hellenistic era, first introduced to Greece by Eudoxus of Cnidus in the 4th century BC. The original work of Eudoxus is lost, but it survives as a versification by Aratus, dating to the 3rd century BC. The most complete existing works dealing with the mythical origins of the constellations are by the Hellenistic writer termed pseudo-Eratosthenes and an early Roman writer styled pseudo-Hyginus.

The basis of western astronomy as taught during Late Antiquity and until the Early Modern period is the Almagest by Ptolemy, written in the 2nd century. Indian astronomy is also based on Hellenistic tradition, via transmission by the Indo-Greek kingdoms.
[edit]
Classical Chinese constellations
Main article: Chinese constellation
Further information: Treatise on Astrology of the Kaiyuan Era

In classical Chinese astronomy, the northern sky is divided geometrically, into five "enclosures" and twenty-eight mansions along the ecliptic, grouped into Four Symbols of seven asterisms each. The 28 lunar mansions are one of the most important and also the most ancient structures in the Chinese sky, attested from the 5th century BC. Parallels to the earliest Babylonian (Sumerian) star catalogues suggest that the ancient Chinese system did not arise independently from that of the Ancient Near East.[10] Classical Chinese astronomy is recorded in the Han period and appears in the form of three schools, which are attributed to astronomers of the Zhanguo period. The constellations of the three schools were conflated into a single system by Chen Zhuo, an astronomer of the 3rd century (Three Kingdoms period). Chen Zhuo's work has been lost, but information on his system of constellations survives in Tang period records, notably by Qutan Xida. The oldest extant Chinese star chart dates to the Tang period and was preserved as part of the Dunhuang Manuscripts. Native Chinese astronomy flourished during the Song Dynasty, and during the Yuan Dynasty became increasingly influenced by medieval Islamic astronomy.[11]
[edit]
Early Modern era

The constellations around the South Pole were not observable from north of the equator, by either Babylonians, Greeks, Chinese or Arabs.

The modern constellations in this region were defined during the Age of exploration, notably by Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman at the end of sixteenth century. They were depicted by Johann Bayer in his star atlas Uranometria of 1603. Several more were created by Nicolas Louis de Lacaille in his star catalogue, published in 1756.

Some modern proposals for new constellations were not successful; an example is Quadrans, eponymous of the Quadrantid meteors, now divided between Boötes and Draco. The classical constellation of Argo Navis was broken up into several different constellations, for the convenience of stellar cartographers.

By the end of the Ming Dynasty, Xu Guangqi introduced 23 asterisms of the southern sky based on the knowledge of western star charts.[12] These asterisms have since been incorporated into the traditional Chinese star maps.
[edit]
IAU constellations

In 1922, Henry Norris Russell aided the IAU in dividing the celestial sphere into 88 official constellations.[13] Where possible, these modern constellations usually share the names of their Graeco-Roman predecessors, such as Orion, Leo or Scorpius. The aim of this system is area-mapping, i.e. the division of the celestial sphere into contiguous fields.[14] Out of the 88 modern constellations, 36 lie predominantly in the northern sky, and the other 52 predominantly in the southern.

In 1930, the boundaries between the 88 constellations were devised by Eugène Delporte along vertical and horizontal lines of right ascension and declination.[15] However, the data he used originated back to epoch B1875.0, which was when Benjamin A. Gould first made the proposal to designate boundaries for the celestial sphere, a suggestion upon which Delporte would base his work. The consequence of this early date is that due to the precession of the equinoxes, the borders on a modern star map, such as epoch J2000, are already somewhat skewed and no longer perfectly vertical or horizontal.[16] This effect will increase over the years and centuries to come.
[edit]
Asterisms

Much of the dark space between stars, as seen in the sky of the image above, is due to the human eye's low light sensitivity. Other images (like the Hubble Deep Field – not pictured) detect far more stars.
Main article: Asterism (astronomy)

The stars of the main asterism within a constellation are usually given Greek letters in their order of brightness, the so-called Bayer designation introduced by Johann Bayer in 1603. A total of 1,564 stars are so identified, out of approximately 10,000 stars visible to the naked eye.[17]

The brightest stars, usually the stars that make up the constellation's eponymous asterism, also retain proper names, often from Arabic. For example, the "Little Dipper" asterism of the constellation Ursa Minor has ten stars with Bayer designation, α UMi to π UMi. Of these ten stars, seven have a proper name, viz. Polaris (α UMi), Kochab (β UMi), Pherkad (γ UMi), Yildun (δ UMi), Urodelus (ε UMi), Ahfa al Farkadain (ζ UMi) and Anwar al Farkadain (η UMi).

The stars within an asterism rarely have any substantial astrophysical relationship to each other, and their apparent proximity when viewed from Earth disguises the fact that they are far apart, some being much farther from Earth than others. However, there are some exceptions: many of the stars in the constellation of Ursa Major (including most of the Big Dipper) are genuinely close to one another, travel through the galaxy with similar velocities, and are likely to have formed together as part of a cluster that is slowly dispersing. These stars form the Ursa Major moving group.
[edit]
Dark cloud constellations

The "Emu in the sky," a constellation defined by dark clouds rather than the stars. An IAU interpretation would recognise Crux (the Southern Cross) above the emu's head and Scorpius on the left. The head of the emu is the Coalsack.
Further information: Great Rift (astronomy)

The Great Rift, a series of dark patches in the Milky Way, is more visible and striking in the southern hemisphere than in the northern. It vividly stands out when conditions are otherwise so dark that the Milky Way's central region casts shadows on the ground. Some cultures have discerned shapes in these patches and have given names to these "dark cloud constellations." Members of the Inca civilization identified various dark areas or dark nebulae in the Milky Way as animals, and associated their appearance with the seasonal rains.[18] Australian Aboriginal astronomy also describes dark cloud constellations, the most famous being the "emu in the sky" whose head is formed by the Coalsack.
[edit]
See also Book: Guide to the Constellations
Wikipedia books are collections of articles that can be downloaded or ordered in print.
Star portal
Astronomy portal

Asterism (astronomy)
Zodiacal constellation
Astrological sign
Former constellations
List of constellations
List of constellations by area
List of stars by constellation
Planisphere
List of all constellations in 15 languages (German WP, with interwiki-links to other languages)
[edit]
Notes
^ Steele, Joel Dorman (1884), The story of the stars: New desscriptive astronomy, Science series, American Book Company, p. 220
^ Harbord, John Bradley; Goodwin, H. B. (1897), Glossary of navigation: a vade mecum for practical navigators (3rd ed.), Griffin, p. 142
^ International Astronomical Union. "The Constellations".
^ Ian Ridpath. "Constellation names, abbreviations and sizes".
^ E.W. Bullinger, The Witness of the Stars
^ D. James Kennedy, The Real Meaning of the Zodiac.
^ Richard Hinckley Allen, Star Names: Their Lore and Meaning, Vol. 1 (New York: Dover Publications, 1899, p. 213-215.) argued for Scorpio having previously been called Eagle.
^ Gesenius, Hebrew Lexicon
^ The Origin of the Greek Constellations, by Bradley E. Schaefer. Scientific American, November 2006.
^ Xiaochun Sun, Jacob Kistemaker, The Chinese sky during the Han, vol. 38 of Sinica Leidensia, BRILL, 1997, ISBN 978-90-04-10737-3, p. 18, note 9.
^ Xiaochun Sun, Jacob Kistemaker, The Chinese sky during the Han, vol. 38 of Sinica Leidensia, BRILL, 1997, ISBN 978-90-04-10737-3, chapter 2, 15-36.
^ Sun, Xiaochun (1997). Helaine Selin. ed. Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures. Kluwer Academic Publishers. pp. 910. ISBN 0-7923-4066-3 (HB).
^ "The original names and abbreviations for constellations from 1922.". Retrieved 2010-01-31.
^ "The IAU on constellations". Retrieved 2010-01-31.
^ "Constellation boundaries.". Retrieved 2011-05-24.
^ A.C. Davenhall & S.K. Leggett, "A Catalogue of Constellation Boundary Data", (Centre de Donneés astronomiques de Strasbourg, February 1990).
^ The Bright Star Catalogue lists 9,110 objects of the night sky which are visible to the naked eye (apparent magnitude of 6.5 or brighter). 9,096 of these are stars, all of them well within our galaxy.
^ The Incan View of the Night Sky
[edit]
Further reading
[edit]
Mythology, lore, history, and archaeoastronomy
Allen, Richard Hinckley. (1899) Star-Names And Their Meanings, G. E. Stechert, New York, New York, U.S.A., hardcover; reprint 1963 as Star Names: Their Lore and Meaning, Dover Publications, Inc., Mineola, New York, U.S.A., ISBN 978-0-486-21079-7 softcover.
Olcott, William Tyler. (1911); Star Lore of All Ages, G. P. Putnam's Sons, New York, New York, U.S.A., hardcover; reprint 2004 as Star Lore: Myths, Legends, and Facts, Dover Publications, Inc., Mineola, New York, U.S.A., ISBN 978-0-486-43581-7 softcover.
Kelley, David H. and Milone, Eugene F. (2004) Exploring Ancient Skies: An Encyclopedic Survey of Archaeoastronomy, Springer, ISBN 978-0-387-95310-6 hardcover.
Ridpath, Ian. (1989) Star Tales, Lutterworth Press, ISBN 0-7188-2695-7 hardcover.
Staal, Julius D. W. (1988) The New Patterns in the Sky: Myths and Legends of the Stars, McDonald & Woodward Publishing Co., ISBN 0-939923-10-6 hardcover, ISBN 0-939923-04-1 softcover.
John H. Rogers, "Origins of the Ancient Contellations: I. The Mesopotamian Traditions", Journal of the British Astronomical Association 108 (1998) 9–28.
John H. Rogers, "Origins of the Ancient Contellations: II. The Mediterranean Traditions", Journal of the British Astronomical Association 108 (1998) 79–89.
[edit]
Atlases and celestial maps

Celestial map, signs of the Zodiac and lunar mansions.

General & Nonspecialized – Entire Celestial Heavens:
Becvar, Antonin. Atlas Coeli. Published as Atlas of the Heavens, Sky Publishing Corporation, Cambridge, Massachusetts, U.S.A.; with coordinate grid transparency overlay.
Norton, Arthur Philip. (1910) Norton's Star Atlas, 20th Edition 2003 as Norton's Star Atlas and Reference Handbook, edited by Ridpath, Ian, Pi Press, ISBN 978-0-13-145164-3, hardcover.
National Geographic Society. (1957, 1970, 2001, 2007) The Heavens (1970), Cartographic Division of the National Geographic Society (NGS), Washington, D.C., U.S.A., two sided large map chart depicting the constellations of the heavens; as special supplement to the August 1970 issue of National Geographic. Forerunner map as A Map of The Heavens, as special supplement to the December 1957 issue. Current version 2001 (Tirion), with 2007 reprint.
Sinnott, Roger W. and Perryman, Michael A.C. (1997) Millennium Star Atlas, Epoch 2000.0, Sky Publishing Corporation, Cambridge, Massachusetts, U.S.A., and European Space Agency (ESA), ESTEC, Noordwijk, The Netherlands. Subtitle: "An All-Sky Atlas Comprising One Million Stars to Visual Magnitude Eleven from the Hipparcos and Tycho Catalogues and Ten Thousand Nonstellar Objects". 3 volumes, hardcover, in hardcover slipcase, set ISBN 0-933346-84-0. Vol. 1, 0–8 Hours (Right Ascension), ISBN 0-933346-81-6 hardcover; Vol. 2, 8–16 Hours, ISBN 0-933346-82-4 hardcover; Vol. 3, 16–24 Hours, ISBN 0-933346-83-2 hardcover. Softcover version available. Supplemental separate purchasable coordinate grid transparent overlays.
Tirion, Wil; et al. (1987) Uranometria 2000.0, Willmann-Bell, Inc., Richmond, Virginia, U.S.A., 3 volumes, hardcover. Vol. 1 (1987): "The Northern Hemisphere to −6°", by Wil Tirion, Barry Rappaport, and George Lovi, ISBN 0-943396-14-X hardcover, printed boards (blue). Vol. 2 (1988): "The Southern Hemisphere to +6°", by Wil Tirion, Barry Rappaport and George Lovi, ISBN 0-943396-15-8 hardcover, printed boards (red). Vol. 3 (1993) as a separate added work: The Deep Sky Field Guide to Uranometria 2000.0, by Murray Cragin, James Lucyk, and Barry Rappaport, ISBN 0-943396-38-7 hardcover, printed boards (gray). 2nd Edition 2001 (black or dark background) as collective set of 3 volumes – Vol. 1: Uranometria 2000.0 Deep Sky Atlas, by Wil Tirion, Barry Rappaport, and Will Remaklus, ISBN 978-0-943396-71-2 hardcover, printed boards (blue edging); Vol. 2: Uranometria 2000.0 Deep Sky Atlas, by Wil Tirion, Barry Rappaport, and Will Remaklus, ISBN 978-0-943396-72-9 hardcover, printed boards (green edging); Vol. 3: Uranometria 2000.0 Deep Sky Field Guide by Murray Cragin and Emil Bonanno, ISBN 978-0-943396-73-6, hardcover, printed boards (teal green).
Tirion, Wil and Sinnott, Roger W. (1998) Sky Atlas 2000.0, various editions. 2nd Deluxe Edition, Cambridge University Press, Cambridge, England (UK).

Northern Celestial Hemisphere & North Circumpolar Region:
Becvar, Antonin. (1962) Atlas Borealis 1950.0, Czechoslovak Academy of Sciences (Ceskoslovenske Akademie Ved), Praha, Czechoslovakia, 1st Edition, elephant folio hardcover, with small transparency overlay coordinate grid square and separate paper magnitude legend ruler. 2nd Edition 1972 and 1978 reprint, Czechoslovak Academy of Sciences (Ceskoslovenske Akademie Ved), Prague, Czechoslovakia, and Sky Publishing Corporation, Cambridge, Massachusetts, U.S.A., ISBN 0-933346-01-8 oversize folio softcover spiral bound, with transparency overlay coordinate grid ruler.

Equatorial, Ecliptic, & Zodiacal Celestial Sky:
Becvar, Antonin. (1958) Atlas Eclipticalis 1950.0, Czechoslovak Academy of Sciences (Ceskoslovenske Akademie Ved), Praha, Czechoslovakia, 1st Edition, elephant folio hardcover, with small transparency overlay coordinate grid square and separate paper magnitude legend ruler. 2nd Edition 1974, Czechoslovak Academy of Sciences (Ceskoslovenske Akademie Ved), Prague, Czechoslovakia, and Sky Publishing Corporation, Cambridge, Massachusetts, U.S.A., oversize folio softcover spiral bound, with transparency overlay coordinate grid ruler.

Southern Celestial Hemisphere & South Circumpolar Region:
Becvar, Antonin. Atlas Australis 1950.0, Czechoslovak Academy of Sciences (Ceskoslovenske Akademie Ved), Praha, Czechoslovakia, 1st Edition, elephant folio hardcover, with small transparency overlay coordinate grid square and separate paper magnitude legend ruler. 2nd Edition, Czechoslovak Academy of Sciences (Ceskoslovenske Akademie Ved), Prague, Czechoslovakia, and Sky Publishing Corporation, Cambridge, Massachusetts, U.S.A., oversize folio softcover spiral bound, with transparency overlay coordinate grid ruler.
[edit]
Catalogs
Becvar, Antonin. (1959) Atlas Coeli II Katalog 1950.0, Praha, 1960 Prague. Published 1964 as Atlas of the Heavens - II Catalogue 1950.0, Sky Publishing Corporation, Cambridge, Massachusetts, U.S.A.
Hirshfeld, Alan and Sinnott, Roger W. (1982) Sky Catalogue 2000.0, Cambridge University Press and Sky Publishing Corporation, 1st Edition, 2 volumes. LCCN 81017975 both vols., and LCCN 83240310 vol. 1. "Volume 1: Stars to Magnitude 8.0", ISBN 0-521-24710-1 (Cambridge) and 0-933346-35-2 (Sky) hardcover, ISBN 0-933346-34-4 (Sky) softcover. Vol. 2 (1985) - "Volume 2: Double Stars, Variable Stars, and Nonstellar Objects", ISBN 0-521-25818-9 (Cambridge) hardcover, ISBN 0-521-27721-3 (Cambridge) softcover. 2nd Edition (1991) with additional third author Frangois Ochsenbein, 2 volumes, LCCN 91026764. Vol. 1: ISBN 0-521-41743-0 (Cambridge) hardcover (black binding); ISBN 0-521-42736-3 (Cambridge) softcover (red lettering with Hans Vehrenberg astrophoto). Vol. 2 (1999): ISBN 0-521-27721-3 (Cambridge) softcover and 0-933346-38-7 (Sky) softcover - reprint of 1985 edition (blue lettering with Hans Vehrenberg astrophoto).
Yale University Observatory. (1908, et al.) Catalogue of Bright Stars, New Haven, Connecticut, U.S.A. Referred to commonly as "Bright Star Catalogue". Various editions with various authors historically, the longest term revising author as (Ellen) Dorrit Hoffleit. 1st Edition 1908. 2nd Edition 1940 by Frank Schlesinger and Louise F. Jenkins. 3rd Edition (1964), 4th Edition, 5th Edition (1991), and 6th Edition (pending posthumous) by Hoffleit.





 Moon
The Moon (Latin: luna) is the only natural satellite of the Earth,[d][7] and the fifth largest satellite in the Solar System. It is the largest natural satellite of a planet in the Solar System relative to the size of its primary,[e] having 27% the diameter and 60% the density of Earth, resulting in 1⁄81 its mass. The Moon is the second densest satellite after Io, a satellite of Jupiter. It is in synchronous rotation with Earth, always showing the same face with its near side marked by dark volcanic maria that fill between the bright ancient crustal highlands and the prominent impact craters. The Moon is the brightest object in the sky after the Sun, although its surface is actually very dark, with a reflectance similar to that of coal. Its prominence in the sky and its regular cycle of phases have, since ancient times, made the Moon an important cultural influence on language, calendars, art and mythology. The Moon's gravitational influence produces the ocean tides and the minute lengthening of the day. The Moon's current orbital distance, about thirty times the diameter of the Earth, causes it to appear almost the same size in the sky as the Sun, allowing it to cover the Sun nearly precisely in total solar eclipses. This matching of apparent visual size is a coincidence. Earlier in Earth's history, the Moon was closer to Earth, and had an apparent visual size greater than that of the Sun.

The Moon is thought to have formed nearly 4.5 billion years ago, not long after the Earth. Although there have been several hypotheses for its origin in the past, the current most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body. The Moon is the only celestial body other than Earth on which humans have set foot. The Soviet Union's Luna programme was the first to reach the Moon with unmanned spacecraft in 1959; the United States' NASA Apollo program achieved the only manned missions to date, beginning with the first manned lunar orbiting mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972, with the first being Apollo 11. These missions returned over 380 kg of lunar rocks, which have been used to develop a geological understanding of the Moon's origins, the formation of its internal structure, and its subsequent history.

After the Apollo 17 mission in 1972, the Moon has been visited only by unmanned spacecraft, notably by the final Soviet Lunokhod rover. Since 2004, Japan, China, India, the United States, and the European Space Agency have each sent lunar orbiters. These spacecraft have contributed to confirming the discovery of lunar water ice in permanently shadowed craters at the poles and bound into the lunar regolith. Future manned missions to the Moon have been planned, including government as well as privately funded efforts. The Moon remains, under the Outer Space Treaty, free to all nations to explore for peaceful purposes.


The evolution of the Moon and a tour of the Moon.Contents [hide]
1 Name and etymology
2 Formation
3 Physical characteristics
3.1 Internal structure
3.2 Surface geology
3.2.1 Volcanic features
3.2.2 Impact craters
3.2.3 Presence of water
3.3 Gravity and magnetic fields
3.4 Atmosphere
3.5 Seasons
4 Relationship to Earth
4.1 Orbit
4.2 Relative size
4.3 Appearance from Earth
4.4 Tidal effects
4.5 Eclipses
5 Study and exploration
5.1 Early studies
5.2 First direct exploration: 1959–1976
5.2.1 Soviet missions
5.2.2 United States missions
5.3 Current era: 1990–present
6 Astronomy from the Moon
7 Legal status
8 In culture
9 See also
10 References
11 Further reading
12 External links
12.1 Cartographic resources
12.2 Observation tools

Name and etymology

The English proper name for Earth's natural satellite is "the Moon".[8][9] The noun moon derives from moone (around 1380), which developed from mone (1135), which derives from Old English mōna (dating from before 725), which, like all Germanic language cognates, ultimately stems from Proto-Germanic *mǣnōn.[10]

The principal modern English adjective pertaining to the Moon is lunar, derived from the Latin Luna. Another less common adjective is selenic, derived from the Ancient Greek Selene (Σελήνη), from which the prefix "seleno-" (as in selenography) is derived.[11]
Formation
Main article: Giant impact hypothesis


Explore what the discovery of lunar valleys tells us about the moon's evolution.

Several mechanisms have been proposed for the Moon's formation 4.527 ± 0.010 billion years ago,[f] some 30–50 million years after the origin of the Solar System.[12] These included the fission of the Moon from the Earth's crust through centrifugal force,[13] (which would require too great an initial spin of the Earth),[14] the gravitational capture of a pre-formed Moon,[15] (which would require an unfeasibly extended atmosphere of the Earth to dissipate the energy of the passing Moon),[14] and the co-formation of the Earth and the Moon together in the primordial accretion disk (which does not explain the depletion of metallic iron in the Moon).[14] These hypotheses also cannot account for the high angular momentum of the Earth–Moon system.[16]

The prevailing hypothesis today is that the Earth–Moon system formed as a result of a giant impact: a Mars-sized body hitting the newly formed proto-Earth, blasting material into orbit around it, which accreted to form the Moon.[17] Giant impacts are thought to have been common in the early Solar System. Computer simulations modelling a giant impact are consistent with measurements of the angular momentum of the Earth–Moon system and the small size of the lunar core; they also show that most of the Moon came from the impactor, not from the proto-Earth.[18] More recent tests suggest more of the Moon coalesced from the Earth and not the impactor.[19][20][21] Meteorites show that other inner Solar System bodies such as Mars and Vesta have very different oxygen and tungsten isotopic compositions to the Earth, while the Earth and Moon have near-identical isotopic compositions. Post-impact mixing of the vaporized material between the forming Earth and Moon could have equalized their isotopic compositions,[22] although this is debated.[23]

The large amount of energy released in the giant impact event and the subsequent reaccretion of material in Earth orbit would have melted the outer shell of the Earth, forming a magma ocean.[24][25] The newly formed Moon would also have had its own lunar magma ocean; estimates for its depth range from about 500 km to the entire radius of the Moon.[24]

Despite its accuracy in explaining many lines of evidence, there are still some difficulties that are not fully explained by the giant impact hypothesis, most of them involving the Moon's composition. Published in 2012, an analysis of titanium isotopes in Apollo lunar samples showed that the Moon has the same composition as the Earth,[26] which conflicts with the moon forming far from Earth's orbit.
Physical characteristics
Internal structure
Main article: Internal structure of the Moon

Internal structure of the Moon
Chemical composition of the lunar surface regolith (derived from crustal rocks)[27]Compound Formula Composition (wt %)
Maria Highlands
silica SiO2 45.4% 45.5%
alumina Al2O3 14.9% 24.0%
lime CaO 11.8% 15.9%
iron(II) oxide FeO 14.1% 5.9%
magnesia MgO 9.2% 7.5%
titanium dioxide TiO2 3.9% 0.6%
sodium oxide Na2O 0.6% 0.6%
Total 99.9% 100.0%


The Moon is a differentiated body: it has a geochemically distinct crust, mantle, and core. The Moon has a solid iron-rich inner core with a radius of 240 kilometers and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometers. Around the core is a partially molten boundary layer with a radius of about 500 kilometers.[28] This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation 4.5 billion years ago.[29] Crystallization of this magma ocean would have created a mafic mantle from the precipitation and sinking of the minerals olivine, clinopyroxene, and orthopyroxene; after about three-quarters of the magma ocean had crystallised, lower-density plagioclase minerals could form and float into a crust on top.[30] The final liquids to crystallise would have been initially sandwiched between the crust and mantle, with a high abundance of incompatible and heat-producing elements.[1] Consistent with this, geochemical mapping from orbit shows the crust is mostly anorthosite,[6] and moon rock samples of the flood lavas erupted on the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron rich than that of Earth.[1] Geophysical techniques suggest that the crust is on average ~50 km thick.[1]

The Moon is the second densest satellite in the Solar System after Io.[31] However, the inner core of the Moon is small, with a radius of about 350 km or less;[1] this is only ~20% the size of the Moon, in contrast to the ~50% of most other terrestrial bodies[clarification needed]. Its composition is not well constrained, but it is probably metallic iron alloyed with a small amount of sulphur and nickel; analyses of the Moon's time-variable rotation indicate that it is at least partly molten.[32]
Surface geology
Main articles: Geology of the Moon and Moon rocks
See also: Topography of the Moon and List of features on the Moon

Near side of the Moon

Far side of the Moon. Note the almost complete lack of dark maria.[33]

Topography of the Moon.

