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Formation and evolution of Stars

Stars are formed within molecular clouds; large regions of high density in the interstellar medium (though still less dense than the inside of an earthly vacuum chamber). These clouds consist of mostly hydrogen with about 23–28% helium and a few percent heavier elements. One example of such a star-forming nebula is the Orion Nebula.[13] As massive stars are formed from these clouds, they powerfully illuminate the clouds from which they formed, creating an H II region.

Molecular Cloud

A molecular cloud is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2). This molecule is difficult to detect, and the molecule most used to trace the H2 is CO (carbon monoxide). The ratio between CO luminosity and H2 mass is roughly constant,

although there are reasons to doubt this assumption in observations of some other galaxies.

Jeans Instability

The theory of fragmentation and gravitational collapse of molecular clouds because their own gravity was developed by James Jeans about the year 1902 and although at present the processes of stellar formation are known by major great precision, the theory of Jeans constitutes the first good approach.

Jeans calculated that under certain conditions a molecular cloud could contract for gravitational attraction. Only it was necessary that it was it sufficiently massive and cold. A stable cloud, if it is compressed, increases his pressure more rapidly than his gravity and returns spontaneously his original state.

But if the cloud overcomes certain critical mass of that time it will unbecome stable and will collapse in all its volume. This is the motive for which the instabilities usually take place in the biggest clouds giving place to intense sprouts of stellar formation. This critical mass of Jeans is a function dependent on the thickness and the temperature.

Protostar

A Protostar is the name given to a stage in the development of a star and it is a period after clouds of hydrogen, helium and dust begin to contract and before the star reaches the main sequence.Protostars of around the mass of the Sun typically take 10 million years to evolve from a condensing cloud to a main-sequence star. A protostar of 15 solar masses evolves much more quickly, typically taking only 100,000 years to reach the main sequence. A protostar forms from the contraction within a giant molecular cloud in the interstellar medium. Observations reveal that giant molecular clouds are approximately in a state of virial equilibrium—on the whole, the gravitational binding energy of the cloud is balanced by the kinetic energy of the cloud's constituent molecules.

Any disturbance to the cloud may upset its state of equilibrium. Examples of disturbances are shock waves from supernovae; spiral density waves within galaxies and the close approach or collision of another cloud. Whatever the source of the disturbance, if it is sufficiently large it may cause the force due to gravity to become greater than the force due to thermal kinetic energy within a particular region of the cloud.

The British physicist Sir James Jeans considered the above phenomenon in detail. He was able to show that, under appropriate conditions, a cloud, or part of one, would start to contract as described above. He derived a formula for calculating the mass and size that a cloud would have to reach as a function of its density and temperature before gravitational contraction would begin. This critical mass is known as the Jeans mass.

Supermassive stars

A supermassive star is a hypothetical star whose mass exceeds 60 solar masses. Most astronomers believe that supermassive stars do not exist, instead postulating the existence of supermassive black holes.

The largest stable star is determined by the Eddington limit. If a star exceeds this mass, the excess luminosity will exceed the gravitational force at the outer surface, and the star will shed gas until it reaches the limiting mass. The maximum such mass is thought to be about 100 solar masses.

It has been hypothesized that the first stars in the early universe, sometimes called population III stars, may have achieved supermassive dimensions. The lack of heavy elements in the early universe allowed significantly larger stars to form.

What is a Star?

A star is a massive, compact body of plasma in outer space that is held together by its own gravity and, unlike a planet, is sufficiently massive to sustain nuclear fusion in a very dense, hot core region. This fusion of atomic nuclei generates the energy that is continuously radiated from the outer layers of the star during much of its life span.

Individual stars differ in their total mass, composition, and age. The total mass of a star is the principal determinant in its evolution and eventual fate. A Hertzsprung-Russell diagram (HR-diagram) shows the pattern of the temperature of stars against their absolute magnitude, and can be used to determine the age of a star and the stage in its evolution. Initially, stars are composed primarily of hydrogen, with some helium and heavier trace elements that determine their metallicity. Over the course of a star's evolution, a portion of the hydrogen is converted into helium and smaller quantities of heavier elements through the process of nuclear fusion. Part of the matter is then recycled into the interstellar environment and used to form a new generation of more metal-rich stars.

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. For example, a nova occurs when a white dwarf accretes matter from a companion star.

During the major part of their lives, most stars on the Main Sequence will create their energy by the process of hydrogen fusion - the process of fusing two hydrogen atoms to create one helium atom *. Energy is created because a helium atom weighs slightly less than the two hydrogen atoms, and the excess mass is converted into energy, as related by Einstein's famous equation E=m*c2. Our sun is currently in this stage of converting hydrogen to helium.

two protons join together to form a deuterium nucleus, which is also known as "heavy water." A positron and a neutrino are released as by-

products. The deuterium nucleus is bombarded by another proton, creating a helium-3 nucleus. The by-product of this is a photon in the form of a gamma ray (a very high-energy form of light). Then, the helium-3 nucleus in bombarded by another helium-3 nucleus, creating a normal helium-4 nucleus. The by-product of this are two

protons, which are free to start the whole process over again. The positron will be destroyed and form another gamma ray; the energy from this in the form of gamma rays is radiated out of sun's core.

Since our sun is currently in this stage, the numbers here are for it alone, although since it is like most other stars, the are representative of how all stars work. Each second, the sun converts 500 million metric tons of hydrogen to helium. In turn, every second 5 million metric tons of excess material is converted into energy. This means that every year, 157,680,000,000,000 metric tons are converted into energy. The material from one second energy is about 1x1027 (one octillion thousand) watts of energy. On Earth, we receive about 2/1,000,000,000 (two billionths) of that energy, or about 2x1018 (two quintillion) watts. This is enough energy to power 100 average light bulbs for about 5