The topography of the Moon has been measured with laser altimetry and stereo image analysis.[34] The most visible topographic feature is the giant far side South Pole – Aitken basin, some 2,240 km in diameter, the largest crater on the Moon and the largest known crater in the Solar System.[35][36] At 13 km deep, its floor is the lowest elevation on the Moon.[35][37] The highest elevations are found just to its north-east, and it has been suggested that this area might have been thickened by the oblique formation impact of South Pole – Aitken.[38] Other large impact basins, such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale, also possess regionally low elevations and elevated rims.[35] The lunar far side is on average about 1.9 km higher than the near side.[1]
Volcanic features
Main article: Lunar mare

The dark and relatively featureless lunar plains which can clearly be seen with the naked eye are called maria (Latin for "seas"; singular mare), since they were believed by ancient astronomers to be filled with water.[39] They are now known to be vast solidified pools of ancient basaltic lava. While similar to terrestrial basalts, the mare basalts have much higher abundances of iron and are completely lacking in minerals altered by water.[40][41] The majority of these lavas erupted or flowed into the depressions associated with impact basins. Several geologic provinces containing shield volcanoes and volcanic domes are found within the near side maria.[42]

Maria are found almost exclusively on the near side of the Moon, covering 31% of the surface on the near side,[43] compared with a few scattered patches on the far side covering only 2%.[44] This is thought to be due to a concentration of heat-producing elements under the crust on the near side, seen on geochemical maps obtained by Lunar Prospector's gamma-ray spectrometer, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt.[30][45][46] Most of the Moon's mare basalts erupted during the Imbrian period, 3.0–3.5 billion years ago, although some radiometrically dated samples are as old as 4.2 billion years,[47] and the youngest eruptions, dated by crater counting, appear to have been only 1.2 billion years ago.[48]

The lighter-coloured regions of the Moon are called terrae, or more commonly highlands, since they are higher than most maria. They have been radiometrically dated as forming 4.4 billion years ago, and may represent plagioclase cumulates of the lunar magma ocean.[47][48] In contrast to the Earth, no major lunar mountains are believed to have formed as a result of tectonic events.[49]

The concentration of mare on the Near Side likely reflects the substantially thicker crust of the highlands of the Far Side, which may have formed in a slow-velocity impact of a second terran moon a few tens of millions of years after the formations of the moons themselves.[50][51]
Impact craters
See also: List of craters on the Moon

The other major geologic process that has affected the Moon's surface is impact cratering,[52] with craters formed when asteroids and comets collide with the lunar surface. There are estimated to be roughly 300,000 craters wider than 1 km on the Moon's near side alone.[53] Some of these are named for scholars, scientists, artists and explorers.[54] The lunar geologic timescale is based on the most prominent impact events, including Nectaris, Imbrium, and Orientale, structures characterized by multiple rings of uplifted material, typically hundreds to thousands of kilometres in diameter and associated with a broad apron of ejecta deposits that form a regional stratigraphic horizon.[55] The lack of an atmosphere, weather and recent geological processes mean that many of these craters are well-preserved. While only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages. Since impact craters accumulate at a nearly constant rate, counting the number of craters per unit area can be used to estimate the age of the surface.[55] The radiometric ages of impact-melted rocks collected during the Apollo missions cluster between 3.8 and 4.1 billion years old: this has been used to propose a Late Heavy Bombardment of impacts.[56]

Blanketed on top of the Moon's crust is a highly comminuted (broken into ever smaller particles) and impact gardened surface layer called regolith, formed by impact processes. The finer regolith, the lunar soil of silicon dioxide glass, has a texture like snow and smell like spent gunpowder.[57] The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 10–20 m in the highlands and 3–5 m in the maria.[58] Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock many kilometres thick.[59]
Presence of water
Main article: Lunar water

Mosaic image of the lunar south pole as taken by Clementine: note permanent polar shadow.

Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation and is lost to space. However since the 1960s, scientists have hypothesized that water ice may be deposited by impacting comets or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from solar wind, leaving traces of water which could possibly survive in cold, permanently shadowed craters at either pole on the Moon.[60][61] Computer simulations suggest that up to 14,000 km2 of the surface may be in permanent shadow.[62] The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive.[63]

In years since, signatures of water have been found to exist on the lunar surface.[64] In 1994, the bistatic radar experiment located on the Clementine spacecraft, indicated the existence of small, frozen pockets of water close to the surface. However, later radar observations by Arecibo, suggest these findings may rather be rocks ejected from young impact craters.[65] In 1998, the neutron spectrometer located on the Lunar Prospector spacecraft, indicated that high concentrations of hydrogen are present in the first meter of depth in the regolith near the polar regions.[66] In 2008, an analysis of volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water to exist in the interior of the beads.[67]

The 2008, Chandrayaan-1 spacecraft has since confirmed the existence of surface water ice, using the on-board Moon Mineralogy Mapper. The spectrometer observed absorption lines common to hydroxyl, in reflected sunlight, providing evidence of large quantities of water ice, on the lunar surface. The spacecraft showed that concentrations may possibly be as high as 1,000 ppm.[68] In 2009, LCROSS sent a 2300 kg impactor into a permanently shadowed polar crater, and detected at least 100 kg of water in a plume of ejected material.[69][70] Another examination of the LCROSS data showed the amount of detected water, to be closer to 155 kilograms (± 12 kg).[71]

In May 2011, Erik Hauri et al. reported[72] 615–1410 ppm water in melt inclusions in lunar sample 74220, the famous high-titanium "orange glass soil" of volcanic origin collected during the Apollo 17 mission in 1972. The inclusions were formed during explosive eruptions on the Moon approximately 3.7 billion years ago. This concentration is comparable with that of magma in Earth's upper mantle. While of considerable selenological interest, Hauri's announcement affords little comfort to would-be lunar colonists—the sample originated many kilometers below the surface, and the inclusions are so difficult to access that it took 39 years to find them with a state-of-the-art ion microprobe instrument.
Gravity and magnetic fields
Main articles: Gravity of the Moon and Magnetic field of the Moon

The gravitational field of the Moon has been measured through tracking the Doppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features are mascons, large positive gravitational anomalies associated with some of the giant impact basins, partly caused by the dense mare basaltic lava flows that fill these basins.[73] These anomalies greatly influence the orbit of spacecraft about the Moon. There are some puzzles: lava flows by themselves cannot explain all of the gravitational signature, and some mascons exist that are not linked to mare volcanism.[74]

The Moon has an external magnetic field of the order of one to a hundred nanoteslas, less than one-hundredth that of the Earth. It does not currently have a global dipolar magnetic field, as would be generated by a liquid metal core geodynamo, and only has crustal magnetization, probably acquired early in lunar history when a geodynamo was still operating.[75][76] Alternatively, some of the remnant magnetization may be from transient magnetic fields generated during large impact events, through the expansion of an impact-generated plasma cloud in the presence of an ambient magnetic field—this is supported by the apparent location of the largest crustal magnetizations near the antipodes of the giant impact basins.[77]
Atmosphere
Main article: Atmosphere of the Moon

The Moon has an atmosphere so tenuous as to be nearly vacuum, with a total mass of less than 10 metric tons.[78] The surface pressure of this small mass is around 3 × 10−15 atm (0.3 nPa); it varies with the lunar day. Its sources include outgassing and sputtering, the release of atoms from the bombardment of lunar soil by solar wind ions.[6][79] Elements that have been detected include sodium and potassium, produced by sputtering, which are also found in the atmospheres of Mercury and Io; helium-4 from the solar wind; and argon-40, radon-222, and polonium-210, outgassed after their creation by radioactive decay within the crust and mantle.[80][81] The absence of such neutral species (atoms or molecules) as oxygen, nitrogen, carbon, hydrogen and magnesium, which are present in the regolith, is not understood.[80] Water vapour has been detected by Chandrayaan-1 and found to vary with latitude, with a maximum at ~60–70 degrees; it is possibly generated from the sublimation of water ice in the regolith.[82] These gases can either return into the regolith due to the Moon's gravity, or be lost to space: either through solar radiation pressure, or if they are ionised, by being swept away by the solar wind's magnetic field.[80]
Seasons

The Moon's north pole during summer.

The Moon's axial tilt with respect to the ecliptic is only 1.54°,[83] much less than the 23.44° of the Earth. Because of this, the Moon's solar illumination varies much less with season, and topographical details play a crucial role in seasonal effects.[84] From images taken by Clementine in 1994, it appears that four mountainous regions on the rim of Peary crater at the Moon's north pole remain illuminated for the entire lunar day, creating peaks of eternal light. No such regions exist at the south pole. Similarly, there are places that remain in permanent shadow at the bottoms of many polar craters,[62] and these dark craters are extremely cold: Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (−238 °C),[85] and just 26 K close to the winter solstice in north polar Hermite Crater. This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of Pluto.[84]
Relationship to Earth

Earth and Moon, showing their sizes and distance to scale. The yellow bar represents a pulse of light traveling from Earth to Moon in 1.26 seconds.

Schematic of the Earth–Moon system (without a consistent scale)
Orbit
Main articles: Orbit of the Moon and Lunar theory

The Moon makes a complete orbit around the Earth with respect to the fixed stars about once every 27.3 days[g] (its sidereal period). However, since the Earth is moving in its orbit about the Sun at the same time, it takes slightly longer for the Moon to show the same phase to Earth, which is about 29.5 days[h] (its synodic period).[43] Unlike most satellites of other planets, the Moon orbits nearer the ecliptic plane than to the planet's equatorial plane. The Moon's orbit is subtly perturbed by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon's orbital motion gradually rotates, which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini's laws.[86]
Relative size

Comparative sizes of the Earth and the Moon, as imaged by Deep Impact in September 2008 at a separation of 50 million km[87]

The Moon is exceptionally large relative to the Earth: a quarter the diameter of the planet and 1/81 its mass.[43] It is the largest moon in the Solar System relative to the size of its planet, though Charon is larger relative to the dwarf planet Pluto, at 1/9 Pluto's mass.[88]

However, the Earth and Moon are still considered a planet–satellite system, rather than a double-planet system, as their barycentre, the common centre of mass, is located 1,700 km (about a quarter of the Earth's radius) beneath the surface of the Earth.[89]
Appearance from Earth
See also: Lunar phase, Earthshine, and Observing the Moon

The Moon is in synchronous rotation: it rotates about its axis in about the same time it takes to orbit the Earth. This results in it nearly always keeping the same face turned towards the Earth. The Moon used to rotate at a faster rate, but early in its history, its rotation slowed and became tidally locked in this orientation as a result of frictional effects associated with tidal deformations caused by the Earth.[90] The side of the Moon that faces Earth is called the near side, and the opposite side the far side. The far side is often called the "dark side", but in fact, it is illuminated as often as the near side: once per lunar day, during the new moon phase we observe on Earth when the near side is dark.[91]

The Moon has an exceptionally low albedo, giving it a similar reflectance to coal. Despite this, it is the second brightest object in the sky after the Sun.[43][i] This is partly due to the brightness enhancement of the opposition effect; at quarter phase, the Moon is only one-tenth as bright, rather than half as bright, as at full moon.[92]

Additionally, colour constancy in the visual system recalibrates the relations between the colours of an object and its surroundings, and since the surrounding sky is comparatively dark, the sunlit Moon is perceived as a bright object. The edges of the full moon seem as bright as the centre, with no limb darkening, due to the reflective properties of lunar soil, which reflects more light back towards the Sun than in other directions. The Moon does appear larger when close to the horizon, but this is a purely psychological effect, known as the Moon illusion, first described in the 7th century BC.[93] The full moon subtends an arc of about 0.52° (on average) in the sky, roughly the same apparent size as the Sun (see eclipses).

The monthly changes of angle between the direction of illumination by the Sun and viewing from Earth, and the phases of the Moon that result

The highest altitude of the Moon in the sky varies: while it has nearly the same limit as the Sun, it alters with the lunar phase and with the season of the year, with the full moon highest during winter. The 18.6-year nodes cycle also has an influence: when the ascending node of the lunar orbit is in the vernal equinox, the lunar declination can go as far as 28° each month. This means the Moon can go overhead at latitudes up to 28° from the equator, instead of only 18°. The orientation of the Moon's crescent also depends on the latitude of the observation site: close to the equator, an observer can see a smile-shaped crescent Moon.[94]

The distance between the Moon and the Earth varies from around 356,400 km to 406,700 km at the extreme perigees (closest) and apogees (farthest). On 19 March 2011, it was closer to the Earth while at full phase than it has been since 1993.[95] Reported as a "super moon", this closest point coincides within an hour of a full moon, and it thus appeared 30 percent brighter, and 14 percent larger than when at its greatest distance.[96][97][98]

There has been historical controversy over whether features on the Moon's surface change over time. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or inadequate drawings. However, outgassing does occasionally occur, and could be responsible for a minor percentage of the reported lunar transient phenomena. Recently, it has been suggested that a roughly 3 km diameter region of the lunar surface was modified by a gas release event about a million years ago.[99][100] The Moon's appearance, like that of the Sun, can be affected by Earth's atmosphere: common effects are a 22° halo ring formed when the Moon's light is refracted through the ice crystals of high cirrostratus cloud, and smaller coronal rings when the Moon is seen through thin clouds.[101]
Tidal effects
Main articles: Tidal force, Tidal acceleration, Tide, and Theory of tides

The tides on the Earth are mostly generated by the gradient in intensity of the Moon's gravitational pull from one side of the Earth to the other, the tidal forces. This forms two tidal bulges on the Earth, which are most clearly seen in elevated sea level as ocean tides.[102] Since the Earth spins about 27 times faster than the Moon moves around it, the bulges are dragged along with the Earth's surface faster than the Moon moves, rotating around the Earth once a day as it spins on its axis.[102] The ocean tides are magnified by other effects: frictional coupling of water to Earth's rotation through the ocean floors, the inertia of water's movement, ocean basins that get shallower near land, and oscillations between different ocean basins.[103] The gravitational attraction of the Sun on the Earth's oceans is almost half that of the Moon, and their gravitational interplay is responsible for spring and neap tides.[102]

The libration of the Moon over a single lunar month.

Gravitational coupling between the Moon and the bulge nearest the Moon acts as a torque on the Earth's rotation, draining angular momentum and rotational kinetic energy from the Earth's spin.[102][104] In turn, angular momentum is added to the Moon's orbit, accelerating it, which lifts the Moon into a higher orbit with a longer period. As a result, the distance between the Earth and Moon is increasing, and the Earth's spin slowing down.[104] Measurements from lunar ranging experiments with laser reflectors left during the Apollo missions have found that the Moon's distance to the Earth increases by 38 mm per year[105] (though this is only 0.10 ppb/year of the radius of the Moon's orbit). Atomic clocks also show that the Earth's day lengthens by about 15 microseconds every year,[106] slowly increasing the rate at which UTC is adjusted by leap seconds. Left to run its course, this tidal drag would continue until the spin of the Earth and the orbital period of the Moon matched. However, the Sun will become a red giant long before that, engulfing the Earth.[107][108]

The lunar surface also experiences tides of amplitude ~10 cm over 27 days, with two components: a fixed one due to the Earth, as they are in synchronous rotation, and a varying component from the Sun.[104] The Earth-induced component arises from libration, a result of the Moon's orbital eccentricity; if the Moon's orbit were perfectly circular, there would only be solar tides.[104] Libration also changes the angle from which the Moon is seen, allowing about 59% of its surface to be seen from the Earth (but only half at any instant).[43] The cumulative effects of stress built up by these tidal forces produces moonquakes. Moonquakes are much less common and weaker than earthquakes, although they can last for up to an hour—a significantly longer time than terrestrial earthquakes—because of the absence of water to damp out the seismic vibrations. The existence of moonquakes was an unexpected discovery from seismometers placed on the Moon by Apollo astronauts from 1969 through 1972.[109]
Eclipses
Main articles: Solar eclipse, Lunar eclipse, and Eclipse cycle

The 1999 solar eclipse

The Moon passing in front of the Sun, from the STEREO-B spacecraft.[110]
From the Earth, the Moon and Sun appear the same size. From a satellite in an Earth-trailing orbit, the Moon may appear smaller than the Sun.

Eclipses can only occur when the Sun, Earth, and Moon are all in a straight line (termed "syzygy"). Solar eclipses occur at new moon, when the Moon is between the Sun and Earth. In contrast, lunar eclipses occur at full moon, when the Earth is between the Sun and Moon. The apparent size of the Moon is roughly the same as that of the Sun, with both being viewed at close to one-half a degree wide. The Sun is much larger than the Moon but it is the precise vastly greater distance that coincidentally gives it the same apparent size as the much closer and much smaller Moon from the perspective of the Earth. The variations in apparent size, due to the non-circular orbits, are nearly the same as well, though occurring in different cycles. This makes possible both total (with the Moon appearing larger than the Sun) and annular (with the Moon appearing smaller than the Sun) solar eclipses.[111] In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Since the distance between the Moon and the Earth is very slowly increasing over time,[102] the angular diameter of the Moon is decreasing. This means that hundreds of millions of years ago the Moon would always completely cover the Sun on solar eclipses, and no annular eclipses were possible. Likewise, about 600 million years from now (if the angular diameter of the Sun does not change), the Moon will no longer cover the Sun completely, and only annular eclipses will occur.[112]

Because the Moon's orbit around the Earth is inclined by about 5° to the orbit of the Earth around the Sun, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes.[112] The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by the Earth, is described by the saros cycle, which has a period of approximately 18 years.[113]

As the Moon is continuously blocking our view of a half-degree-wide circular area of the sky,[j][114] the related phenomenon of occultation occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to the Earth, occultations of individual stars are not visible everywhere on the planet, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted.[115]
Study and exploration
See also: Robotic exploration of the Moon, List of current and future lunar missions, Colonization of the Moon, and List of man-made objects on the Moon

Map of the Moon by Johannes Hevelius from his Selenographia (1647), the first map to include the libration zones.
Early studies
Main articles: Exploration of the Moon: Early history, Selenography, and Lunar theory

Understanding of the Moon's cycles was an early development of astronomy: by the 5th century BC, Babylonian astronomers had recorded the 18-year Saros cycle of lunar eclipses,[116] and Indian astronomers had described the Moon’s monthly elongation.[117] The Chinese astronomer Shi Shen (fl. 4th century BC) gave instructions for predicting solar and lunar eclipses.[118] Later, the physical form of the Moon and the cause of moonlight became understood. The ancient Greek philosopher Anaxagoras (d. 428 BC) reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former.[119][120] Although the Chinese of the Han Dynasty believed the Moon to be energy equated to qi, their 'radiating influence' theory also recognized that the light of the Moon was merely a reflection of the Sun, and Jing Fang (78–37 BC) noted the sphericity of the Moon.[121] In 499 AD, the Indian astronomer Aryabhata mentioned in his Aryabhatiya that reflected sunlight is the cause of the shining of the Moon.[122] The astronomer and physicist Alhazen (965–1039) found that sunlight was not reflected from the Moon like a mirror, but that light was emitted from every part of the Moon's sunlit surface in all directions.[123] Shen Kuo (1031–1095) of the Song Dynasty created an allegory equating the waxing and waning of the Moon to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent.[124]

In Aristotle's (384–322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether, an influential philosophy that would dominate for centuries.[125] However, in the 2nd century BC, Seleucus of Seleucia correctly theorized that tides were due to the attraction of the Moon, and that their height depends on the Moon's position relative to the Sun.[126] In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the Earth radius for the distance. These figures were greatly improved by Ptolemy (90–168 AD): his values of a mean distance of 59 times the Earth's radius and a diameter of 0.292 Earth diameters were close to the correct values of about 60 and 0.273 respectively.[127] Archimedes (287–212 BC) invented a planetarium calculating motions of the Moon and the known planets.[128]

During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognised as a sphere, though many believed that it was "perfectly smooth".[129] In 1609, Galileo Galilei drew one of the first telescopic drawings of the Moon in his book Sidereus Nuncius and noted that it was not smooth but had mountains and craters. Telescopic mapping of the Moon followed: later in the 17th century, the efforts of Giovanni Battista Riccioli and Francesco Maria Grimaldi led to the system of naming of lunar features in use today. The more exact 1834-6 Mappa Selenographica of Wilhelm Beer and Johann Heinrich Mädler, and their associated 1837 book Der Mond, the first trigonometrically accurate study of lunar features, included the heights of more than a thousand mountains, and introduced the study of the Moon at accuracies possible in earthly geography.[130] Lunar craters, first noted by Galileo, were thought to be volcanic until the 1870s proposal of Richard Proctor that they were formed by collisions.[43] This view gained support in 1892 from the experimentation of geologist Grove Karl Gilbert, and from comparative studies from 1920 to the 1940s,[131] leading to the development of lunar stratigraphy, which by the 1950s was becoming a new and growing branch of astrogeology.[43]
First direct exploration: 1959–1976
Soviet missions
Main articles: Luna program and Lunokhod programme

Lunokhod 1 (lit. moonwalker), the first successful space rover.

The Cold War-inspired Space Race between the Soviet Union and the U.S. led to an acceleration of interest in exploration of the Moon. Once launchers had the necessary capabilities, these nations sent unmanned probes on both flyby and impact/lander missions. Spacecraft from the Soviet Union's Luna program were the first to accomplish a number of goals: following three unnamed, failed missions in 1958,[132] the first man-made object to escape Earth's gravity and pass near the Moon was Luna 1; the first man-made object to impact the lunar surface was Luna 2, and the first photographs of the normally occluded far side of the Moon were made by Luna 3, all in 1959.

The first spacecraft to perform a successful lunar soft landing was Luna 9 and the first unmanned vehicle to orbit the Moon was Luna 10, both in 1966.[43] Rock and soil samples were brought back to Earth by three Luna sample return missions (Luna 16 in 1970, Luna 20 in 1972, and Luna 24 in 1976), which returned 0.3 kg total.[133] Two pioneering robotic rovers landed on the Moon in 1970 and 1973 as a part of Soviet Lunokhod programme.
United States missions
Main articles: Apollo program and Moon landing

Earth as viewed from Lunar orbit during the Apollo 8 mission, Christmas Eve, 1968. Africa is at the sunset terminator, both Americas are under cloud, and Antarctica is at the left end of the terminator.

Astronaut Buzz Aldrin photographed by Neil Armstrong during the first Moon landing on 20 July 1969

A US flag from Apollo 11.

American lunar exploration began with robotic missions aimed at developing understanding of the lunar surface for an eventual manned landing: the Jet Propulsion Laboratory's Surveyor program landed its first spacecraft four months after Luna 9. NASA's manned Apollo program was developed in parallel; after a series of unmanned and manned tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet lunar flight, in 1968 Apollo 8 made the first crewed mission to lunar orbit. The subsequent landing of the first humans on the Moon in 1969 is seen by many as the culmination of the Space Race.[134] Neil Armstrong became the first person to walk on the Moon as the commander of the American mission Apollo 11 by first setting foot on the Moon at 02:56 UTC on 21 July 1969.[135] The Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing) returned 382 kg of lunar rock and soil in 2,196 separate samples.[136] The American Moon landing and return was enabled by considerable technological advances in the early 1960s, in domains such as ablation chemistry, software engineering and atmospheric re-entry technology, and by highly competent management of the enormous technical undertaking.[137][138]

Scientific instrument packages were installed on the lunar surface during all the Apollo landings. Long-lived instrument stations, including heat flow probes, seismometers, and magnetometers, were installed at the Apollo 12, 14, 15, 16, and 17 landing sites. Direct transmission of data to Earth concluded in late 1977 due to budgetary considerations,[139][140] but as the stations' lunar laser ranging corner-cube retroreflector arrays are passive instruments, they are still being used. Ranging to the stations is routinely performed from Earth-based stations with an accuracy of a few centimetres, and data from this experiment are being used to place constraints on the size of the lunar core.[141]
Current era: 1990–present

Post-Apollo and Luna, many more countries have become involved in direct exploration of the Moon. In 1990, Japan became the third country to place a spacecraft into lunar orbit with its Hiten spacecraft. The spacecraft released a smaller probe, Hagoromo, in lunar orbit, but the transmitter failed, preventing further scientific use of the mission.[142] In 1994, the U.S. sent the joint Defense Department/NASA spacecraft Clementine to lunar orbit. This mission obtained the first near-global topographic map of the Moon, and the first global multispectral images of the lunar surface.[143] This was followed in 1998 by the Lunar Prospector mission, whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters.[144]

The European spacecraft SMART-1, the second ion-propelled spacecraft, was in lunar orbit from 15 November 2004 until its lunar impact on 3 September 2006, and made the first detailed survey of chemical elements on the lunar surface.[145] China has expressed ambitious plans for exploring the Moon, and successfully orbited its first spacecraft, Chang'e-1, from 5 November 2007 until its controlled lunar impact on 1 March 2008.[146] In its sixteen-month mission, it obtained a full image map of the Moon. Between 4 October 2007 and 10 June 2009, the Japan Aerospace Exploration Agency's Kaguya (Selene) mission, a lunar orbiter fitted with a high-definition video camera, and two small radio-transmitter satellites, obtained lunar geophysics data and took the first high-definition movies from beyond Earth orbit.[147][148] India's first lunar mission, Chandrayaan I, orbited from 8 November 2008 until loss of contact on 27 August 2009, creating a high resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of water molecules in lunar soil.[149] The Indian Space Research Organisation plans to launch Chandrayaan II in 2013, which is slated to include a Russian robotic lunar rover.[150][151] The U.S. co-launched the Lunar Reconnaissance Orbiter (LRO) and the LCROSS impactor and follow-up observation orbiter on 18 June 2009; LCROSS completed its mission by making a planned and widely observed impact in the crater Cabeus on 9 October 2009,[152] while LRO is currently in operation, obtaining precise lunar altimetry and high-resolution imagery. In November 2011, the LRO passed over the Aristarchus crater, which spans 40 kilometres and sinks more than 3.5 kilometres deep. The crater is one of the most visible ones from Earth. "The Aristarchus plateau is one of the most geologically diverse places on the Moon: a mysterious raised flat plateau, a giant rille carved by enormous outpourings of lava, fields of explosive volcanic ash, and all surrounded by massive flood basalts", said Mark Robinson, principal investigator of the Lunar Reconnaissance Orbiter Camera at Arizona State University. NASA released photos of the crater on 25 December 2011.[153]

Two GRAIL spacecraft begin orbiting the Moon around 1 January 2012.[154]

Other upcoming lunar missions include Russia's Luna-Glob: an unmanned lander, set of seismometers, and an orbiter based on its Martian Fobos-Grunt mission, which is slated to launch in 2012.[155][156] Privately funded lunar exploration has been promoted by the Google Lunar X Prize, announced 13 September 2007, which offers US$20 million to anyone who can land a robotic rover on the Moon and meet other specified criteria.[157] Shackleton Energy Company is building a program to establish operations on the south pole of the Moon to harvest water and supply their Propellant Depots.[158]

NASA began to plan to resume manned missions following the call by U.S. President George W. Bush on 14 January 2004 for a manned mission to the Moon by 2019 and the construction of a lunar base by 2024.[159] The Constellation program was funded and construction and testing begun on a manned spacecraft and launch vehicle,[160] and design studies for a lunar base.[161] However, that program has been cancelled in favour of a manned asteroid landing by 2025 and a manned Mars orbit by 2035.[162] India has also expressed its hope to send a manned mission to the Moon by 2020.[163]
Astronomy from the Moon

For many years, the Moon has been recognized as an excellent site for telescopes.[164] It is relatively nearby; astronomical seeing is not a concern; certain craters near the poles are permanently dark and cold, and thus especially useful for infrared telescopes; and radio telescopes on the far side would be shielded from the radio chatter of Earth.[165] The lunar soil, although it poses a problem for any moving parts of telescopes, can be mixed with carbon nanotubes and epoxies in the construction of mirrors up to 50 meters in diameter.[166] A lunar zenith telescope can be made cheaply with ionic liquid.[167]
Legal status
Main article: Space law

Although Luna landers scattered pennants of the Soviet Union on the Moon, and U.S. flags were symbolically planted at their landing sites by the Apollo astronauts, no nation currently claims ownership of any part of the Moon's surface.[168] Russia and the U.S. are party to the 1967 Outer Space Treaty,[169] which defines the Moon and all outer space as the "province of all mankind".[168] This treaty also restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction.[170] The 1979 Moon Agreement was created to restrict the exploitation of the Moon's resources by any single nation, but it has not been signed by any of the space-faring nations.[171] While several individuals have made claims to the Moon in whole or in part, none of these are considered credible.[172][173][174]
In culture
See also: Moon in fiction, Lunar calendar, Metonic cycle, Lunar deity, Lunar effect, and Blue moon

Luna, the Moon, from a 1550 edition of Guido Bonatti's Liber astronomiae.

The Moon's regular phases make it a very convenient timepiece, and the periods of its waxing and waning form the basis of many of the oldest calendars. Tally sticks, notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon.[175][176][177] The ~30-day month is an approximation of the lunar cycle. The English noun month and its cognates in other Germanic languages stem from Proto-Germanic *mǣnṓth-, which is connected to the above mentioned Proto-Germanic *mǣnōn, indicating the usage of a lunar calendar among the Germanic peoples (Germanic calendar) prior to the adoption of a solar calendar.[178] The same Indo-European root as moon led, via Latin, to measure and menstrual, words which echo the Moon's importance to many ancient cultures in measuring time (see Latin mensis and Ancient Greek μήνας (mēnas), meaning "month").[179][180]

A crescent Moon and "star" (here the planet Venus) are a common symbol of Islam, appearing in flags like: (Turkey), (Tunisia) and (Pakistan).

The Moon has been the subject of many works of art and literature and the inspiration for countless others. It is a motif in the visual arts, the performing arts, poetry, prose and music. A 5,000-year-old rock carving at Knowth, Ireland, may represent the Moon, which would be the earliest depiction discovered.[181] The contrast between the brighter highlands and darker maria create the patterns seen by different cultures as the Man in the Moon, the rabbit and the buffalo, among others. In many prehistoric and ancient cultures, the Moon was personified as a deity or other supernatural phenomenon, and astrological views of the Moon continue to be propagated today.

The Moon has a long association with insanity and irrationality; the words lunacy and lunatic (popular shortening loony) are derived from the Latin name for the Moon, Luna. Philosophers such as Aristotle and Pliny the Elder argued that the full moon induced insanity in susceptible individuals, believing that the brain, which is mostly water, must be affected by the Moon and its power over the tides, but the Moon's gravity is too slight to affect any single person.[182] Even today, people insist that admissions to psychiatric hospitals, traffic accidents, homicides or suicides increase during a full moon, although there is no scientific evidence to support such claims.[182]
See also Solar System portal
Moon portal
Book: The Moon
Book: Solar System
Wikipedia books are collections of articles that can be downloaded or ordered in print.

Other moons of Earth
Moon illusion
References

Notes
^ The maximum value is given based on scaling of the brightness from the value of −12.74 given for an equator to Moon-centre distance of 378 000 km in the NASA factsheet reference to the minimum Earth–Moon distance given there, after the latter is corrected for the Earth's equatorial radius of 6 378 km, giving 350 600 km. The minimum value (for a distant new moon) is based on a similar scaling using the maximum Earth–Moon distance of 407 000 km (given in the factsheet) and by calculating the brightness of the earthshine onto such a new moon. The brightness of the earthshine is [ Earth albedo × (Earth radius / Radius of Moon's orbit)2 ] relative to the direct solar illumination that occurs for a full moon. (Earth albedo = 0.367; Earth radius = (polar radius × equatorial radius)½ = 6 367 km.)
^ The range of angular size values given are based on simple scaling of the following values given in the fact sheet reference: at an Earth-equator to Moon-centre distance of 378 000 km, the angular size is 1896 arcseconds. The same fact sheet gives extreme Earth–Moon distances of 407 000 km and 357 000 km. For the maximum angular size, the minimum distance has to be corrected for the Earth's equatorial radius of 6 378 km, giving 350 600 km.
^ Lucey et al. (2006) give 107 particles cm−3 by day and 105 particles cm−3 by night. Along with equatorial surface temperatures of 390 K by day and 100 K by night, the ideal gas law yields the pressures given in the infobox (rounded to the nearest order of magnitude): 10−7 Pa by day and 10−10 Pa by night.
^ There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term (Morais et al, 2002). These are quasi-satellites – they are not moons as they do not orbit the Earth. For more information, see Other moons of Earth.
^ Charon is proportionally larger in comparison to Pluto, but Pluto has been reclassified as a dwarf planet.
^ This age is calculated from isotope dating of lunar rocks.
^ More accurately, the Moon's mean sidereal period (fixed star to fixed star) is 27.321661 days (27d 07h 43m 11.5s), and its mean tropical orbital period (from equinox to equinox) is 27.321582 days (27d 07h 43m 04.7s) (Explanatory Supplement to the Astronomical Ephemeris, 1961, at p.107).
^ More accurately, the Moon's mean synodic period (between mean solar conjunctions) is 29.530589 days (29d 12h 44m 02.9s) (Explanatory Supplement to the Astronomical Ephemeris, 1961, at p.107).
^ The Sun's apparent magnitude is −26.7, and the full moon's apparent magnitude is −12.7.
^ On average, the Moon covers an area of 0.21078 square degrees on the night sky.

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^ Sarma, K. V. (2008). "Astronomy in India". In Helaine Selin. Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures (2 ed.). Springer. pp. 317–321. ISBN 978-1-4020-4559-2.
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^ O'Connor, J.J.; Robertson, E.F. (February 1999). "Anaxagoras of Clazomenae". University of St Andrews. Retrieved 12 April 2007.
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^ Needham 1986, p. 415–416.
^ Lewis, C. S. (1964). The Discarded Image. Cambridge: Cambridge University Press. p. 108. ISBN 978-0-521-47735-2.
^ van der Waerden, Bartel Leendert (1987). "The Heliocentric System in Greek, Persian and Hindu Astronomy". Annals of the New York Academy of Sciences 500: 1–569. Bibcode 1987NYASA.500....1A. doi:10.1111/j.1749-6632.1987.tb37193.x. PMID 3296915.
^ Evans, James (1998). The History and Practice of Ancient Astronomy. Oxford & New York: Oxford University Press. pp. 71, 386. ISBN 978-0-19-509539-5.
^ "Discovering How Greeks Computed in 100 B.C.". The New York Times. 31 July 2008. Retrieved 27 March 2010.
^ Van Helden, A. (1995). "The Moon". Galileo Project. Retrieved 12 April 2007.
^ Consolmagno, Guy J. (1996). "Astronomy, Science Fiction and Popular Culture: 1277 to 2001 (And beyond)". Leonardo (The MIT Press) 29 (2): 128. JSTOR 1576348.
^ Hall, R. Cargill (1977). "Appendix A: LUNAR THEORY BEFORE 1964". NASA History Series. LUNAR IMPACT: A History of Project Ranger.. Washington, D.C.: Scientific and Technical Information Office, NATIONAL AERONAUTICS AND SPACE ADMINISTRATION. Retrieved 13 April 2010.
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^ "Record of Lunar Events, 24 July 1969". Apollo 11 30th anniversary. NASA.. Retrieved 13 April 2010.
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^ Wall, Mike (14 January 2011). "Mining the Moon's Water: Q&A with Shackleton Energy's Bill Stone". Space News.
^ "President Bush Offers New Vision For NASA" (Press release). NASA. 14 December 2004. Retrieved 12 April 2007.
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^ NASAtelevision (15 April 2010). "President Obama Pledges Total Commitment to NASA". YouTube. Retrieved 7 May 2012.
^ "India's Space Agency Proposes Manned Spaceflight Program". SPACE.com. 10 November 2006. Retrieved 23 October 2008.
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^ Chandler, David (15 February 2008). "MIT to lead development of new telescopes on moon". MIT News. Retrieved 27 March 2011.
^ Naeye, Robert (6 April 2008). "NASA Scientists Pioneer Method for Making Giant Lunar Telescopes". Goddard Space Flight Center. Retrieved 27 March 2011.
^ Bell, Trudy (9 October 2008). "Liquid Mirror Telescopes on the Moon". Science News. NASA. Retrieved 27 March 2011.
^ a b "Can any State claim a part of outer space as its own?". United Nations Office for Outer Space Affairs. Retrieved 28 March 2010.
^ "How many States have signed and ratified the five international treaties governing outer space?". United Nations Office for Outer Space Affairs. 1 January 2006. Retrieved 28 March 2010.
^ "Do the five international treaties regulate military activities in outer space?". United Nations Office for Outer Space Affairs. Retrieved 28 March 2010.
^ "Agreement Governing the Activities of States on the Moon and Other Celestial Bodies". United Nations Office for Outer Space Affairs. Retrieved 28 March 2010.
^ "The treaties control space-related activities of States. What about non-governmental entities active in outer space, like companies and even individuals?". United Nations Office for Outer Space Affairs. Retrieved 28 March 2010.
^ "Statement by the Board of Directors of the IISL On Claims to Property Rights Regarding The Moon and Other Celestial Bodies (2004)". International Institute of Space Law. 2004. Retrieved 28 March 2010.
^ "Further Statement by the Board of Directors of the IISL On Claims to Lunar Property Rights (2009)". International Institute of Space Law. 22 March 2009. Retrieved 28 March 2010.
^ Marshack, Alexander (1991): The Roots of Civilization, Colonial Hill, Mount Kisco, NY.
^ Brooks, A. S. and Smith, C. C. (1987): "Ishango revisited: new age determinations and cultural interpretations", The African Archaeological Review, 5 : 65–78.
^ Duncan, David Ewing (1998). The Calendar. Fourth Estate Ltd.. pp. 10–11. ISBN 978-1-85702-721-1.
^ For etymology, see Barnhart, Robert K. (1995). The Barnhart Concise Dictionary of Etymology. Harper Collins. p. 487. ISBN 978-0-06-270084-1. For the lunar calendar of the Germanic peoples, see Birley, A. R. (Trans.) (1999). Agricola and Germany. Oxford World's Classics. USA: Oxford. p. 108. ISBN 978-0-19-283300-6.
^ Smith, William George (1849). Dictionary of Greek and Roman Biography and Mythology: Oarses-Zygia. 3. J. Walton. p. 768. Retrieved 29 March 2010.
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^ a b Lilienfeld, Scott O.; Arkowitz, Hal (2009). "Lunacy and the Full Moon". Scientific American. Retrieved 13 April 2010.

Bibliography
Needham, Joseph (1986). Science and Civilization in China, Volume III: Mathematics and the Sciences of the Heavens and Earth. Taipei: Caves Books. ISBN 978-0-521-05801-8.
Further reading
The Moon. Discovery 2008. BBC World Service.
Bussey, B.; Spudis, P.D. (2004). The Clementine Atlas of the Moon. Cambridge University Press. ISBN 0-521-81528-2.
Cain, Fraser. "Where does the Moon Come From?". Universe Today. Retrieved 1 April 2008. (podcast and transcript)
Jolliff, B.; Wieczorek, M.; Shearer, C.; Neal, C. (eds.) (2006). "New views of the Moon". Rev. Mineral. Geochem. (Chantilly, Virginia: Min. Soc. Amer.) 60 (1): 721. doi:10.2138/rmg.2006.60.0. ISBN 0-939950-72-3. Retrieved 12 April 2007.
Jones, E.M. (2006). "Apollo Lunar Surface Journal". NASA. Retrieved 12 April 2007.
"Exploring the Moon". Lunar and Planetary Institute. Retrieved 12 April 2007.
Mackenzie, Dana (2003). The Big Splat, or How Our Moon Came to Be. Hoboken, New Jersey: John Wiley & Sons, Inc. ISBN 0-471-15057-6.
Moore, P. (2001). On the Moon. Tucson, Arizona: Sterling Publishing Co.. ISBN 0-304-35469-4.
"Moon Articles". Planetary Science Research Discoveries.
Spudis, P. D. (1996). The Once and Future Moon. Smithsonian Institution Press. ISBN 1-56098-634-4.
Taylor, S.R. (1992). Solar system evolution. Cambridge Univ. Press. p. 307. ISBN 0-521-37212-7.
Teague, K. (2006). "The Project Apollo Archive". Retrieved 12 April 2007.
Wilhelms, D.E. (1987). "Geologic History of the Moon". U.S. Geological Survey Professional paper 1348. Retrieved 12 April 2007.
Wilhelms, D.E. (1993). To a Rocky Moon: A Geologist's History of Lunar Exploration. Tucson, Arizona: University of Arizona Press. ISBN 0-8165-1065-2. Retrieved 10 March 2009


                               
A star is a massive, luminous sphere of plasma held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on the planet. Other stars are visible from Earth during the night when they are not obscured by atmospheric phenomena, appearing as a multitude of fixed luminous points because of their immense distance. Historically, the most prominent stars on the celestial sphere were grouped together into constellations and asterisms, and the brightest stars gained proper names. Extensive catalogues of stars have been assembled by astronomers, which provide standardized star designations.
For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium are created by stars, either via stellar nucleosynthesis during their lifetimes or by supernova nucleosynthesis when very massive stars explode. Near the end of its life, a star can also contain a proportion of degenerate matter. Astronomers can determine the mass, age, chemical composition and many other properties of a star by observing its spectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined.
A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.[1] The remainder of the star's interior carries energy away from the core through a combination of radiative and convective processes. The star's internal pressure prevents it from collapsing further under its own gravity. Once the hydrogen fuel at the core is exhausted, a star with at least 0.4 times the mass of the Sun[2] expands to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. The star then evolves into a degenerate form, recycling a portion of its matter into the interstellar environment, where it will form a new generation of stars with a higher proportion of heavy elements.[3] Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or (if it is sufficiently massive) a black hole.
Binary and multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.[4] Stars can form part of a much larger gravitationally bound structure, such as a cluster or a galaxy.
                                                                        
Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and used for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere, and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.[5] The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices.[7] The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun.
The oldest accurately dated star chart appeared in ancient Egyptian astronomy in 1534 BC.[8] The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period (ca. 1531–1155 BC).[9]
The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help of Timocharis.[10] The star catalog of Hipparchus (2nd century BC) included 1020 stars and was used to assemble Ptolemy's star catalogue.[11] Hipparchus is known for the discovery of the first recorded nova (new star).[12] Many of the constellations and star names in use today derive from Greek astronomy.
In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear.[13] In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185.[14] The brightest stellar event in recorded history was the SN 1006 supernova, which was observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers.[15] The SN 1054 supernova, which gave birth to the Crab Nebula, was also observed by Chinese and Islamic astronomers.[16][17][18]
Medieval Islamic astronomers gave Arabic names to many stars that are still used today, and they invented numerous astronomical instruments that could compute the positions of the stars. They built the first large observatory research institutes, mainly for the purpose of producing Zij star catalogues.[19] Among these, the Book of Fixed Stars (964) was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters (including the Omicron Velorum and Brocchi's Clusters) and galaxies (including the Andromeda Galaxy).[20] According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019.[21]
According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars which almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence.[22]



Andromeda as depicted in Urania's Mirror, set of constellation cards published in London c.1825
Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable. In 1584 Giordano Bruno suggested that the stars were like the Sun, and may have other planets, possibly even Earth-like, in orbit around them,[23] an idea that had been suggested earlier by the ancient Greek philosophers, Democritus and Epicurus,[24] and by medieval Islamic cosmologists[25] such as Fakhr al-Din al-Razi.[26] By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.[27]
The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions from the time of the ancient Greek astronomers Ptolemy and Hipparchus. The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.[23]
William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he performed a series of gauges in 600 directions, and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction.[28] In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems.
The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in a stellar spectra due to the absorption of specific frequencies by the atmosphere. In 1865 Secchi began classifying stars into spectral types.[29] However, the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s.
Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius, and inferred a hidden companion. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104 day period. Detailed observations of many binary star systems were collected by astronomers such as William Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of the orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.[30] The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschild discovered that the color of a star, and hence its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. The development of the photoelectric photometer allowed very precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope.[31]
Important conceptual work on the physical basis of stars occurred during the first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. The spectra of stars were also successfully explained through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.[32]
With the exception of supernovae, individual stars have primarily been observed in our Local Group of galaxies,[33] and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our galaxy).[34] But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth.[35] In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Cluster—the most distant stars resolved have up to hundred million light years away[36] (see Cepheids). However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located one billion light years away[37]—ten times the distance of the most distant star cluster previously observed.
                                            

Designations

The concept of the constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology.[38] Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.
As well as certain constellations and the Sun itself, stars as a whole have their own myths.[39] To the Ancient Greeks, some "stars", known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken.[39] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)
Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star's right ascension was invented and added to John Flamsteed's star catalogue in his book "Historia coelestis Britannica" (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.[40][41]
Under space law, the only internationally recognized authority for naming celestial bodies is the International Astronomical Union (IAU).[42] A number of private companies sell names of stars, which the British Library calls an unregulated commercial enterprise.[43][44] However, the IAU has disassociated itself from this commercial practice, and these names are neither recognized by the IAU nor used by them.[45] One such star naming company is the International Star Registry, which, during the 1980s, was accused of deceptive practice for making it appear that the assigned name was official. This now-discontinued ISR practice was informally labeled a scam and a fraud,[46][47][48][49] and the New York City Department of Consumer Affairs issued a violation against ISR for engaging in a deceptive trade practice.[50][51]

Units of measurement

 

Although stellar parameters can be expressed in SI units or CGS units, it is often most convenient to express mass, luminosity, and radii in solar units, based on the characteristics of the Sun:
solar mass: M = 1.9891 × 1030 kg[52]
solar luminosity: L = 3.827 × 1026 watts[52]
solar radius R = 6.960 × 108 m[53]
Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit (AU)—approximately the mean distance between the Earth and the Sun (150 million km or 93 million miles).

Formation and evolution

      
Stars are formed within extended regions of higher density in the interstellar medium, although the density is still lower than the inside of an earthly vacuum chamber. These regions are called molecular clouds and consist mostly of hydrogen, with about 23–28% helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.[54] As massive stars are formed from molecular clouds, they powerfully illuminate those clouds. They also ionize the hydrogen, creating an H II region.

Protostar formation

The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density often triggered by shock waves from supernovae (massive stellar explosions), the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy). Once a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability, it begins to collapse under its own gravitational force.[55]


As the cloud collapses, individual conglomerations of dense dust and gas form what are known as Bok globules. As a globule collapses and the density increases, the gravitational energy is converted into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.[56] These pre–main sequence stars are often surrounded by a protoplanetary disk. The period of gravitational contraction lasts for about 10–15 million years.
Early stars of less than 2 solar masses are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly born stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig–Haro objects.[57][58] These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud in which the star was formed.[59]

Main sequence

Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase. As a consequence, in order to maintain the required rate of nuclear fusion at the core, the star will slowly increase in temperature and luminosity[60]–the Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6 × 109) years ago.[61]
Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the amount of mass lost is negligible. The Sun loses 10−14 solar masses every year,[62] or about 0.01% of its total mass over its entire lifespan. However very massive stars can lose 10−7 to 10−5 solar masses each year, significantly affecting their evolution.[63] Stars that begin with more than 50 solar masses can lose over half their total mass while they remain on the main sequence.[64]

An example of a Hertzsprung–Russell diagram for a set of stars that includes the Sun (center). (See "Classification" below.)
The duration that a star spends on the main sequence depends primarily on the amount of fuel it has to fuse and the rate at which it fuses that fuel, i.e. its initial mass and its luminosity. For the Sun, this is estimated to be about 1010 years. Large stars consume their fuel very rapidly and are short-lived. Small stars (called red dwarfs) consume their fuel very slowly and last tens to hundreds of billions of years. At the end of their lives, they simply become dimmer and dimmer.[2] However, since the lifespan of such stars is greater than the current age of the universe (13.7 billion years), no stars under about 85% of solar mass,[65] including all red dwarfs, are expected to have moved off of the main sequence.
Besides mass, the portion of elements heavier than helium can play a significant role in the evolution of stars. In astronomy all elements heavier than helium are considered a "metal", and the chemical concentration of these elements is called the metallicity. The metallicity can influence the duration that a star will burn its fuel, control the formation of magnetic fields[66] and modify the strength of the stellar wind.[67] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. (Over time these clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.)

Post-main sequence

As stars of at least 0.4 solar masses[2] exhaust their supply of hydrogen at their core, their outer layers expand greatly and cool to form a red giant. For example, in about 5 billion years, when the Sun is a red giant, it will expand out to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size. As a giant, the Sun will lose roughly 30% of its current mass.[61][68]
In a red giant of up to 2.25 solar masses, hydrogen fusion proceeds in a shell-layer surrounding the core.[69] Eventually the core is compressed enough to start helium fusion, and the star now gradually shrinks in radius and increases its surface temperature. For larger stars, the core region transitions directly from fusing hydrogen to fusing helium.[4]
After the star has consumed the helium at the core, fusion continues in a shell around a hot core of carbon and oxygen. The star then follows an evolutionary path that parallels the original red giant phase, but at a higher surface temperature.

Massive stars


Betelgeuse is a red supergiant star approaching the end of its life cycle.
During their helium-burning phase, very high mass stars with more than nine solar masses expand to form red supergiants. Once this fuel is exhausted at the core, they can continue to fuse elements heavier than helium.
The core contracts until the temperature and pressure are sufficient to fuse carbon (see carbon burning process). This process continues, with the successive stages being fueled by neon (see neon burning process), oxygen (see oxygen burning process), and silicon (see silicon burning process). Near the end of the star's life, fusion can occur along a series of onion-layer shells within the star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.[70]
The final stage is reached when the star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, if they are fused they do not release energy—the process would, on the contrary, consume energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission.[69] In relatively old, very massive stars, a large core of inert iron will accumulate in the center of the star. The heavier elements in these stars can work their way up to the surface, forming evolved objects known as Wolf-Rayet stars that have a dense stellar wind which sheds the outer atmosphere.

Collapse

An evolved, average-size star will now shed its outer layers as a planetary nebula. If what remains after the outer atmosphere has been shed is less than 1.4 solar masses, it shrinks to a relatively tiny object (about the size of Earth) that is not massive enough for further compression to take place, known as a white dwarf.[71] The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. White dwarfs will eventually fade into black dwarfs over a very long stretch of time.

The Crab Nebula, remnants of a supernova that was first observed around 1050 AD
In larger stars, fusion continues until the iron core has grown so large (more than 1.4 solar masses) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse beta decay, or electron capture. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae are so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none existed before.[72]
Most of the matter in the star is blown away by the supernova explosion (forming nebulae such as the Crab Nebula)[72] and what remains will be a neutron star (which sometimes manifests itself as a pulsar or X-ray burster) or, in the case of the largest stars (large enough to leave a stellar remnant greater than roughly 4 solar masses), a black hole.[73] In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole the matter is in a state that is not currently understood.
The blown-off outer layers of dying stars include heavy elements which may be recycled during new star formation. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.[72]

In addition to isolated stars, a multi-star system can consist of two or more gravitationally bound stars that orbit around each other. The most common multi-star system is a binary star, but systems of three or more stars are also found. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of co-orbiting binary stars.[74] Larger groups called star clusters also exist. These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars.
It has been a long-held assumption that the majority of stars occur in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, where 80% of the systems are believed to be multiple. However the proportion of single star systems increases for smaller stars, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.[75]
Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 100 billion (1011) galaxies in the observable universe.[76] A 2010 star count estimate was 300 sextillion (3 × 1023) in the observable universe.[77] While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.[78]
The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres, or 4.2 light-years away. Travelling at the orbital speed of the Space Shuttle (8 kilometres per second—almost 30,000 kilometres per hour), it would take about 150,000 years to get there.[79] Distances like this are typical inside galactic discs, including in the vicinity of the solar system.[80] Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos.
Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common.[81] Such collisions can produce what are known as blue stragglers. These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity in the cluster .[82]

Characteristics


The Sun is the nearest star to Earth.
Almost everything about a star is determined by its initial mass, including essential characteristics such as luminosity and size, as well as the star's evolution, lifespan, and eventual fate.

Age

Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old—the observed age of the universe. The oldest star yet discovered, HE 1523-0901, is an estimated 13.2 billion years old.[83][84]
The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years.[85][86]

Chemical composition

When stars form in the present Milky Way galaxy they are composed of about 71% hydrogen and 27% helium,[87] as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. Because the molecular clouds where stars form are steadily enriched by heavier elements from supernovae explosions, a measurement of the chemical composition of a star can be used to infer its age.[88] The portion of heavier elements may also be an indicator of the likelihood that the star has a planetary system.[89]
The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun.[90] By contrast, the super-metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron.[91] There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements.[92]

Diameter


Stars vary widely in size. In each image in the sequence, the right-most object appears as the left-most object in the next panel. The Earth appears at right in panel 1 and the Sun is second from the right in panel 3.
Due to their great distance from the Earth, all stars except the Sun appear to the human eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.[93]
The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.[94]
Stars range in size from neutron stars, which vary anywhere from 20 to 40 km (25 mi) in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter approximately 650 times larger than the Sun—about 900,000,000 km (560,000,000 mi). However, Betelgeuse has a much lower density than the Sun.[95]

Kinematics


The Pleiades, an open cluster of stars in the constellation of Taurus. These stars share a common motion through space.[96] NASA photo
The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy. The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion.
Radial velocity is measured by the doppler shift of the star's spectral lines, and is given in units of km/s. The proper motion of a star is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. By determining the parallax of a star, the proper motion can then be converted into units of velocity. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.[97]
Once both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy.[98] Comparison of the kinematics of nearby stars has also led to the identification of stellar associations. These are most likely groups of stars that share a common point of origin in giant molecular clouds.[99]

Magnetic field


Surface magnetic field of SU Aur (a young star of T Tauri type), reconstructed by means of Zeeman-Doppler imaging
The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, generating magnetic fields that extend throughout the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic fields that reach out into the corona from active regions. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.[100]
Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, however, functioning as a brake to gradually slow the rate of rotation as the star grows older. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods.[101] During the Maunder minimum, for example, the Sun underwent a 70-year period with almost no sunspot activity.

Mass

One of the most massive stars known is Eta Carinae,[102] with 100–150 times as much mass as the Sun; its lifespan is very short—only several million years at most. A study of the Arches cluster suggests that 150 solar masses is the upper limit for stars in the current era of the universe.[103] The reason for this limit is not precisely known, but it is partially due to the Eddington luminosity which defines the maximum amount of luminosity that can pass through the atmosphere of a star without ejecting the gases into space. However, a star named R136a1 in the RMC 136a star cluster has been measured at 265 solar masses, which put this limit into question.[104] A study determined that stars larger than 150 solar masses in R136 were created through the collision and merger of massive stars in close binary systems, providing a way to sidestep the 150 solar mass limit.[105]

The reflection nebula NGC 1999 is brilliantly illuminated by V380 Orionis (center), a variable star with about 3.5 times the mass of the Sun. The black patch of sky is a vast hole of empty space and not a dark nebula as previously thought. NASA image
The first stars to form after the Big Bang may have been larger, up to 300 solar masses or more,[106] due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive, population III stars is long extinct, however, and currently only theoretical.
With a mass only 93 times that of Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core.[107] For stars with similar metallicity to the Sun, the theoretical minimum mass the star can have, and still undergo fusion at the core, is estimated to be about 75 times the mass of Jupiter.[108][109] When the metallicity is very low, however, a recent study of the faintest stars found that the minimum star size seems to be about 8.3% of the solar mass, or about 87 times the mass of Jupiter.[109][110] Smaller bodies are called brown dwarfs, which occupy a poorly defined grey area between stars and gas giants.
The combination of the radius and the mass of a star determines the surface gravity. Giant stars have a much lower surface gravity than main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.[32]
Stars are sometimes grouped by mass based upon their evolutionary behavior as they approach the end of their nuclear fusion lifetimes. Very low mass stars with masses below 0.5 solar masses do not enter the asymptotic giant branch (AGB) but evolve directly into white dwarfs. Low mass stars with a mass below about 1.8–2.2 solar masses (depending on composition) do enter the AGB, where they develop a degenerate helium core. Intermediate-mass stars undergo helium fusion and develop a degenerate carbon-oxygen core. Massive stars have a minimum mass of 7–10 solar masses, but this may be as low as 5–6 solar masses. These stars undergo carbon fusion, with their lives ending in a core-collapse supernova explosion.[111]

Rotation

The rotation rate of stars can be approximated through spectroscopic measurement, or more exactly determined by tracking the rotation rate of starspots. Young stars can have a rapid rate of rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial rotation velocity of about 225 km/s or greater, giving it an equatorial diameter that is more than 50% larger than the distance between the poles. This rate of rotation is just below the critical velocity of 300 km/s where the star would break apart.[112] By contrast, the Sun only rotates once every 25 – 35 days, with an equatorial velocity of 1.994 km/s. The star's magnetic field and the stellar wind serve to slow down a main sequence star's rate of rotation by a significant amount as it evolves on the main sequence.[113]
Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.[114] In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second.[115] The rotation rate of the pulsar will gradually slow due to the emission of radiation.

Temperature

The surface temperature of a main sequence star is determined by the rate of energy production at the core and the radius of the star and is often estimated from the star's color index.[116] It is normally given as the effective temperature, which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. Note that the effective temperature is only a representative value, however, as stars actually have a temperature gradient that decreases with increasing distance from the core.[117] The temperature in the core region of a star is several million kelvins.[118]
The stellar temperature will determine the rate of energization or ionization of different elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below).[32]
Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3,600 K, but they also have a high luminosity due to their large exterior surface area.[119]

Radiation

The energy produced by stars, as a by-product of nuclear fusion, radiates into space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind[120] (which exists as a steady stream of electrically charged particles, such as free protons, alpha particles, and beta particles, emanating from the star's outer layers) and as a steady stream of neutrinos emanating from the star's core.
The production of energy at the core is the reason why stars shine so brightly: every time two or more atomic nuclei of one element fuse together to form an atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion reaction. This energy is converted to other forms of electromagnetic energy, including visible light, by the time it reaches the star's outer layers.
The color of a star, as determined by the peak frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.[121] Besides visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves and infrared to the shortest wavelengths of ultraviolet, X-rays, and gamma rays. All components of stellar electromagnetic radiation, both visible and invisible, are typically significant.
Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is known, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be measured directly for stars in binary systems. The technique of gravitational microlensing will also yield the mass of a star)[122] With these parameters, astronomers can also estimate the age of the star.[123]

Luminosity

In astronomy, luminosity is the amount of light, and other forms of radiant energy, a star radiates per unit of time. The luminosity of a star is determined by the radius and the surface temperature. However, many stars do not radiate a uniform flux—the amount of energy radiated per unit area—across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux at its poles than along its equator.[124]
Surface patches with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as the Sun generally have essentially featureless disks with only small starspots. Larger, giant stars have much bigger, much more obvious starspots,[125] and they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.[126] Red dwarf flare stars such as UV Ceti may also possess prominent starspot features.[127]

Magnitude

The apparent brightness of a star is measured by its apparent magnitude, which is the brightness of a star with respect to the star's luminosity, distance from Earth, and the altering of the star's light as it passes through Earth's atmosphere. Intrinsic or absolute magnitude is directly related to a star's luminosity and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years).
Number of stars brighter than magnitude
Apparent
magnitude
Number
of Stars[128]
0 4
1 15
2 48
3 171
4 513
5 1,602
6 4,800
7 14,000
Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times[129] (the 5th root of 100 or approximately 2.512). This means that a first magnitude (+1.00) star is about 2.5 times brighter than a second magnitude (+2.00) star, and approximately 100 times brighter than a sixth magnitude (+6.00) star. The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.
On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness (ΔL) between two stars is calculated by subtracting the magnitude number of the brighter star (mb) from the magnitude number of the fainter star (mf), then using the difference as an exponent for the base number 2.512; that is to say:
 \Delta{m} = m_\mathrm{f} - m_\mathrm{b}
2.512^{\Delta{m}} = \Delta{L}
Relative to both luminosity and distance from Earth, absolute magnitude (M) and apparent magnitude (m) are not equivalent for an individual star;[129] for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41.
The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.
As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14.2. This star is at least 5,000,000 times more luminous than the Sun.[130] The least luminous stars that are currently known are located in the NGC 6397 cluster. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.[131]

Classification

Surface Temperature Ranges for
Different Stellar Classes
[132]
Class Temperature Sample star
O 33,000 K or more Zeta Ophiuchi
B 10,500–30,000 K Rigel
A 7,500–10,000 K Altair
F 6,000–7,200 K Procyon A
G 5,500–6,000 K Sun
K 4,000–5,250 K Epsilon Indi
M 2,600–3,850 K Proxima Centauri
The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.[133] It was not known at the time that the major influence on the line strength was temperature; the hydrogen line strength reaches a peak at over 9000 K, and is weaker at both hotter and cooler temperatures. When the classifications were reordered by temperature, it more closely resembled the modern scheme.[134]
There are different single-letter classifications of stars according to their spectra, ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types have special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures: class O0 and O1 stars may not exist.[135]
In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by the surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs); some authors add VII (white dwarfs). Most stars belong to the main sequence, which consists of ordinary hydrogen-burning stars. These fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.[135] The Sun is a main sequence G2V yellow dwarf, being of intermediate temperature and ordinary size.
Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.[135]
White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature index.[136]

Variable stars


The asymmetrical appearance of Mira, an oscillating variable star. NASA HST image
Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.
During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and cepheid-like stars, and long-period variables such as Mira.[137]
Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.[137] This group includes protostars, Wolf-Rayet stars, and Flare stars, as well as giant and supergiant stars.
Cataclysmic or explosive variables undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.[4] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.[138] Some novae are also recurrent, having periodic outbursts of moderate amplitude.[137]
Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.[137] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.3 to 3.5 over a period of 2.87 days.

Structure


Internal structures of main sequence stars, convection zones with arrowed cycles and radiative zones with red flashes. To the left a low-mass red dwarf, in the center a mid-sized yellow dwarf and at the right a massive blue-white main sequence star.
The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.[139][140]
As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant and energy production ceases at the core. Instead, for stars of more than 0.4 solar masses, fusion occurs in a slowly expanding shell around the degenerate helium core.[141]
In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.
The radiation zone is the region within the stellar interior where radiative transfer is sufficiently efficient to maintain the flux of energy. In this region the plasma will not be perturbed and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity as in the outer envelope.[140]
The occurrence of convection in the outer envelope of a main sequence star depends on the mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.[142] Red dwarf stars with less than 0.4 solar masses are convective throughout, which prevents the accumulation of a helium core.[2] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.[140]

This diagram shows a cross-section of the Sun. NASA image
The portion of a star that is visible to an observer is called the photosphere. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate out into space. It is within the photosphere that sun spots, or regions of lower than average temperature, appear.
Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere is the thin chromosphere region, where spicules appear and stellar flares begin. This is surrounded by a transition region, where the temperature rapidly increases within a distance of only 100 km (62 mi). Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.[143] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.[142] Despite its high temperature, the corona emits very little light. The corona region of the Sun is normally only visible during a solar eclipse.
From the corona, a stellar wind of plasma particles expands outward from the star, propagating until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout the bubble-shaped region of the heliosphere.[144]

Nuclear fusion reaction pathways

Overview of the proton-proton chain
The carbon-nitrogen-oxygen cycle
A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition, as part of stellar nucleosynthesis. The net mass of the fused atomic nuclei is smaller than the sum of the constituents. This lost mass is released as electromagnetic energy, according to the mass-energy equivalence relationship E = mc2.[1]
The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. As a result the core temperature of main sequence stars only varies from 4 million kelvin for a small M-class star to 40 million kelvin for a massive O-class star.[118]
In the Sun, with a 10-million-kelvin core, hydrogen fuses to form helium in the proton-proton chain reaction:[145]
41H → 22H + 2e+ + 2νe (4.0 MeV + 1.0 MeV)
21H + 22H → 23He + 2γ (5.5 MeV)
23He → 4He + 21H (12.9 MeV)
These reactions result in the overall reaction:
41H → 4He + 2e+ + 2γ + 2νe (26.7 MeV)
where e+ is a positron, γ is a gamma ray photon, νe is a neutrino, and H and He are isotopes of hydrogen and helium, respectively. The energy released by this reaction is in millions of electron volts, which is actually only a tiny amount of energy. However enormous numbers of these reactions occur constantly, producing all the energy necessary to sustain the star's radiation output.
Minimum stellar mass required for fusion
Element Solar
masses
Hydrogen 0.01
Helium 0.4
Carbon 5[146]
Neon 8
In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon—the carbon-nitrogen-oxygen cycle.[145]
In evolved stars with cores at 100 million kelvin and masses between 0.5 and 10 solar masses, helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium:[145]
4He + 4He + 92 keV → 8*Be
4He + 8*Be + 67 keV → 12*C
12*C → 12C + γ + 7.4 MeV
For an overall reaction of:
34He → 12C + γ + 7.2 MeV
In massive stars, heavier elements can also be burned in a contracting core through the neon burning process and oxygen burning process. The final stage in the stellar nucleosynthesis process is the silicon burning process that results in the production of the stable isotope iron-56. Fusion can not proceed any further except through an endothermic process, and so further energy can only be produced through gravitational collapse.[145]
The example below shows the amount of time required for a star of 20 solar masses to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity.[147]
                                                                                            astronaut

An astronaut or cosmonaut is a person trained by a human spaceflight program to command, pilot, or serve as a crew member of a spacecraft. While generally reserved for professional space travelers, the terms are sometimes applied to anyone who travels into space, including scientists, politicians, journalists, and tourists.[1][2]
Until 2002, astronauts were sponsored and trained exclusively by governments, either by the military, or by civilian space agencies. With the sub-orbital flight of the privately funded SpaceShipOne in 2004, a new category of astronaut was created: the commercial astronaut.

Definition

The criteria for what constitutes human spaceflight vary. The Fédération Aéronautique Internationale (FAI) Sporting Code for astronautics recognizes only flights that exceed an altitude of 100 kilometers (62 mi).[3] In the United States, professional, military, and commercial astronauts who travel above an altitude of 50 miles (80 km)[4] are awarded astronaut wings.
As of June 20, 2011, a total of 523 people from 38 countries[5] have reached 100 km (62 mi) or more in altitude, of which 520 reached low Earth orbit or beyond.[6][7] Of these, 24 people have traveled beyond Low Earth orbit, to either lunar or trans-lunar orbit or to the surface of the moon; three of the 24 did so twice: Jim Lovell, John Young and Eugene Cernan.[8] The three astronauts who have not reached low Earth orbit are spaceplane pilots Joe Walker, Mike Melvill, and Brian Binnie.
Under the U.S. definition, as of June 20, 2011, 529 people qualify as having reached space, above 50 miles (80 km) altitude. Of eight X-15 pilots who exceeded 50 miles (80 km) in altitude, only one exceeded 100 kilometers (about 62 miles).[9] Space travelers have spent over 30,400 man-days (83 man-years) in space, including over 100 astronaut-days of spacewalks.[9][10] As of 2008, the man with the longest cumulative time in space is Sergei K. Krikalev, who has spent 803 days, 9 hours and 39 minutes, or 2.2 years, in space.[11][12] Peggy A. Whitson holds the record for the most time in space by a woman, 377 days.[13]

Terminology

English

In the United States, Canada, Ireland, the United Kingdom, and many other English-speaking nations, a professional space traveler is called an astronaut.[14] The term derives from the Greek words ástron (ἄστρον), meaning "star", and nautes (ναύτης), meaning "sailor". The first known use of the term "astronaut" in the modern sense was by Neil R. Jones in his short story "The Death's Head Meteor" in 1930. The word itself had been known earlier. For example, in Percy Greg's 1880 book Across the Zodiac, "astronaut" referred to a spacecraft. In Les Navigateurs de l'Infini (1925) of J.-H. Rosny aîné, the word astronautique (astronautic) was used. The word may have been inspired by "aeronaut", an older term for an air traveler first applied (in 1784) to balloonists. An early use in a non-fiction publication is Eric Frank Russell's poem "The Astronaut" in the November 1934 Bulletin of the British Interplanetary Society.[15]
The first known formal use of the term astronautics in the scientific community was the establishment of the annual International Astronautical Congress in 1950 and the subsequent founding of the International Astronautical Federation the following year.[16]
NASA applies the term astronaut to any crew member aboard NASA spacecraft bound for Earth orbit or beyond. NASA also uses the term as a title for those selected to join its Astronaut Corps.[17] The European Space Agency similarly uses the term astronaut for members of its Astronaut Corps.[18]

Russian

By convention, an astronaut employed by the Russian Federal Space Agency (or its Soviet predecessor) is called a cosmonaut in English texts.[17] The word is an anglicisation of the Russian word kosmonavt (Russian: космонавт Russian pronunciation: [kəsmɐˈnaft]), one who works in space outside the Earth's atmosphere, a space traveler,[19] which derives from the Greek words kosmos (κόσμος), meaning "universe", and nautes (ναύτης), meaning "sailor".
The Soviet Air Force pilot Yuri Gagarin was the first cosmonaut—indeed the first person—in space. Valentina Tereshkova, a Russian factory worker, was the first female in space, as well as arguably the first civilian to make it there (see below for a further discussion of civilians in space). On March 14, 1995, Norman Thagard became the first American to ride to space on board a Russian launch vehicle, and thus became the first "American cosmonaut".

Chinese

Official English-language texts issued by the government of the People's Republic of China use astronaut while texts in Russian use космонавт (kosmonavt).[20][21] In official Chinese-language texts, the terms "yǔhángyuán" (宇航员, "sailing personnel in universe") for cosmonaut and "hángtiānyuán" (航天员, "sailing personnel in sky") for astronaut have long been used. The phrase "tàikōng rén" (太空人, "spaceman") is often used in Taiwan and Hong Kong.
The term taikonaut is used by some English-language news media organizations for professional space travelers from China.[22] The word has featured in the Longman and Oxford English dictionaries, the latter of which describes it as "a hybrid of the Chinese term taikong (space) and the Greek naut (sailor)"; the term became more common in 2003 when China sent its first astronaut Yang Liwei into space aboard the Shenzhou 5 spacecraft.[23] This is the term used by Xinhua in the English version of the Chinese People's Daily since the advent of the Chinese space program.[24] The origin of the term is unclear; as early as May 1998, Chiew Lee Yih (趙裡昱) from Malaysia, used it in newsgroups.[25][26]

Other terms

With the rise of space tourism, NASA and the Russian Federal Space Agency agreed to use the term "spaceflight participant" to distinguish those space travelers from professional astronauts on missions coordinated by those two agencies.
While no nation other than the Russian Federation (and previously the former Soviet Union), the United States, and China have launched a manned spacecraft, several other nations have sent people into space in cooperation with one of these countries. Inspired partly by these missions, other synonyms for astronaut have entered occasional English usage. For example, the term spationaut (French spelling: spationaute) is sometimes used to describe French space travelers, from the Latin word spatium or "space", and the Malay or Korean term angkasawan or gong-gan was used to describe participants in the Angkasawan program. In Hungarian the word describing astronauts is űrhajós (from űr meaning "space" and hajós meaning "sailor".)

Space travel milestones


Yuri Gagarin, first human in space (1961)

Valentina Tereshkova, 1963 first woman in space.

Neil Armstrong, first person to walk on the moon (1969).
The first human in space was Soviet Yuri Gagarin, who was launched on April 12, 1961 aboard Vostok 1 and orbited around the Earth for 108 minutes. The first woman in space was Soviet Valentina Tereshkova, who launched on June 16, 1963 aboard Vostok 6 and orbited Earth for almost three days.
Alan Shepard became the first American and second person in space on May 5, 1961 on a 15-minute sub-orbital flight. The first American woman in space was Sally Ride, during Space Shuttle Challenger's mission STS-7, on June 18, 1983.[27] In 1992 Mae Jemison became the first African American woman to travel in space aboard STS-47.
The first manned mission to orbit the moon, Apollo 8, included American William Anders who was born in Hong Kong, making him the first Asian-born astronaut in 1968. In April 1985, Taylor Wang became the first ethnic Chinese person in space.[28][29] On 15 October 2003, Yang Liwei became China's first astronaut on the Shenzhou 5 spacecraft.
The Soviet Union, through its Intercosmos program, allowed people from other "socialist" (i.e. Warsaw Pact and other Soviet-allied) countries to fly on its missions. An example is Czechoslovak Vladimír Remek, the first cosmonaut from a country other than the Soviet Union or the United States, who flew to space in 1978 on a Soyuz-U rocket.[30] On July 23, 1980, Pham Tuan of Vietnam became the first Asian in space when he flew aboard Soyuz 37.[31]
Also in 1980, Cuban Arnaldo Tamayo Méndez became the first person of Hispanic and black African descent to fly in space, and Guion Bluford became the first African American to fly into space. The first person born in Africa to fly in space was Patrick Baudry, in 1985.[32][33] In 1985, Saudi Arabian Prince Sultan Bin Salman Bin AbdulAziz Al-Saud became the first Arab Muslim astronaut in space.[34] In 1988, Abdul Ahad Mohmand became the first Afghan to reach space, spending nine days aboard the Mir space station.[35]
With the larger number of seats available on the Space Shuttle, the U.S. began taking international astronauts. In 1983, Ulf Merbold of West Germany became the first non-US citizen to fly in a US spacecraft. In 1984, Marc Garneau became the first of 8 Canadian astronauts to fly in space (through 2010).[36] In 1985, Rodolfo Neri Vela became the first Mexican-born person in space.[37] In 1991, Helen Sharman became the first Briton to fly in space.[38] In 2002, Mark Shuttleworth became the first citizen of an African country to fly in space, as a paying spaceflight participant.[39] In 2003, Ilan Ramon became the first Israeli to fly in space, although he died during a re-entry accident.

Age milestones

The youngest person to fly in space is Gherman Titov, who was 25 years old when he flew Vostok 2. (Titov was also the first person to suffer space sickness).[40][41] The oldest person who has flown in space is John Glenn, who was 77 when he flew on STS-95.[42]

Duration and distance milestones

The longest stay in space thus far has been 438 days, by Russian Valeri Polyakov.[9] As of 2006, the most spaceflights by an individual astronaut is seven, a record held by both Jerry L. Ross and Franklin Chang-Diaz. The farthest distance from Earth an astronaut has traveled was 401,056 km (249,205 mi), when Jim Lovell, Jack Swigert, and Fred Haise went around the Moon during the Apollo 13 emergency.[9]

Civilian and non-government milestones

Depending on the exact definition of 'civilian', the first civilian in space was either Valentina Tereshkova[43] aboard Vostok 6 (she also became the first woman in space on that mission) or Joseph Albert Walker[44][45] on X-15 Flight 90 a month later. Tereshkova was only honorarily inducted into the USSR's Air Force, which had no female pilots whatsoever at that time. Joe Walker had joined the US Army Air Force but was not a member during his flight. The first people in space who had never been a member of any country's armed forces were both Konstantin Feoktistov and Boris Yegorov aboard Voskhod 1.
The first non-governmental space traveler was Byron K. Lichtenberg, a researcher from the Massachusetts Institute of Technology who flew on STS-9 in 1983.[46] In December 1990, Toyohiro Akiyama became the first paying space traveler as a reporter for Tokyo Broadcasting System, a visit to Mir as part of an estimated $12 million (USD) deal with a Japanese TV station, although at the time, the term used to refer to Akiyama was "Research Cosmonaut".[47][48][49] Akiyama suffered severe space sickness during his mission, which affected his productivity.[48]
The first self-funded space tourist was Dennis Tito on board the Russian spacecraft Soyuz TM-3 on 28 April 2001.

Self-funded travelers

The first person to fly on an entirely privately funded mission was Mike Melvill, piloting SpaceShipOne flight 15P on a suborbital journey, although he was a test pilot employed by Scaled Composites and not an actual paying space tourist.[50][51] Seven others have paid to fly into space:
  1. Dennis Tito (American): April 28 – May 6, 2001 (ISS)
  2. Mark Shuttleworth (South African): April 25 – May 5, 2002 (ISS)
  3. Gregory Olsen (American): October 1–11, 2005 (ISS)
  4. Anousheh Ansari (Iranian / American): September 18–29, 2006 (ISS)
  5. Charles Simonyi (Hungarian / American): April 7–21, 2007 (ISS), March 26 – April 8, 2009 (ISS)
  6. Richard Garriott (American): October 12–24, 2008 (ISS)
  7. Guy Laliberté (Canadian): September 30, 2009 – October 11, 2009 (ISS)

Training

The first NASA astronauts were selected for training in 1959.[52] Early in the space program, military jet test piloting and engineering training were often cited as prerequisites for selection as an astronaut at NASA, although neither John Glenn nor Scott Carpenter (of the Mercury Seven) had any university degree, in engineering or any other discipline at the time of their selection. Selection was initially limited to military pilots.[53][54] The earliest astronauts for both America and the USSR tended to be jet fighter pilots, and were often test pilots.
Once selected, NASA astronauts go through twenty months of training in a variety of areas, including training for extra-vehicular activity in a facility such as NASA's Neutral Buoyancy Laboratory.[1][53] Astronauts-in-training may also experience short periods of weightlessness in aircraft called the "vomit comet", the nickname given to a pair of modified KC-135s (retired in 2000 and 2004 respectively, and replaced in 2005 with a C-9) which perform parabolic flights.[52] Astronauts are also required to accumulate a number of flight hours in high-performance jet aircraft. This is mostly done in T-38 jet aircraft out of Ellington Field, due to its proximity to the Johnson Space Center. Ellington Field is also where the Shuttle Training Aircraft is maintained and developed, although most flights of the aircraft are done out of Edwards Air Force Base.

NASA candidacy requirements

  • Be citizens of the United States.[52][55]
  • Pass a strict physical examination, and have a near and distant visual acuity correctable to 20/20 (6/6). Blood pressure, while sitting, must be no greater than 140 over 90.

Commander and Pilot

  • A bachelor's degree in engineering, biological science, physical science or mathematics is required.
  • At least 1,000 hours' flying time as pilot-in-command in jet aircraft. Experience as a test pilot is desirable.
  • Height must be 5 ft 2 in to 6 ft 2 in (1.58 to 1.88 m).
  • Distant visual acuity must be correctable to 20/20 in each eye.
  • The refractive surgical procedures of the eye, PRK (Photorefractive keratectomy) and LASIK, are now allowed, providing at least 1 year has passed since the date of the procedure with no permanent adverse aftereffects. For those applicants under final consideration, an operative report on the surgical procedure will be requested.

Mission Specialist

  • A bachelor's degree in engineering, biological science, physical science or mathematics, as well as at least three years of related professional experience (graduate work or studies) and an advanced degree, such as a master's degree (one to three years) or a doctoral degree (three years or more).
  • Applicant's height must be between 58.5 and 76 inches

Mission Specialist Educator


Mission Specialist Educators Lindenberger, Arnold, and Acaba during a parabolic flight.
  • Applicants must have a bachelor's degree with teaching experience, including work at the kindergarten through twelfth grade level. An advanced degree, such as a master's degree or a doctoral degree, is not required, but is strongly desired.[56]
Mission Specialist Educators, or "Educator Astronauts", were first selected in 2004, and as of 2007, there are three NASA Educator astronauts: Joseph M. Acaba, Richard R. Arnold, and Dorothy Metcalf-Lindenburger.[57][58] Barbara Morgan, selected as back-up teacher to Christa McAuliffe in 1985, is considered to be the first Educator astronaut by the media, but she trained as a mission specialist.[59] The Educator Astronaut program is a successor to the Teacher in Space program from the 1980s.[60][61]

Health risks of space travel

Astronauts are susceptible to a variety of health risks including decompression sickness, barotrauma, immunodeficiencies, loss of bone and muscle, orthostatic intolerance due to volume loss, sleep disturbances, and radiation injury. A variety of large scale medical studies are being conducted in space via the National Space and Biomedical Research Institute (NSBRI) to address these issues. Prominent among these is the Advanced Diagnostic Ultrasound in Microgravity Study in which astronauts (including former ISS commanders Leroy Chiao and Gennady Padalka) perform ultrasound scans under the guidance of remote experts to diagnose and potentially treat hundreds of medical conditions in space. This study's techniques are now being applied to cover professional and Olympic sports injuries as well as ultrasound performed by non-expert operators in medical and high school students. It is anticipated that remote guided ultrasound will have application on Earth in emergency and rural care situations, where access to a trained physician is often rare.[62][63][64]

Insignia

In Russia, cosmonauts are awarded Pilot-Cosmonaut of the Russian Federation upon completion of their missions, often accompanied with the award of Hero of the Russian Federation. This follows the practice established in the Soviet Union.
At NASA, those who complete astronaut candidate training receive a silver lapel pin. Once they have flown in space, they receive a gold pin. U.S. astronauts who also have active-duty military status receive a special qualification badge, known as the Astronaut Badge, after participation on a spaceflight. The United States Air Force also presents an Astronaut Badge to its pilots who exceed 50 miles (80 km) in altitude.

Space Mirror Memorial


A planet (from Ancient Greek αστήρ πλανήτης (astēr planētēs), meaning "wandering star") is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.[a][1][2] The term planet is ancient, with ties to history, science, mythology, and religion. The planets were originally seen by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union (IAU) officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticized, and remains disputed by some scientists since it excludes many objects of planetary mass based on where or what they orbit. While eight of the planetary bodies discovered before 1950 remain "planets" under modern definition, some celestial bodies, such as Ceres, Pallas, Juno, Vesta (each an object in the Solar asteroid belt) and Pluto (the first-discovered trans-Neptunian object), that were once considered planets by the scientific community are no longer viewed as such.
The planets were thought by Ptolemy to orbit the Earth in deferent and epicycle motions. Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. By careful analysis of the observation data, Johannes Kepler found the planets' orbits were not circular, but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology.
Planets are generally divided into two main types: large, low-density gas giants, and smaller, rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System. In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four gas giants, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites. Additionally, the Solar System also contains at least five dwarf planets[3][4][dubious ] and hundreds of thousands of known small Solar System bodies.
Since 1992, hundreds of planets around other stars ("extrasolar planets" or "exoplanets") in the Milky Way Galaxy have been discovered. As of November 8, 2012, 846 known extrasolar planets (in 665 planetary systems and 126 multiple planetary systems) are listed in the Extrasolar Planets Encyclopaedia, ranging in size from that of terrestrial planets similar to Earth to that of gas giants larger than Jupiter.[5] On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e[6] and Kepler-20f,[7] orbiting a Sun-like star, Kepler-20.[8][9][10] A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.[11]

Contents

History

Printed rendition of a geocentric cosmological model from Cosmographia, Antwerp, 1539
The idea of planets has evolved over its history, from the divine wandering stars of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy.
The five classical planets, being visible to the naked eye, have been known since ancient times, and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. Ancient Greeks called these lights πλάνητες ἀστέρες (planetes asteres "wandering stars") or simply "πλανήτοι" (planētoi "wanderers"),[12] from which today's word "planet" was derived.[13][14] In ancient Greece, China, Babylon and indeed all pre-modern civilizations,[15][16] it was almost universally believed that Earth was in the center of the Universe and that all the "planets" circled the Earth. The reasons for this perception were that stars and planets appeared to revolve around the Earth each day,[17] and the apparently common-sense perception that the Earth was solid and stable, and that it was not moving but at rest.

Babylon

The first civilization known to possess a functional theory of the planets were the Babylonians, who lived in Mesopotamia in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th century BC copy of a list of observations of the motions of the planet Venus, that probably dates as early as the second millennium BC.[18] The MUL.APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun, Moon and planets over the course of the year.[19] The Babylonian astrologers also laid the foundations of what would eventually become Western astrology.[20] The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC,[21] comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.[22][23] Venus, Mercury and the outer planets Mars, Jupiter and Saturn were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope in early modern times.[24]

Greco-Roman astronomy

Ptolemy's 7 planetary spheres
1
Moon
☾
2
Mercury
☿
3
Venus
♀
4
Sun
☉
5
Mars
♂
6
Jupiter
♃
7
Saturn
♄
The ancient Greeks initially did not attach as much significance to the planets as the Babylonians. The Pythagoreans, in the 6th and 5th centuries BC appear to have developed their own independent planetary theory, which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe. Pythagoras or Parmenides is said to have been the first to identify the evening star and morning star (Venus) as one and the same.[25] In the 3rd century BC, Aristarchus of Samos proposed a heliocentric system, according to which the Earth and planets revolved around the sun. However, the geocentric system would remain dominant until the Scientific Revolution.
By the 1st century BC, during the Hellenistic period, the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness, and account for most of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the Almagest written by Ptolemy in the 2nd century CE. So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries.[18][26] To the Greeks and Romans there were seven known planets, each presumed to be circling the Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.[14][26][27]

Medieval Muslim astronomy

In the 11th century, the transit of Venus was observed by Avicenna, who established that Venus was, at least sometimes, below the Sun.[32] In the 12th century, Ibn Bajjah observed "two planets as black spots on the face of the Sun", which was later identified as a transit of Mercury and Venus by the Maragha astronomer Qotb al-Din Shirazi in the 13th century.[33] However, Ibn Bajjah could not have observed a transit of Venus, as none occurred in his lifetime.[34]

European Renaissance

Renaissance planets, ca. 1543 to 1781
1
Mercury
☿
2
Venus
♀
3
Earth
⊕
4
Mars
♂
5
Jupiter
♃
6
Saturn
♄
With the advent of the Scientific Revolution, understanding of the term "planet" changed from something that moved across the sky (in relation to the star field); to a body that orbited the Earth (or that were believed to do so at the time); and in the 16th century to something that directly orbited the Sun when the heliocentric model of Copernicus, Galileo and Kepler gained sway.
Thus, the Earth became included in the list of planets,[35] while the Sun and Moon were excluded. At first, when the first satellites of Jupiter and Saturn were discovered in the 17th century, the terms "planet" and "satellite" were used interchangeably – although the latter would gradually become more prevalent in the following century.[36] Until the mid-19th century, the number of "planets" rose rapidly since any newly discovered object directly orbiting the Sun was listed as a planet by the scientific community.

19th century

New planets, 1807–1845
1
Mercury
☿
2
Venus
♀
3
Earth
⊕
4
Mars
♂
5
Vesta
⚶
6
Juno
⚵
7
Ceres
⚳
8
Pallas
⚴
9
Jupiter
♃
10
Saturn
♄
11
Uranus
♅
In the 19th century astronomers began to realize that recently discovered bodies that had been classified as planets for almost half a century (such as Ceres, Pallas, and Vesta) were very different from the traditional ones. These bodies shared the same region of space between Mars and Jupiter (the asteroid belt), and had a much smaller mass; as a result they were reclassified as "asteroids". In the absence of any formal definition, a "planet" came to be understood as any "large" body that orbited the Sun. Since there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846, there was no apparent need to have a formal definition.[37]

20th century

Planets 1854–1930, 2006–present
1
Mercury
☿
2
Venus
♀
3
Earth
⊕
4
Mars
♂
5
Jupiter
♃
6
Saturn
♄
7
Uranus
♅
8
Neptune
♆
However, in the 20th century, Pluto was discovered. After initial observations led to the belief it was larger than Earth,[38] the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in 1936, Raymond Lyttleton suggested that Pluto may be an escaped satellite of Neptune,[39] and Fred Whipple suggested in 1964 that Pluto may be a comet.[40] However, as it was still larger than all known asteroids and seemingly did not exist within a larger population,[41] it kept its status until 2006.
Planets 1930–2006
1
Mercury
☿
2
Venus
♀
3
Earth
⊕
4
Mars
♂
5
Jupiter
♃
6
Saturn
♄
7
Uranus
♅
8
Neptune
♆
9
Pluto
♇
In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12.[42] This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[43]
The discovery of extrasolar planets led to another ambiguity in defining a planet; the point at which a planet becomes a star. Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as "brown dwarfs".[44] Brown dwarfs are generally considered stars due to their ability to fuse deuterium, a heavier isotope of hydrogen. While stars more massive than 75 times that of Jupiter fuse hydrogen, stars of only 13 Jupiter masses can fuse deuterium. However, deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.[45]

21st century

With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There were particular disagreements over whether an object should be considered a planet if it was part of a distinct population such as a belt, or if it was large enough to generate energy by the thermonuclear fusion of deuterium.
A growing number of astronomers argued for Pluto to be declassified as a planet, since many similar objects approaching its size had been found in the same region of the Solar System (the Kuiper belt) during the 1990s and early 2000s. Pluto was found to be just one small body in a population of thousands.
Some of them including Quaoar, Sedna, and Eris were heralded in the popular press as the tenth planet, failing however to receive widespread scientific recognition. The announcement of Eris in 2005, an object 27% more massive than Pluto, created the necessity and public desire for an official definition of a planet.
Acknowledging the problem, the IAU set about creating the definition of planet, and produced one in August 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris).[46]

Extrasolar planet definition



Earth Dysnomia Eris Charon Nix Hydra S/2011 (134340) 1 Pluto Makemake Namaka Hi'iaka Haumea Sedna 2007 OR10 Weywot Quaoar Vanth Orcus File:EightTNOs.png
Artistic comparison of Eris, Pluto, Makemake, Haumea, Sedna, 2007 OR10, Quaoar, Orcus, and Earth. These eight trans-Neptunian objects have the brightest absolute magnitudes, although several other TNOs have been found to be physically larger than Orcus, and several more may yet be found.
  1. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter for objects with the same isotopic abundance as the Sun[47]) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass and size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System.
  2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located.
  3. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).
This definition has since been widely used by astronomers when publishing discoveries of exoplanets in academic journals.[48] Although temporary, it remains an effective working definition until a more permanent one is formally adopted. However, it does not address the dispute over the lower mass limit,[49] and so it steered clear of the controversy regarding objects within the Solar System. This definition also makes no comment on the planetary status of objects orbiting brown dwarfs, such as 2M1207b.
One definition of a sub-brown dwarf is a planet-mass object that formed through cloud-collapse rather than accretion. This formation distinction between a sub-brown dwarf and a planet is not universally agreed upon; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification.[50] One reason for the dissent is that oftentimes, it may not be possible to determine the formation process: for example an accretion-formed planet around a star may get ejected from the system to become free-floating, and likewise a cloud-collapse-formed sub-brown dwarf formed on its own in a star cluster may get captured into orbit around a star.
Dwarf planets 2006–present
Ceres Pluto Makemake Haumea Eris
The 13 Jupiter-mass cutoff is a rule of thumb rather than something of precise physical significance. The question arises: what is meant by deuterium burning? This question arises because large objects will burn most of their deuterium and smaller ones will burn only a little, and the 13 MJ value is somewhere in between. The amount of deuterium burnt depends not only on mass but also on the composition of the planet, on the amount of helium and deuterium present.[51]
Another criterion for separating planets and brown dwarfs, rather than deuterium burning, formation process or location is whether the core pressure is dominated by coulomb pressure or electron degeneracy pressure.[52][53]

2006 definition

The matter of the lower limit was addressed during the 2006 meeting of the IAU's General Assembly. After much debate and one failed proposal, the assembly voted to pass a resolution that defined planets within the Solar System as:[54]
A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
Under this definition, the Solar System is considered to have eight planets. Bodies which fulfill the first two conditions but not the third (such as Pluto, Makemake and Eris) are classified as dwarf planets, provided they are not also natural satellites of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion.[55] After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.[56]
This definition is based in theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described by astronomer Steven Soter:[57]
The end product of secondary disk accretion is a small number of relatively large bodies (planets) in either non-intersecting or resonant orbits, which prevent collisions between them. Minor planets and comets, including KBOs [Kuiper belt objects], differ from planets in that they can collide with each other and with planets.
In the aftermath of the IAU's 2006 vote, there has been controversy and debate about the definition,[58][59] and many astronomers have stated that they will not use it.[60] Part of the dispute centres around the belief that point (c) (clearing its orbit) should not have been listed, and that those objects now categorised as dwarf planets should actually be part of a broader planetary definition.
Beyond the scientific community, Pluto has held a strong cultural significance for many in the general public considering its planetary status since its discovery in 1930. The discovery of Eris was widely reported in the media as the tenth planet and therefore the reclassification of all three objects as dwarf planets has attracted a lot of media and public attention as well.[61]

Former classifications

The table below lists Solar System bodies formerly considered to be planets:
Body (current classification) Notes
Star Dwarf planet Asteroid Moon
Sun

The Moon Classified as planets in antiquity, in accordance with the now disproved geocentric model.



Io, Europa, Ganymede, and Callisto The four largest moons of Jupiter, known as the Galilean moons after their discoverer Galileo Galilei. He referred to them as the "Medicean Planets" in honor of his patron, the Medici family.



Titan,[b] Iapetus,[c] Rhea,[c] Tethys,[d] and Dione[d] Five of Saturn's larger moons, discovered by Christiaan Huygens and Giovanni Domenico Cassini.

Ceres[e] Pallas, Juno, and Vesta
The first known asteroids, from their discoveries between 1801 and 1807 until their reclassification as asteroids during the 1850s.[62] Ceres has subsequently been classified as a dwarf planet in 2006.


Astrea, Hebe, Iris, Flora, Metis, Hygeia, Parthenope, Victoria, Egeria, Irene, Eunomia
More asteroids, discovered between 1845 and 1851. The rapidly expanding list of planets prompted their reclassification as asteroids by astronomers, and this was widely accepted by 1854.[63]

Pluto[f]

The first known trans-Neptunian object (i.e. minor planet with a semi-major axis beyond Neptune). In 2006, Pluto was reclassified as a dwarf planet.

Eris

Discovered in 2003, this trans-Neptunian object (i.e. minor planet with a semi-major axis beyond Neptune) was recognized in 2005, before, like Pluto, in 2006 getting classified as a dwarf planet.

Mythology and naming

The gods of Olympus, after whom the Solar System's planets are named
The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In ancient Greece, the two great luminaries the Sun and the Moon were called Helios and Selene; the farthest planet was called Phainon, the shiner; followed by Phaethon, "bright"; the red planet was known as Pyroeis, the "fiery"; the brightest was known as Phosphoros, the light bringer; and the fleeting final planet was called Stilbon, the gleamer. The Greeks also made each planet sacred to one among their pantheon of gods, the Olympians: Helios and Selene were the names of both planets and gods; Phainon was sacred to Cronus, the Titan who fathered the Olympians; Phaethon was sacred to Zeus, Cronus's son who deposed him as king; Pyroeis was given to Ares, son of Zeus and god of war; Phosphoros was ruled by Aphrodite, the goddess of love; and Hermes, messenger of the gods and god of learning and wit, ruled over Stilbon.[18]
The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphoros after their goddess of love, Ishtar; Pyroeis after their god of war, Nergal, Stilbon after their god of wisdom Nabu, and Phaethon after their chief god, Marduk.[64] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.[18] The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. However, unlike Ares, Nergal was also god of pestilence and the underworld.[65]
Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. While modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire and, later, the Catholic Church, use the Roman (or Latin) names rather than the Greek ones. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable.[66] When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius (for Hermes), Venus (Aphrodite), Mars (Ares), Iuppiter (Zeus) and Saturnus (Cronus). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained with Neptūnus (Poseidon). Uranus is unique in that it is named for a Greek deity rather than his Roman counterpart.
Some Romans, following a belief possibly originating in Mesopotamia but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet).[67] Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Since each day was named by the god that started it, this is also the order of the days of the week in the Roman calendar after the Nundinal cycle was rejected – and still preserved in many modern languages.[68] Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, (Tuesday) Wóden (Wednesday), Thunor (Thursday), and Fríge (Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively.
Earth is the only planet whose name in English is not derived from Greco-Roman mythology. Since it was only generally accepted as a planet in the 17th century,[35] there is no tradition of naming it after a god (the same is true, in English at least, of the Sun and the Moon, though they are no longer considered planets). The name originates from the 8th century Anglo-Saxon word erda, which means ground or soil and was first used in writing as the name of the sphere of the Earth perhaps around 1300.[69][70] As with its equivalents in the other Germanic languages, it derives ultimately from the Proto-Germanic word ertho, "ground",[70] as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jord. Many of the Romance languages retain the old Roman word terra (or some variation of it) that was used with the meaning of "dry land" (as opposed to "sea").[71] However, the non-Romance languages use their own respective native words. The Greeks retain their original name, Γή (Ge or Yi).
Non-European cultures use other planetary naming systems. India uses a naming system based on the Navagraha, which incorporates the seven traditional planets (Surya for the Sun, Chandra for the Moon, and Budha, Shukra, Mangala, Bṛhaspati and Shani for the traditional planets Mercury, Venus, Mars, Jupiter and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. China and the countries of eastern Asia historically subject to Chinese cultural influence (such as Japan, Korea and Vietnam) use a naming system based on the five Chinese elements: water (Mercury), metal (Venus), fire (Mars), wood (Jupiter) and earth (Saturn).[68]

Formation

It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets.[72] After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of atmospheric drag.[73]

An artist's impression of protoplanetary disk
When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting–Robertson drag and other effects.[74][75] Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb.[76] Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Meanwhile, protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies.
The energetic impacts of the smaller planetesimals (as well as radioactive decay) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core.[77] Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets.[78] (Smaller planets will lose any atmosphere they gain through various escape mechanisms.)
With the discovery and observation of planetary systems around stars other than our own, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity – an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 (helium) – is now believed to determine the likelihood that a star will have planets.[79] Hence, it is thought that a metal-rich population I star will likely possess a more substantial planetary system than a metal-poor, population II star.

Solar System


Planets and dwarf planets of the Solar System (Sizes to scale, distances not to scale)

The inner planets. From left to right: Mercury, Venus, Earth and Mars in true-color. (Sizes to scale, distances not to scale)

The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune (Sizes to scale, distances not to scale)
According to the IAU, there are eight planets and five recognized dwarf planets in the Solar System. In increasing distance from the Sun, the planets are:
  1. ☿ Mercury
  2. ♀ Venus
  3.   ^  Earth
  4. ♂ Mars
  5. ♃ Jupiter
  6. ♄ Saturn
  7. ♅ Uranus
  8. ♆ Neptune
Jupiter is the largest, at 318 Earth masses, while Mercury is smallest, at 0.055 Earth masses.
The planets of the Solar System can be divided into categories based on their composition:
  • Terrestrials: Planets that are similar to Earth, with bodies largely composed of rock: Mercury, Venus, Earth and Mars. At 0.055 Earth masses, Mercury is the smallest terrestrial planet (and smallest planet) in the Solar System, while Earth is the largest terrestrial planet.
  • Gas giants (Jovians): Planets largely composed of gaseous material and significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Jupiter, at 318 Earth masses, is the largest planet in the Solar System, while Saturn is one third as big, at 95 Earth masses.
    • Ice giants, comprising Uranus and Neptune, are a sub-class of gas giants, distinguished from gas giants by their significantly lower mass (only 14 and 17 Earth masses), and by depletion in hydrogen and helium in their atmospheres together with a significantly higher proportion of rock and ice.
  • Dwarf planets: Before the August 2006 decision, several objects were proposed by astronomers, including at one stage by the IAU, as planets. However in 2006 several of these objects were reclassified as dwarf planets, objects distinct from planets. Currently five dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto, Haumea, Makemake and Eris. Several other objects in both the asteroid belt and the Kuiper belt are under consideration, with as many as 50 that could eventually qualify. There may be as many as 200 that could be discovered once the Kuiper belt has been fully explored. Dwarf planets share many of the same characteristics as planets, although notable differences remain – namely that they are not dominant in their orbits. By definition, all dwarf planets are members of larger populations. Ceres is the largest body in the asteroid belt, while Pluto, Haumea, and Makemake are members of the Kuiper belt and Eris is a member of the scattered disc. Scientists such as Mike Brown believe that there are probably over one hundred trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition.[80]

Planetary attributes

Type Name Equatorial
diameter[a]
Mass[a] Orbital
radius (AU)
Orbital period
(years)[a]
Inclination
to Sun's equator
(°)
Orbital
eccentricity
Rotation period
(days)
Confirmed
moons[c]
Rings Atmosphere
Terrestrial planet Mercury 0.382 0.06 0.31-0.47 0.24 3.38 0.206 58.64 0 no minimal
Terrestrial planetVenus 0.949 0.82 0.72 0.62 3.86 0.007 −243.02 0 no CO2, N2
Terrestrial planetEarth[b] 1.00 1.00 1.00 1.00 7.25 0.017 1.00 1 no N2, O2
Terrestrial planetMars 0.532 0.11 1.52 1.88 5.65 0.093 1.03 2 no CO2, N2
Gas giant Jupiter 11.209 317.8 5.20 11.86 6.09 0.048 0.41 66 yes H2, He
Gas giantSaturn 9.449 95.2 9.54 29.46 5.51 0.054 0.43 62 yes H2, He
Gas giantUranus 4.007 14.6 19.22 84.01 6.48 0.047 −0.72 27 yes H2, He
Gas giantNeptune 3.883 17.2 30.06 164.8 6.43 0.009 0.67 13 yes H2, He
Dwarf planet Ceres 0.08 0.000 2 2.5–3.0 4.60 10.59 0.080 0.38 0 no none
Dwarf planetPluto 0.18 0.002 2 29.7–49.3 248.09 17.14 0.249 −6.39 5 no temporary
Dwarf planetHaumea 0.15×0.12×0.08 0.000 7 35.2–51.5 282.76 28.19 0.189 0.16 2  ?  ?
Dwarf planetMakemake ~0.12 0.000 7 38.5–53.1 309.88 28.96 0.159  ? 0  ?  ? [d]
Dwarf planetEris 0.19 0.002 5 37.8–97.6 ~557 44.19 0.442 ~0.3 1  ?  ? [d]
a Measured relative to the Earth.
b See Earth article for absolute values.
c Jupiter has the most verified satellites (66) in the Solar System.[81]
d Like Pluto, when near perihelion, a temporary atmosphere is suspected.

Extrasolar planets


Exoplanets, by year of discovery, through 2011-07-10.
In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[42] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi. Of the 846 extrasolar planets discovered by November 8, 2012,[5] most have masses which are comparable to or larger than Jupiter's, though masses ranging from just below that of Mercury to many times Jupiter's mass have been observed.[5] The smallest extrasolar planets found to date have been discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12.[82]
There have been roughly a dozen extrasolar planets found of between 10 and 20 Earth masses,[5] such as those orbiting the stars Mu Arae, 55 Cancri and GJ 436.[83]
Another new category are the so-called "super-Earths", possibly terrestrial planets larger than Earth but smaller than Neptune or Uranus. To date, about twenty possible super-Earths (depending on mass limits) have been found, including OGLE-2005-BLG-390Lb and MOA-2007-BLG-192Lb, frigid icy worlds discovered through gravitational microlensing,[84][85] Kepler 10b, a planet with a diameter roughly 1.4 times that of Earth, (making it the smallest super-Earth yet measured)[86] and five of the six planets orbiting the nearby red dwarf Gliese 581. Gliese 581 d is roughly 7.7 times Earth's mass,[87] while Gliese 581 c is five times Earth's mass and was initially thought to be the first terrestrial planet found within a star's habitable zone.[88] However, more detailed studies revealed that it was slightly too close to its star to be habitable, and that the farther planet in the system, Gliese 581 d, though it is much colder than Earth, could potentially be habitable if its atmosphere contained sufficient amounts of greenhouse gases.[89] Another super-Earth, Kepler-22b, was later confirmed to be orbiting comfortably within the habitable zone of its star.[90] On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-size extrasolar planets, Kepler-20e[6] and Kepler-20f,[7] orbiting a Sun-like star, Kepler-20.[8][9][10]

Comparison of Kepler-20e[6] and Kepler-20f[7] with Venus and Earth.
It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all. Also, a class of hot Jupiters may exist called Chthonian planets, that orbit so close to their star that their atmospheres have been blown away completely by stellar radiation. While many hot Jupiters have been found in the process of losing their atmospheres, as of 2008, no genuine Chthonian planets have been discovered.[91]

Size comparison of HR 8799 c (gray) with Jupiter. Most exoplanets discovered thus far are larger than Jupiter.
More detailed observation of extrasolar planets will require a new generation of instruments, including space telescopes. Currently the COROT and Kepler spacecraft are searching for stellar luminosity variations due to transiting planets. Several projects have also been proposed to create an array of space telescopes to search for extrasolar planets with masses comparable to the Earth. These include the proposed NASA's, Terrestrial Planet Finder, and Space Interferometry Mission programs, and the CNES' PEGASE.[92] The New Worlds Mission is an occulting device that may work in conjunction with the James Webb Space Telescope. However, funding for some of these projects remains uncertain. The first spectra of extrasolar planets were reported in February 2007 (HD 209458 b and HD 189733 b).[93][94] The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy.[95]

Planetary-mass objects

A planetary-mass object, PMO, or planemo is a celestial object with a mass that falls within the range of the definition of a planet: massive enough to achieve hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain core fusion like a star.[96] By definition, all planets are planetary-mass objects, but the purpose of the term is to describe objects which do not conform to typical expectations for a planet. These include dwarf planets, the larger moons, free-floating planets not orbiting a star, such as rogue planets ejected from their system, and objects that formed through cloud-collapse rather than accretion (sometimes called sub-brown dwarfs).

Rogue planets

Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.[97] Some scientists have argued that such objects found roaming in deep space should be classed as "planets", although others have suggested that they could be low-mass stars.[98][99]

Sub-brown dwarfs

Stars form via the gravitational collapse of gas clouds, but smaller objects can also form via cloud-collapse. Planetary-mass objects formed this way are sometimes called sub-brown dwarfs. Sub-brown dwarfs may be free-floating such as Cha 110913-773444, or orbiting a larger object such as 2MASS J04414489+2301513.
For a brief time in 2006, astronomers believed they had found a binary system of such objects, Oph 162225-240515, which the discoverers described as "planemos", or "planetary-mass objects". However, recent analysis of the objects has determined that their masses are probably each greater than 13 Jupiter-masses, making the pair brown dwarfs.[100][101][102]

Former stars

In close binary star systems one of the stars can lose mass to a heavier companion. See accretion-powered pulsars. The shrinking star can then become a planetary-mass object. An example is a Jupiter-mass object orbiting the pulsar PSR J1719-1438.[103]

Satellite planets and belt planets

Some large satellites are of similar size or larger than the planet Mercury, e.g. Jupiter's Galilean moons and Titan. Alan Stern has argued that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet, and proposes the term satellite planet for a planet-sized satellite. Likewise, dwarf planets in the asteroid belt and Kuiper belt should be considered planets according to Stern.[104]

Attributes

Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whilst others are also common to extrasolar planets.

Dynamic characteristics

Orbit


The orbit of the planet Neptune compared to that of Pluto. Note the elongation of Pluto's orbit in relation to Neptune's (eccentricity), as well as its large angle to the ecliptic (inclination).
According to current definitions, all planets must revolve around stars; thus, any potential "rogue planets" are excluded. In the Solar System, all the planets orbit the Sun in the same direction as the Sun rotates (counter-clockwise as seen from above the Sun's north pole). At least one extrasolar planet, WASP-17b, has been found to orbit in the opposite direction to its star's rotation.[105] The period of one revolution of a planet's orbit is known as its sidereal period or year.[106] A planet's year depends on its distance from its star; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the star's gravity. Because no planet's orbit is perfectly circular, the distance of each varies over the course of its year. The closest approach to its star is called its periastron (perihelion in the Solar System), while its farthest separation from the star is called its apastron (aphelion). As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy, just as a falling object on Earth accelerates as it falls; as the planet reaches apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory.[107]
Each planet's orbit is delineated by a set of elements:
  • The eccentricity of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, while planets with high eccentricities have more elliptical orbits. The planets in the Solar System have very low eccentricities, and thus nearly circular orbits.[106] Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits.[108][109]

  • Illustration of the semi-major axis
    The semi-major axis is the distance from a planet to the half-way point along the longest diameter of its elliptical orbit (see image). This distance is not the same as its apastron, as no planet's orbit has its star at its exact centre.[106]
  • The inclination of a planet tells how far above or below an established reference plane its orbit lies. In the Solar System, the reference plane is the plane of Earth's orbit, called the ecliptic. For extrasolar planets, the plane, known as the sky plane or plane of the sky, is the plane of the observer's line of sight from Earth.[110] The eight planets of the Solar System all lie very close to the ecliptic; comets and Kuiper belt objects like Pluto are at far more extreme angles to it.[111] The points at which a planet crosses above and below its reference plane are called its ascending and descending nodes.[106] The longitude of the ascending node is the angle between the reference plane's 0 longitude and the planet's ascending node. The argument of periapsis (or perihelion in the Solar System) is the angle between a planet's ascending node and its closest approach to its star.[106]

Axial tilt


Earth's axial tilt is about 23°.
Planets also have varying degrees of axial tilt; they lie at an angle to the plane of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa. Each planet therefore possesses seasons; changes to the climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its solstice. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices.[112] Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to possess negligible to no axial tilt, as a result of their proximity to their stars.[113]

Rotation

The planets rotate around invisible axes through their centres. A planet's rotation period is known as a stellar day. Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the sun's north pole, the exceptions being Venus[114] and Uranus[115] which rotate clockwise, though Uranus's extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise.[116] However, regardless of which convention is used, Uranus has a retrograde rotation relative to its orbit.
The rotation of a planet can be induced by several factors during formation. A net angular momentum can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the gas giants can also contribute to the angular momentum. Finally, during the last stages of planet building, a stochastic process of protoplanetary accretion can randomly alter the spin axis of the planet.[117] There is great variation in the length of day between the planets, with Venus taking 243 Earth days to rotate, and the gas giants only a few hours.[118] The rotational periods of extrasolar planets are not known; however their proximity to their stars means that hot Jupiters are tidally locked (their orbits are in sync with their rotations). This means they only ever show one face to their stars, with one side in perpetual day, the other in perpetual night.[119]

Orbital clearing

The defining dynamic characteristic of a planet is that it has cleared its neighborhood. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the planetesimals in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. This characteristic was mandated as part of the IAU's official definition of a planet in August, 2006. This criterion excludes such planetary bodies as Pluto, Eris and Ceres from full-fledged planethood, making them instead dwarf planets.[1] Although to date this criterion only applies to the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs.[120]

Physical characteristics

Mass

A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of hydrostatic equilibrium. This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until the object collapses into a sphere.[121]
Mass is also the prime attribute by which planets are distinguished from stars. The upper mass limit for planethood is roughly 13 times Jupiter's mass for objects with solar-type isotopic abundance, beyond which it achieves conditions suitable for nuclear fusion. Other than the Sun, no objects of such mass exist in the Solar System; but there are exoplanets of this size. The 13MJ limit is not universally agreed upon and the Extrasolar Planets Encyclopaedia includes objects up to 20 Jupiter masses,[122] and the Exoplanet Data Explorer up to 24 Jupiter masses.[123]
The smallest known planet, excluding dwarf planets and satellites, is PSR B1257+12A, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. Its mass is roughly half that of the planet Mercury.[5] The smallest planet orbiting a main-sequence star other than the Sun is Kepler-20e, with a mass roughly similar to that of Venus.

Internal differentiation


Illustration of the interior of Jupiter, with a rocky core overlaid by a deep layer of metallic hydrogen
Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a differentiated interior consisting of a dense planetary core surrounded by a mantle which either is or was a fluid. The terrestrial planets are sealed within hard crusts,[124] but in the gas giants the mantle simply dissolves into the upper cloud layers. The terrestrial planets possess cores of magnetic elements such as iron and nickel, and mantles of silicates. Jupiter and Saturn are believed to possess cores of rock and metal surrounded by mantles of metallic hydrogen.[125] Uranus and Neptune, which are smaller, possess rocky cores surrounded by mantles of water, ammonia, methane and other ices.[126] The fluid action within these planets' cores creates a geodynamo that generates a magnetic field.[124]

Atmosphere


Earth's atmosphere
All of the Solar System planets except Mercury[127] have substantial atmospheres as their large masses mean gravity is strong enough to keep gases close to the surface. The larger gas giants are massive enough to keep large amounts of the light gases hydrogen and helium close by, while the smaller planets lose these gases into space.[128] The composition of the Earth's atmosphere is different from the other planets because the various life processes that have transpired on the planet have introduced free molecular oxygen.[129]
Planetary atmospheres are affected by the varying insolation or internal energy, leading to the formation of dynamic weather systems such as hurricanes, (on Earth), planet-wide dust storms (on Mars), an Earth-sized anticyclone on Jupiter (called the Great Red Spot), and holes in the atmosphere (on Neptune).[112] At least one extrasolar planet, HD 189733 b, has been claimed to possess such a weather system, similar to the Great Red Spot but twice as large.[130]
Hot Jupiters, due to their extreme proximities to their host stars, have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets.[131][132] These planets may have vast differences in temperature between their day and night sides which produce supersonic winds,[133] although the day and night sides of HD 189733 b appear to have very similar temperatures, indicating that that planet's atmosphere effectively redistributes the star's energy around the planet.[130]

Magnetosphere


Schematic of Earth's magnetosphere
One important characteristic of the planets is their intrinsic magnetic moments which in turn give rise to magnetospheres. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of electrically conducting material in their interiors, which generate their magnetic fields. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which cannot effectively protect the planet.[134]
Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field.[134] In addition, the moon of Jupiter Ganymede also has one. Of the magnetized planets the magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind. Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System (so strong in fact that it poses a serious health risk to future manned missions to its moons). The magnetic fields of the other giant planets are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative the rotational axis and displaced from the centre of the planet.[134]
In 2004, a team of astronomers in Hawaii observed an extrasolar planet around the star HD 179949, which appeared to be creating a sunspot on the surface of its parent star. The team hypothesised that the planet's magnetosphere was transferring energy onto the star's surface, increasing its already high 7,760 °C temperature by an additional 400 °C.[135]

Secondary characteristics

Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in resonance with each other or with smaller bodies (this is also common in satellite systems). All except Mercury and Venus have natural satellites, often called "moons". Earth has one, Mars has two, and the gas giants have numerous moons in complex planetary-type systems. Many gas giant moons have similar features to the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa).[136][137][138]
The four gas giants are also orbited by planetary rings of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny 'moonlets' whose gravity shapes and maintains their structure. Although the origins of planetary rings is not precisely known, they are believed to be the result of natural satellites that fell below their parent planet's Roche limit and were torn apart by tidal forces.[139][140]
No secondary characteristics have been observed around extrasolar planets. However the sub-brown dwarf Cha 110913-773444, which has been described as a rogue planet, is believed to be orbited by a tiny protoplanetary disc.[9
                                                        astronauts


Astronaut Bruce McCandless II using a Manned Maneuvering Unit outside the United States Space Shuttle Challenger in 1984.
An astronaut or cosmonaut is a person trained by a human spaceflight program to command, pilot, or serve as a crew member of a spacecraft. While generally reserved for professional space travelers, the terms are sometimes applied to anyone who travels into space, including scientists, politicians, journalists, and tourists.[1][2]
Until 2002, astronauts were sponsored and trained exclusively by governments, either by the military, or by civilian space agencies. With the sub-orbital flight of the privately funded SpaceShipOne in 2004, a new category of astronaut was created: the commercial astronaut.

Definition

The criteria for what constitutes human spaceflight vary. The Fédération Aéronautique Internationale (FAI) Sporting Code for astronautics recognizes only flights that exceed an altitude of 100 kilometers (62 mi).[3] In the United States, professional, military, and commercial astronauts who travel above an altitude of 50 miles (80 km)[4] are awarded astronaut wings.
As of June 20, 2011, a total of 654 people from 38 countries[5] have reached 100 km (62 mi) or more in altitude, of which 520 reached low Earth orbit or beyond.[6][7] Of these, 24 people have traveled beyond Low Earth orbit, to either lunar or trans-lunar orbit or to the surface of the moon; three of the 24 did so twice: Jim Lovell, John Young and Eugene Cernan.[8] The three astronauts who have not reached low Earth orbit are spaceplane pilots Joe Walker, Mike Melvill, and Brian Binnie.
Under the U.S. definition, as of June 20, 2011, 529 people qualify as having reached space, above 50 miles (80 km) altitude. Of eight X-15 pilots who exceeded 50 miles (80 km) in altitude, only one exceeded 100 kilometers (about 62 miles).[9] Space travelers have spent over 30,400 man-days (83 man-years) in space, including over 100 astronaut-days of spacewalks.[9][10] As of 2008, the man with the longest cumulative time in space is Sergei K. Krikalev, who has spent 803 days, 9 hours and 39 minutes, or 2.2 years, in space.[11][12] Peggy A. Whitson holds the record for the most time in space by a woman, 377 days.[13]

Terminology

English

In the United States, Canada, Ireland, the United Kingdom, and many other English-speaking nations, a professional space traveler is called an astronaut.[14] The term derives from the Greek words ástron (ἄστρον), meaning "star", and nautes (ναύτης), meaning "sailor". The first known use of the term "astronaut" in the modern sense was by Neil R. Jones in his short story "The Death's Head Meteor" in 1930. The word itself had been known earlier. For example, in Percy Greg's 1880 book Across the Zodiac, "astronaut" referred to a spacecraft. In Les Navigateurs de l'Infini (1925) of J.-H. Rosny aîné, the word astronautique (astronautic) was used. The word may have been inspired by "aeronaut", an older term for an air traveler first applied (in 1784) to balloonists. An early use in a non-fiction publication is Eric Frank Russell's poem "The Astronaut" in the November 1934 Bulletin of the British Interplanetary Society.[15]
The first known formal use of the term astronautics in the scientific community was the establishment of the annual International Astronautical Congress in 1950 and the subsequent founding of the International Astronautical Federation the following year.[16]
NASA applies the term astronaut to any crew member aboard NASA spacecraft bound for Earth orbit or beyond. NASA also uses the term as a title for those selected to join its Astronaut Corps.[17] The European Space Agency similarly uses the term astronaut for members of its Astronaut Corps.[18]

Russian

By convention, an astronaut employed by the Russian Federal Space Agency (or its Soviet predecessor) is called a cosmonaut in English texts.[17] The word is an anglicisation of the Russian word kosmonavt (Russian: космонавт Russian pronunciation: [kəsmɐˈnaft]), one who works in space outside the Earth's atmosphere, a space traveler,[19] which derives from the Greek words kosmos (κόσμος), meaning "universe", and nautes (ναύτης), meaning "sailor".
The Soviet Air Force pilot Yuri Gagarin was the first cosmonaut—indeed the first person—in space. Valentina Tereshkova, a Russian factory worker, was the first female in space, as well as arguably the first civilian to make it there (see below for a further discussion of civilians in space). On March 14, 1995, Norman Thagard became the first American to ride to space on board a Russian launch vehicle, and thus became the first "American cosmonaut".

Chinese

Official English-language texts issued by the government of the People's Republic of China use astronaut while texts in Russian use космонавт (kosmonavt).[20][21] In official Chinese-language texts, the terms "yǔhángyuán" (宇航员, "sailing personnel in universe") for cosmonaut and "hángtiānyuán" (航天员, "sailing personnel in sky") for astronaut have long been used. The phrase "tàikōng rén" (太空人, "spaceman") is often used in Taiwan and Hong Kong.
The term taikonaut is used by some English-language news media organizations for professional space travelers from China.[22] The word has featured in the Longman and Oxford English dictionaries, the latter of which describes it as "a hybrid of the Chinese term taikong (space) and the Greek naut (sailor)"; the term became more common in 2003 when China sent its first astronaut Yang Liwei into space aboard the Shenzhou 5 spacecraft.[23] This is the term used by Xinhua in the English version of the Chinese People's Daily since the advent of the Chinese space program.[24] The origin of the term is unclear; as early as May 1998, Chiew Lee Yih (趙裡昱) from Malaysia, used it in newsgroups.[25][26]

Other terms

With the rise of space tourism, NASA and the Russian Federal Space Agency agreed to use the term "spaceflight participant" to distinguish those space travelers from professional astronauts on missions coordinated by those two agencies.
While no nation other than the Russian Federation (and previously the former Soviet Union), the United States, and China have launched a manned spacecraft, several other nations have sent people into space in cooperation with one of these countries. Inspired partly by these missions, other synonyms for astronaut have entered occasional English usage. For example, the term spationaut (French spelling: spationaute) is sometimes used to describe French space travelers, from the Latin word spatium or "space", and the Malay or Korean term angkasawan or gong-gan was used to describe participants in the Angkasawan program. In Hungarian the word describing astronauts is űrhajós (from űr meaning "space" and hajós meaning "sailor".)

Space travel milestones


Yuri Gagarin, first human in space (1961)

Valentina Tereshkova, 1963 first woman in space.

Neil Armstrong, first person to walk on the moon (1969).
The first human in space was Soviet Yuri Gagarin, who was launched on April 12, 1961 aboard Vostok 1 and orbited around the Earth for 108 minutes. The first woman in space was Soviet Valentina Tereshkova, who launched on June 16, 1963 aboard Vostok 6 and orbited Earth for almost three days.
Alan Shepard became the first American and second person in space on May 5, 1961 on a 15-minute sub-orbital flight. The first American woman in space was Sally Ride, during Space Shuttle Challenger's mission STS-7, on June 18, 1983.[27] In 1992 Mae Jemison became the first African American woman to travel in space aboard STS-47.
The first manned mission to orbit the moon, Apollo 8, included American William Anders who was born in Hong Kong, making him the first Asian-born astronaut in 1968. In April 1985, Taylor Wang became the first ethnic Chinese person in space.[28][29] On 15 October 2003, Yang Liwei became China's first astronaut on the Shenzhou 5 spacecraft.
The Soviet Union, through its Intercosmos program, allowed people from other "socialist" (i.e. Warsaw Pact and other Soviet-allied) countries to fly on its missions. An example is Czechoslovak Vladimír Remek, the first cosmonaut from a country other than the Soviet Union or the United States, who flew to space in 1978 on a Soyuz-U rocket.[30] On July 23, 1980, Pham Tuan of Vietnam became the first Asian in space when he flew aboard Soyuz 37.[31]
Also in 1980, Cuban Arnaldo Tamayo Méndez became the first person of Hispanic and black African descent to fly in space, and Guion Bluford became the first African American to fly into space. The first person born in Africa to fly in space was Patrick Baudry, in 1985.[32][33] In 1985, Saudi Arabian Prince Sultan Bin Salman Bin AbdulAziz Al-Saud became the first Arab Muslim astronaut in space.[34] In 1988, Abdul Ahad Mohmand became the first Afghan to reach space, spending nine days aboard the Mir space station.[35]
With the larger number of seats available on the Space Shuttle, the U.S. began taking international astronauts. In 1983, Ulf Merbold of West Germany became the first non-US citizen to fly in a US spacecraft. In 1984, Marc Garneau became the first of 8 Canadian astronauts to fly in space (through 2010).[36] In 1985, Rodolfo Neri Vela became the first Mexican-born person in space.[37] In 1991, Helen Sharman became the first Briton to fly in space.[38] In 2002, Mark Shuttleworth became the first citizen of an African country to fly in space, as a paying spaceflight participant.[39] In 2003, Ilan Ramon became the first Israeli to fly in space, although he died during a re-entry accident.

Age milestones

The youngest person to fly in space is Gherman Titov, who was 25 years old when he flew Vostok 2. (Titov was also the first person to suffer space sickness).[40][41] The oldest person who has flown in space is John Glenn, who was 77 when he flew on STS-95.[42]

Duration and distance milestones

The longest stay in space thus far has been 438 days, by Russian Valeri Polyakov.[9] As of 2006, the most spaceflights by an individual astronaut is seven, a record held by both Jerry L. Ross and Franklin Chang-Diaz. The farthest distance from Earth an astronaut has traveled was 401,056 km (249,205 mi), when Jim Lovell, Jack Swigert, and Fred Haise went around the Moon during the Apollo 13 emergency.[9]

Civilian and non-government milestones

Depending on the exact definition of 'civilian', the first civilian in space was either Valentina Tereshkova[43] aboard Vostok 6 (she also became the first woman in space on that mission) or Joseph Albert Walker[44][45] on X-15 Flight 90 a month later. Tereshkova was only honorarily inducted into the USSR's Air Force, which had no female pilots whatsoever at that time. Joe Walker had joined the US Army Air Force but was not a member during his flight. The first people in space who had never been a member of any country's armed forces were both Konstantin Feoktistov and Boris Yegorov aboard Voskhod 1.
The first non-governmental space traveler was Byron K. Lichtenberg, a researcher from the Massachusetts Institute of Technology who flew on STS-9 in 1983.[46] In December 1990, Toyohiro Akiyama became the first paying space traveler as a reporter for Tokyo Broadcasting System, a visit to Mir as part of an estimated $12 million (USD) deal with a Japanese TV station, although at the time, the term used to refer to Akiyama was "Research Cosmonaut".[47][48][49] Akiyama suffered severe space sickness during his mission, which affected his productivity.[48]
The first self-funded space tourist was Dennis Tito on board the Russian spacecraft Soyuz TM-3 on 28 April 2001.

Self-funded travelers

The first person to fly on an entirely privately funded mission was Mike Melvill, piloting SpaceShipOne flight 15P on a suborbital journey, although he was a test pilot employed by Scaled Composites and not an actual paying space tourist.[50][51] Seven others have paid to fly into space:
  1. Dennis Tito (American): April 28 – May 6, 2001 (ISS)
  2. Mark Shuttleworth (South African): April 25 – May 5, 2002 (ISS)
  3. Gregory Olsen (American): October 1–11, 2005 (ISS)
  4. Anousheh Ansari (Iranian / American): September 18–29, 2006 (ISS)
  5. Charles Simonyi (Hungarian / American): April 7–21, 2007 (ISS), March 26 – April 8, 2009 (ISS)
  6. Richard Garriott (American): October 12–24, 2008 (ISS)
  7. Guy Laliberté (Canadian): September 30, 2009 – October 11, 2009 (ISS)

Training

The first NASA astronauts were selected for training in 1959.[52] Early in the space program, military jet test piloting and engineering training were often cited as prerequisites for selection as an astronaut at NASA, although neither John Glenn nor Scott Carpenter (of the Mercury Seven) had any university degree, in engineering or any other discipline at the time of their selection. Selection was initially limited to military pilots.[53][54] The earliest astronauts for both America and the USSR tended to be jet fighter pilots, and were often test pilots.
Once selected, NASA astronauts go through twenty months of training in a variety of areas, including training for extra-vehicular activity in a facility such as NASA's Neutral Buoyancy Laboratory.[1][53] Astronauts-in-training may also experience short periods of weightlessness in aircraft called the "vomit comet", the nickname given to a pair of modified KC-135s (retired in 2000 and 2004 respectively, and replaced in 2005 with a C-9) which perform parabolic flights.[52] Astronauts are also required to accumulate a number of flight hours in high-performance jet aircraft. This is mostly done in T-38 jet aircraft out of Ellington Field, due to its proximity to the Johnson Space Center. Ellington Field is also where the Shuttle Training Aircraft is maintained and developed, although most flights of the aircraft are done out of Edwards Air Force Base.

NASA candidacy requirements

  • Be citizens of the United States.[52][55]
  • Pass a strict physical examination, and have a near and distant visual acuity correctable to 20/20 (6/6). Blood pressure, while sitting, must be no greater than 140 over 90.

Commander and Pilot

  • A bachelor's degree in engineering, biological science, physical science or mathematics is required.
  • At least 1,000 hours' flying time as pilot-in-command in jet aircraft. Experience as a test pilot is desirable.
  • Height must be 5 ft 2 in to 6 ft 2 in (1.58 to 1.88 m).
  • Distant visual acuity must be correctable to 20/20 in each eye.
  • The refractive surgical procedures of the eye, PRK (Photorefractive keratectomy) and LASIK, are now allowed, providing at least 1 year has passed since the date of the procedure with no permanent adverse aftereffects. For those applicants under final consideration, an operative report on the surgical procedure will be requested.

Mission Specialist

  • A bachelor's degree in engineering, biological science, physical science or mathematics, as well as at least three years of related professional experience (graduate work or studies) and an advanced degree, such as a master's degree (one to three years) or a doctoral degree (three years or more).
  • Applicant's height must be between 58.5 and 76 inches

Mission Specialist Educator


Mission Specialist Educators Lindenberger, Arnold, and Acaba during a parabolic flight.
  • Applicants must have a bachelor's degree with teaching experience, including work at the kindergarten through twelfth grade level. An advanced degree, such as a master's degree or a doctoral degree, is not required, but is strongly desired.[56]
Mission Specialist Educators, or "Educator Astronauts", were first selected in 2004, and as of 2007, there are three NASA Educator astronauts: Joseph M. Acaba, Richard R. Arnold, and Dorothy Metcalf-Lindenburger.[57][58] Barbara Morgan, selected as back-up teacher to Christa McAuliffe in 1985, is considered to be the first Educator astronaut by the media, but she trained as a mission specialist.[59] The Educator Astronaut program is a successor to the Teacher in Space program from the 1980s.[60][61]

Health risks of space travel

Astronauts are susceptible to a variety of health risks including decompression sickness, barotrauma, immunodeficiencies, loss of bone and muscle, orthostatic intolerance due to volume loss, sleep disturbances, and radiation injury. A variety of large scale medical studies are being conducted in space via the National Space and Biomedical Research Institute (NSBRI) to address these issues. Prominent among these is the Advanced Diagnostic Ultrasound in Microgravity Study in which astronauts (including former ISS commanders Leroy Chiao and Gennady Padalka) perform ultrasound scans under the guidance of remote experts to diagnose and potentially treat hundreds of medical conditions in space. This study's techniques are now being applied to cover professional and Olympic sports injuries as well as ultrasound performed by non-expert operators in medical and high school students. It is anticipated that remote guided ultrasound will have application on Earth in emergency and rural care situations, where access to a trained physician is often rare.[62][63][64]

Insignia

In Russia, cosmonauts are awarded Pilot-Cosmonaut of the Russian Federation upon completion of their missions, often accompanied with the award of Hero of the Russian Federation. This follows the practice established in the Soviet Union.
At NASA, those who complete astronaut candidate training receive a silver lapel pin. Once they have flown in space, they receive a gold pin. U.S. astronauts who also have active-duty military status receive a special qualification badge, known as the Astronaut Badge, after participation on a spaceflight. The United States Air Force also presents an Astronaut Badge to its pilots who exceed 50 miles (80 km) in altitude.

Space Mirror Memorial
                                                                  earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the world, the Blue Planet,[21] or by its Latin name, Terra.[note 6]
Earth formed approximately 4.54 billion years ago, and life appeared on its surface within one billion years.[22] Earth's biosphere then significantly altered the atmospheric and other basic physical conditions, which enabled the proliferation of organisms as well as the formation of the ozone layer, which together with Earth's magnetic field blocked harmful solar radiation, and permitted formerly ocean-confined life to move safely to land.[23] The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist. Estimates on how much longer the planet will to be able to continue to support life range from 500 million years (myr), to as long as 2.3 billion years (byr).[24][25][26]
Earth's crust is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. About 71% of the surface is covered by salt water oceans, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with ice that is the solid ice of the Antarctic ice sheet and the sea ice that is the Polar ice packs. The planet's interior remains active, with a solid iron inner core, a liquid outer core that generates the magnetic field, and a thick layer of relatively solid mantle.
Earth interacts with other objects in space, especially the Sun and the Moon. During one orbit around the sun, the Earth rotates about its own axis 366.26 times, creating 365.26 solar days, or one sidereal year.[note 7] The Earth's axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days).[27] The Moon is Earth's only natural satellite. It began orbiting the Earth about 4.53 billion years ago (bya). The Moon's gravitational interaction with Earth stimulates ocean tides, stabilizes the axial tilt, and gradually slows the planet's rotation.
The planet is home to millions of species, including humans.[28] Both the mineral resources of the planet and the products of the biosphere contribute resources that are used to support a global human population.[29] These inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade, and military action. Human cultures have developed many views of the planet, including its personification as a planetary deity, its shape as flat, its position as the center of the universe, and in the modern Gaia Principle, as a single, self-regulating organism in its own right.

Name and etymology

The modern English noun earth developed from Middle English erthe (recorded in 1137), itself from Old English eorthe (dating from before 725), ultimately deriving from Proto-Germanic *erthō. Earth has cognates in all other Germanic languages, including Dutch aarde, German Erde, and Swedish, Norwegian, and Danish jord.[30] The Earth is personified as a goddess in Germanic paganism (appearing as Jörð in Norse mythology, mother of the god Thor).[31]
In general English usage, the name earth can be capitalized or spelled in lowercase interchangeably, either when used absolutely or prefixed with "the" (i.e. "Earth", "the Earth", "earth", or "the earth"). Many deliberately spell the name of the planet with a capital, both as "Earth" or "the Earth". This is to distinguish it as a proper noun, distinct from the senses of the term as a count noun or verb (e.g. referring to soil, the ground, earthing in the electrical sense, etc.). Oxford spelling recognizes the lowercase form as the most common, with the capitalized form as a variant of it. Another convention that is very common is to spell the name with a capital when occurring absolutely (e.g. Earth's atmosphere) and lowercase when preceded by "the" (e.g. the atmosphere of the earth). The term almost exclusively exists in lowercase when appearing in common phrases, even without "the" preceding it (e.g. "It does not cost the earth.", "What on earth are you doing?").[32]

Chronology

Formation

The earliest material found in the Solar System is dated to 4.5672±0.0006 bya;[33] therefore, it is inferred that the Earth must have been forming by accretion around this time. By 4.54±0.04 bya[22] the primordial Earth had formed. The formation and evolution of the Solar System bodies occurred in tandem with the Sun. In theory a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, and then the planets grow out of that in tandem with the star. A nebula contains gas, ice grains and dust (including primordial nuclides). In nebular theory planetesimals commence forming as particulate accrues by cohesive clumping and then by gravity. The assembly of the primordial Earth proceeded for 10–20 myr.[34] The Moon formed shortly thereafter, about 4.53 bya.[35]
The Moon's formation remains a mystery. The working hypothesis is that it formed by accretion from material loosed from the Earth after a Mars-sized object, dubbed Theia, had a giant impact with Earth,[36] but the model is not self-consistent. In this scenario the mass of Theia is 10% of the Earth's mass,[37] it impacts with the Earth in a glancing blow,[38] and some of its mass merges with the Earth. Between approximately 3.8 and 4.1 bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon, and by inference, to the Earth.
Earth's atmosphere and oceans formed by volcanic activity and outgassing that included water vapor. The origin of the world's oceans was condensation augmented by water and ice delivered by asteroids, proto-planets, and comets.[39] In this model, atmospheric "greenhouse gases" kept the oceans from freezing while the newly forming Sun was only at 70% luminosity.[40] By 3.5 bya, the Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.[41]
A crust formed when the molten outer layer of the planet Earth cooled to form a solid as the accumulated water vapor began to act in the atmosphere. The two models[42] that explain land mass propose either a steady growth to the present-day forms[43] or, more likely, a rapid growth[44] early in Earth history[45] followed by a long-term steady continental area.[46][47][48] Continents formed by plate tectonics, a process ultimately driven by the continuous loss of heat from the earth's interior. On time scales lasting hundreds of millions of years, the supercontinents have formed and broken up three times. Roughly 750 mya (million years ago), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which also broke apart 180 mya.[49]

Evolution of life

Highly energetic chemistry is believed to have produced a self-replicating molecule around bya and half a billion years later the last common ancestor of all life existed.[50] The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and formed a layer of ozone (a form of molecular oxygen [O3]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[51] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.[52]
Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.[53]
Following the Cambrian explosion, about 535 mya, there have been five major mass extinctions.[54] The most recent such event was 65 mya, when an asteroid impact triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared some small animals such as mammals, which then resembled shrews. Over the past 65 myr, mammalian life has diversified, and several million years ago an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright.[55] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which allowed the evolution of the human race. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,[56] affecting both the nature and quantity of other life forms.
The present pattern of ice ages began about 40 mya and then intensified during the Pleistocene about 3 mya. High-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last continental glaciation ended 10,000 years ago.[57]

Future

14 billion year timeline showing Sun's present age at 4.6 byr; from 6 byr Sun gradually warming, becoming a red dwarf at 10 byr, "soon" followed by its transformation into a white dwarf star
The life cycle of the Sun
The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium at the Sun's core, the star's total luminosity will slowly increase. The luminosity of the Sun will grow by 10% over the next 1.1 byr and by 40% over the next 3.5 byr.[58] Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the loss of the planet's oceans.[59]
The Earth's increasing surface temperature will accelerate the inorganic CO2 cycle, reducing its concentration to levels lethally low for plants (10 ppm) for C4 photosynthesis) in approximately 500-900 myr.[24] The lack of vegetation will result in the loss of oxygen in the atmosphere, so animal life will become extinct within several million more years.[60] After another billion years all surface water will have disappeared[25] and the mean global temperature will reach 70 °C[60] (158 °F). The Earth is expected to be effectively habitable for about another 500 myr from that point,[24] although this may be extended up to 2.3 byr if the nitrogen is removed from the atmosphere.[26] Even if the Sun were eternal and stable, 27% of the water in the modern oceans will descend to the mantle in one billion years due to reduced steam venting from mid-ocean ridges.[61]
The Sun, as part of its evolution, will become a red giant in about 5 byr. Models predict that the Sun will expand out to about 250 times its present radius, roughly 1 AU (150,000,000 km).[58][62] Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, the Earth will move to an orbit 1.7 AU (250,000,000 km) from the Sun when the star reaches it maximum radius. The planet was therefore initially expected to escape envelopment by the expanded Sun's sparse outer atmosphere, though most, if not all, remaining life would have been destroyed by the Sun's increased luminosity (peaking at about 5000 times its present level).[58] A 2008 simulation indicates that Earth's orbit will decay due to tidal effects and drag, causing it to enter the red giant Sun's atmosphere and be vaporized.[62] After that, the Sun's core will collapse into a white dwarf, as its outer layers are ejected into space as a planetary nebula. The matter that once made up the Earth will be released into interstellar space, where it will one day become incorporated into a new generation of planets and other celestial bodies.

Composition and structure


Size comparison of inner planets (left to right): Mercury, Venus, Earth and Mars in true-color.
Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant like Jupiter. It is the largest of the four solar terrestrial planets in size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity, the strongest magnetic field, and fastest rotation,[63] and is probably the only one with active plate tectonics.[64]

Shape


Chimborazo, Ecuador. The furthermost point on the Earth's surface from its center.[65]
The shape of the Earth approximates an oblate spheroid, a sphere flattened along the axis from pole to pole such that there is a bulge around the equator.[66] This bulge results from the rotation of the Earth, and causes the diameter at the equator to be 43 km (kilometer) larger than the pole-to-pole diameter.[67] For this reason the furthest point on the surface from the Earth's center of mass is the Chimborazo volcano in Ecuador.[68] The average diameter of the reference spheroid is about 12,742 km, which is approximately 40,000 km/π, as the meter was originally defined as 1/10,000,000 of the distance from the equator to the North Pole through Paris, France.[69]
Local topography deviates from this idealized spheroid, although on a global scale, these deviations are small: Earth has a tolerance of about one part in about 584, or 0.17%, from the reference spheroid, which is less than the 0.22% tolerance allowed in billiard balls.[70] The largest local deviations in the rocky surface of the Earth are Mount Everest (8848 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Because of the equatorial bulge, the surface locations farthest from the center of the Earth are the summits of Mount Chimborazo in Ecuador and Huascarán in Peru.[71][72][73]
Chemical composition of the crust[74]
Compound Formula Composition
Continental Oceanic
silica SiO2 60.2% 48.6%
alumina Al2O3 15.2% 16.5%
lime CaO 5.5% 12.3%
magnesia MgO 3.1% 6.8%
iron(II) oxide FeO 3.8% 6.2%
sodium oxide Na2O 3.0% 2.6%
potassium oxide K2O 2.8% 0.4%
iron(III) oxide Fe2O3 2.5% 2.3%
water H2O 1.4% 1.1%
carbon dioxide CO2 1.2% 1.4%
titanium dioxide TiO2 0.7% 1.4%
phosphorus pentoxide P2O5 0.2% 0.3%
Total 99.6% 99.9%

Chemical composition

The mass of the Earth is approximately 5.98×1024 kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.[75]
The geochemist F. W. Clarke calculated that a little more than 47% of the Earth's crust consists of oxygen. The more common rock constituents of the Earth's crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right), with the other constituents occurring in minute quantities.[76]

Internal structure

The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties, but unlike the other terrestrial planets, it has a distinct outer and inner core. The outer layer of the Earth is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging km (kilometers) under the oceans and 30-50 km on the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, and it is of the lithosphere that the tectonic plates are comprised. Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 km below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core.[77] The inner core may rotate at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year.[78]
Geologic layers of the Earth[79]
Earth-crust-cutaway-english.svg

Earth cutaway from core to exosphere. Not to scale.
Depth[80]
km
Component Layer Density
g/cm3
0–60 Lithosphere[note 8]
0–35 Crust[note 9] 2.2–2.9
35–60 Upper mantle 3.4–4.4
  35–2890 Mantle 3.4–5.6
100–700 Asthenosphere
2890–5100 Outer core 9.9–12.2
5100–6378 Inner core 12.8–13.1

Heat

Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).[81] The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.[82] At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa.[83] Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher. This extra heat production, twice present-day at approximately byr,[81] would have increased temperature gradients within the Earth, increasing the rates of mantle convection and plate tectonics, and allowing the production of igneous rocks such as komatiites that are not formed today.[84]
Present-day major heat-producing isotopes[85]
Isotope Heat release
W/kg isotope
Half-life

years
Mean mantle concentration
kg isotope/kg mantle
Heat release
W/kg mantle
238U 9.46 × 10−5 4.47 × 109 30.8 × 10−9 2.91 × 10−12
235U 5.69 × 10−4 7.04 × 108 0.22 × 10−9 1.25 × 10−13
232Th 2.64 × 10−5 1.40 × 1010 124 × 10−9 3.27 × 10−12
40K 2.92 × 10−5 1.25 × 109 36.9 × 10−9 1.08 × 10−12
The mean heat loss from the Earth is 87 mW m−2, for a global heat loss of 4.42 × 1013 W.[86] A portion of the core's thermal energy is transported toward the crust by mantle plumes; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce hotspots and flood basalts.[87] More of the heat in the Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere, the majority of which occurs in the oceans because the crust there is much thinner than that of the continents.[88]

Tectonic plates

Earth's main plates[89]
Shows the extent and boundaries of tectonic plates, with superimposed outlines of the continents they support
Plate name Area
106 km2
103.3
78.0
75.9
67.8
60.9
47.2
43.6
The mechanically rigid outer layer of the Earth, the lithosphere, is broken into pieces called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: Convergent boundaries, at which two plates come together, Divergent boundaries, at which two plates are pulled apart, and Transform boundaries, in which two plates slide past one another laterally. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries.[90] The tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper mantle that can flow and move along with the plates,[91] and their motion is strongly coupled with convection patterns inside the Earth's mantle.
As the tectonic plates migrate across the planet, the ocean floor is subducted under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. The combination of these processes continually recycles the oceanic crust back into the mantle. Because of this recycling, most of the ocean floor is less than 100 myr old in age. The oldest oceanic crust is located in the Western Pacific, and has an estimated age of about 200 myr.[92][93] By comparison, the oldest dated continental crust is 4,030 myr.[94]
The seven major plates are the Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American. Other notable plates include the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate fused with the Indian Plate between 50 and 55 mya. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/year[95] and the Pacific Plate moving 52–69 mm/year. At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/year.[96]

Surface

The Earth's terrain varies greatly from place to place. About 70.8%[97] of the surface is covered by water, with much of the continental shelf below sea level. This equates to 148.94 million km2 (57.51 million sq mi).[98] The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,[67] oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.
The planetary surface undergoes reshaping over geological time periods because of tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts[99] also act to reshape the landscape.
The continental crust consists of lower density material such as the igneous rocks granite and andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors.[100] Sedimentary rock is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust.[101] The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on the Earth's surface include quartz, the feldspars, amphibole, mica, pyroxene and olivine.[102] Common carbonate minerals include calcite (found in limestone) and dolomite.[103]
The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.[14] Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3×107 km2 of cropland and 3.4×107 km2 of pastureland.[104]
The elevation of the land surface of the Earth varies from the low point of −418 m at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m at the top of Mount Everest. The mean height of land above sea level is 840 m.[105]

Hydrosphere


Elevation histogram of the surface of the Earth
The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from others in the Solar System. The Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of −10,911.4 m.[note 11][106]
The mass of the oceans is approximately 1.35×1018 metric tons, or about 1/4400 of the total mass of the Earth. The oceans cover an area of 3.618×108 km2 with a mean depth of 3,682 m, resulting in an estimated volume of 1.332×109 km3.[107] If all the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km.[note 12] About 97.5% of the water is saline, while the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is currently ice.[108]
The average salinity of the Earth's oceans is about 35 grams of salt per kilogram of sea water (35  salt).[109] Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.[110] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[111] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[112] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.[113]

Atmosphere

The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km.[3] It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The height of the troposphere varies with latitude, ranging between 8 km at the poles to 17 km at the equator, with some variation resulting from weather and seasonal factors.[114]
Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 bya, forming the primarily nitrogen-oxygen atmosphere of today. This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer which blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life on Earth include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.[115] This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Water vapor, carbon dioxide, methane and ozone are the primary greenhouse gases in the Earth's atmosphere. Without this heat-retention effect, the average surface would be −18 °C, in contrast to the current +15 °C, and life would likely not exist.[97]

Weather and climate

The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km of the planet's surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower density air then rises, and is replaced by cooler, higher density air. The result is atmospheric circulation that drives the weather and climate through redistribution of heat energy.[116]
The primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°.[117] Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes heat energy from the equatorial oceans to the polar regions.[118]
Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and settles to the surface as precipitation.[116] Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or deposited into lakes. This water cycle is a vital mechanism for supporting life on land, and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topological features and temperature differences determine the average precipitation that falls in each region.[119]
The amount of solar energy reaching the Earth's decreases with increasing latitude. At higher latitudes the sunlight reaches the surface at a lower angles and it must pass through thicker columns of the atmosphere. As a result, the mean annual air temperature at sea level decreases by about 0.4 °C per per degree of latitude away from the equator.[120] The Earth can be sub-divided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), subtropical, temperate and polar climates.[121] Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. The commonly used Köppen climate classification system (as modified by Wladimir Köppen's student Rudolph Geiger) has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes.[117]

Upper atmosphere


This view from orbit shows the full Moon partially obscured and deformed by the Earth's atmosphere. NASA image
Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere.[115] Each layer has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere, where the Earth's magnetic fields interact with the solar wind.[122] Within the stratosphere is the ozone layer, a component that partially shields the surface from ultraviolet light and thus is important for life on Earth. The Kármán line, defined as 100 km above the Earth's surface, is a working definition for the boundary between atmosphere and space.[123]
Thermal energy causes some of the molecules at the outer edge of the Earth's atmosphere to increase their velocity to the point where they can escape from the planet's gravity.. This results in a slow but steady leakage of the atmosphere into space. Because unfixed hydrogen has a low molecular weight, it can achieve escape velocity more readily and it leaks into outer space at a greater rate than other gasses.[124] The leakage of hydrogen into space contributes to the pushing of the Earth from an initially reducing state to its current oxidizing one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as hydrogen is believed to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.[125] Hence the ability of hydrogen to escape from the Earth's atmosphere may have influenced the nature of life that developed on the planet.[126] In the current, oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of methane in the upper atmosphere.[127]

Magnetic field

Diagram showing the magnetic field lines of the Earth's magnetosphere. The lines are swept back in the anti-solar direction under the influence of the solar wind.
Schematic of Earth's magnetosphere. The solar wind flows from left to right
The Earth's magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet's geographic poles. At the equator of the magnetic field, the magnetic field strength at the planet's surface is 3.05 × 10−5 T, with global magnetic dipole moment of 7.91 × 1015 T m3.[128] According to dynamo theory, the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth's magnetic field. The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This results in field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.[129][130]
The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth's atmosphere at the magnetic poles, it forms the aurora.[131]

Orbit and rotation

Rotation


Earth's axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit
Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025 SI seconds).[132] As the Earth's solar day is now slightly longer than it was during the 19th century because of tidal acceleration, each day varies between 0 and 2 SI ms longer.[133][134]
Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86164.098903691 seconds of mean solar time (UT1), or 23h 56m 4.098903691s.[2][note 13] Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is 86164.09053083288 seconds of mean solar time (UT1) (23h 56m 4.09053083288s).[2] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.[135] The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005[136] and 1962–2005.[137]
Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in the Earth's sky is to the west at a rate of 15°/h = 15'/min. For bodies near the celestial equator, this is equivalent to an apparent diameter of the Sun or Moon every two minutes; from the planet's surface, the apparent sizes of the Sun and the Moon are approximately the same.[138][139]

Orbit

Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days, or one sidereal year. From Earth, this gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, or a Sun or Moon diameter, every 12 hours. Because of this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of the Earth averages about 29.8 km/s (107,000 km/h), which is fast enough to cover the planet's diameter (about 12,600 km) in seven minutes, and the distance to the Moon (384,000 km) in four hours.[3]
The Moon revolves with the Earth around a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system's common revolution around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial north pole, the motion of Earth, the Moon and their axial rotations are all counter-clockwise. Viewed from a vantage point above the north poles of both the Sun and the Earth, the Earth appears to revolve in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.4 degrees from the perpendicular to the Earth–Sun plane, and the Earth–Moon plane is tilted about 5 degrees against the Earth-Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.[3][140]
The Hill sphere, or gravitational sphere of influence, of the Earth is about 1.5 Gm (or 1,500,000 kilometers) in radius.[141][note 14] This is maximum distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit the Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.
Barred spiral galaxy
Illustration of the Milky Way Galaxy, showing the location of the Sun
Earth, along with the Solar System, is situated in the Milky Way galaxy, orbiting about 28,000 light years from the center of the galaxy. It is currently about 20 light years above the galaxy's equatorial plane in the Orion spiral arm.[142]

Axial tilt and seasons

Because of the axial tilt of the Earth, the amount of sunlight reaching any given point on the surface varies over the course of the year. This results in seasonal change in climate, with summer in the northern hemisphere occurring when the North Pole is pointing toward the Sun, and winter taking place when the pole is pointed away. During the summer, the day lasts longer and the Sun climbs higher in the sky. In winter, the climate becomes generally cooler and the days shorter. Above the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year—a polar night. In the southern hemisphere the situation is exactly reversed, with the South Pole oriented opposite the direction of the North Pole.
Black space with crescent Earth at lower left, crescent Moon at upper right, 30% of Earth's apparent diameter; five Earth diameters apparent space between; sunlit from right side
Earth and Moon from Mars, imaged by Mars Reconnaissance Orbiter. From space, the Earth can be seen to go through phases similar to the phases of the Moon.
By astronomical convention, the four seasons are determined by the solstices—the point in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. In the northern hemisphere, Winter Solstice occurs on about December 21, Summer Solstice is near June 21, Spring Equinox is around March 20 and Autumnal Equinox is about September 23. In the Southern hemisphere, the situation is reversed, with the Summer and Winter Solstices exchanged and the Spring and Autumnal Equinox dates switched.[143]
The angle of the Earth's tilt is relatively stable over long periods of time. The tilt does undergo nutation; a slight, irregular motion with a main period of 18.6 years.[144] The orientation (rather than the angle) of the Earth's axis also changes over time, precessing around in a complete circle over each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and Moon on the Earth's equatorial bulge. From the perspective of the Earth, the poles also migrate a few meters across the surface. This polar motion has multiple, cyclical components, which collectively are termed quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the Chandler wobble. The rotational velocity of the Earth also varies in a phenomenon known as length of day variation.[145]
In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4. These dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. The changing Earth-Sun distance results in an increase of about 6.9%[note 15] in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. This effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere.[146]

Moon

Characteristics
Diameter 3,474.8 km
Mass 7.349×1022 kg
Semi-major axis 384,400 km
Orbital period 27 d 7 h 43.7 m
The Moon is a relatively large, terrestrial, planet-like satellite, with a diameter about one-quarter of the Earth's. It is the largest moon in the Solar System relative to the size of its planet, although Charon is larger relative to the dwarf planet Pluto. The natural satellites orbiting other planets are called "moons" after Earth's Moon.
The gravitational attraction between the Earth and Moon causes tides on Earth. The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit the Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases; the dark part of the face is separated from the light part by the solar terminator.
Because of their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm a year. Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 µs a year—add up to significant changes.[147] During the Devonian period, for example, (approximately 410 mya) there were 400 days in a year, with each day lasting 21.8 hours.[148]

Details of the Earth-Moon system. Besides the radius of each object, the radius to the Earth-Moon barycenter is shown. Photos from NASA. Data from NASA. The Moon's axis is located by Cassini's third law.
The Moon may have dramatically affected the development of life by moderating the planet's climate. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon.[149] Some theorists believe that without this stabilization against the torques applied by the Sun and planets to the Earth's equatorial bulge, the rotational axis might be chaotically unstable, exhibiting chaotic changes over millions of years, as appears to be the case for Mars.[150]
Viewed from Earth, the Moon is just far enough away to have very nearly the same apparent-sized disk as the Sun. The angular size (or solid angle) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant.[139] This allows total and annular solar eclipses to occur on Earth.
The most widely accepted theory of the Moon's origin, the giant impact theory, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth's crust.[151]
Earth has at least five co-orbital asteroids, including 3753 Cruithne and 2002 AA29.[152][153] As of 2011, there are 931 operational, man-made satellites orbiting the Earth.[154] On July 27, 2011, astronomers reported a trojan asteroid companion, 2010 TK7, librating around the leading Lagrange triangular point, L4, of Earth in Earth's orbit around the Sun.[155][156]

A scale representation of the relative sizes of, and average distance between, Earth and Moon

Habitability

A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain metabolism.[157] The distance of the Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the current climatic conditions at the surface.[158]

Biosphere

The planet's life forms are sometimes said to form a "biosphere". This biosphere is generally believed to have begun evolving about 3.5 bya. The biosphere is divided into a number of biomes, inhabited by broadly similar plants and animals. On land, biomes are separated primarily by differences in latitude, height above sea level and humidity. Terrestrial biomes lying within the Arctic or Antarctic Circles, at high altitudes or in extremely arid areas are relatively barren of plant and animal life; species diversity reaches a peak in humid lowlands at equatorial latitudes.[159]

Natural resources and land use

The Earth provides resources that are exploitable by humans for useful purposes. Some of these are non-renewable resources, such as mineral fuels, that are difficult to replenish on a short time scale.
Large deposits of fossil fuels are obtained from the Earth's crust, consisting of coal, petroleum, natural gas and methane clathrate. These deposits are used by humans both for energy production and as feedstock for chemical production. Mineral ore bodies have also been formed in Earth's crust through a process of Ore genesis, resulting from actions of erosion and plate tectonics.[160] These bodies form concentrated sources for many metals and other useful elements.
The Earth's biosphere produces many useful biological products for humans, including (but far from limited to) food, wood, pharmaceuticals, oxygen, and the recycling of many organic wastes. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land.[161] Humans also live on the land by using building materials to construct shelters. In 1993, human use of land is approximately:
Land use Arable land Permanent crops Permanent pastures Forests and woodland Urban areas Other
Percentage 13.13%[14] 4.71%[14] 26% 32% 1.5% 30%
The estimated amount of irrigated land in 1993 was 2,481,250 km2.[14]

Natural and environmental hazards

Large areas of the Earth's surface are subject to extreme weather such as tropical cyclones, hurricanes, or typhoons that dominate life in those areas. From 1980–2000, these events caused an average of 11,800 deaths per year.[162] Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, wildfires, and other calamities and disasters.
Many localized areas are subject to human-made pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion, erosion, and introduction of invasive species.
According to the United Nations, a scientific consensus exists linking human activities to global warming due to industrial carbon dioxide emissions. This is predicted to produce changes such as the melting of glaciers and ice sheets, more extreme temperature ranges, significant changes in weather and a global rise in average sea levels.[163]

Human geography

Cartography, the study and practice of map making, and vicariously geography, have historically been the disciplines devoted to depicting the Earth. Surveying, the determination of locations and distances, and to a lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.
Earth has reached approximately 7,000,000,000 human inhabitants as of October 31, 2011.[164] Projections indicate that the world's human population will reach 9.2 billion in 2050.[165] Most of the growth is expected to take place in developing nations. Human population density varies widely around the world, but a majority live in Asia. By 2020, 60% of the world's population is expected to be living in urban, rather than rural, areas.[166]
It is estimated that only one-eighth of the surface of the Earth is suitable for humans to live on—three-quarters is covered by oceans, and half of the land area is either desert (14%),[167] high mountains (27%),[168] or other less suitable terrain. The northernmost permanent settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada.[169] (82°28′N) The southernmost is the Amundsen-Scott South Pole Station, in Antarctica, almost exactly at the South Pole. (90°S)
Independent sovereign nations claim the planet's entire land surface, except for some parts of Antarctica and the odd unclaimed area of Bir Tawil between Egypt and Sudan. As of 2011 there are 204 sovereign states, including the 193 United Nations member states. In addition, there are 59 dependent territories, and a number of autonomous areas, territories under dispute and other entities.[14] Historically, Earth has never had a sovereign government with authority over the entire globe, although a number of nation-states have striven for world domination and failed.[170]
The United Nations is a worldwide intergovernmental organization that was created with the goal of intervening in the disputes between nations, thereby avoiding armed conflict.[171] The U.N. serves primarily as a forum for international diplomacy and international law. When the consensus of the membership permits, it provides a mechanism for armed intervention.[172]
The first human to orbit the Earth was Yuri Gagarin on April 12, 1961.[173] In total, about 487 people have visited outer space and reached Earth orbit as of July 30, 2010, and, of these, twelve have walked on the Moon.[174][175][176] Normally the only humans in space are those on the International Space Station. The station's crew, currently six people, is usually replaced every six months.[177] The furthest humans have travelled from Earth is 400,171 km, achieved during the 1970 Apollo 13 mission.[178]
The 7 continents of Earth:[179]      North America,      South America,      Antarctica,      Africa,      Europe,      Asia,      Australia
The Earth at night in 2000, a composite of DMSP/OLS ground illumination data on a simulated night-time image of the world. This image is not photographic and many features are brighter than they would appear to a direct observer.
ISS video beginning just south-east of Alaska. The first city that the ISS passes over (seen approximately 10 seconds into the video) is San Francisco and the surrounding areas. A careful examination shows where the Golden Gate Bridge is located: a smaller strip of lights just before the city of San Francisco, nearest to the clouds on the right of the image. Very obvious lightning storms can be seen on the Pacific Ocean coastline, with clouds overhead. As the video continues, the ISS passes over Central America (green lights can be seen here), with the Yucatan Peninsula on the left. The pass ends as the ISS is over the capital city of Bolivia, La Paz.

Cultural viewpoint


The first photograph ever taken by astronauts of an "Earthrise", from Apollo 8
The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle.[180]
Unlike the rest of the planets in the Solar System, humankind did not begin to view the Earth as a moving object in orbit around the Sun until the 16th century.[181] Earth has often been personified as a deity, in particular a goddess. In many cultures a mother goddess is also portrayed as a fertility deity. Creation myths in many religions recall a story involving the creation of the Earth by a supernatural deity or deities. A variety of religious groups, often associated with fundamentalist branches of Protestantism[182] or Islam,[183] assert that their interpretations of these creation myths in sacred texts are literal truth and should be considered alongside or replace conventional scientific accounts of the formation of the Earth and the origin and development of life.[184] Such assertions are opposed by the scientific community[185][186] and by other religious groups.[187][188][189] A prominent example is the creation-evolution controversy.
In the past there were varying levels of belief in a flat Earth,[190] but this was displaced by the concept of a spherical Earth due to observation and circumnavigation.[191] The human perspective regarding the Earth has changed following the advent of spaceflight, and the biosphere is now widely viewed from a globally integrated perspective.[192][193] This is reflected in a growing environmental movement that is concerned about humankind's effects on the planet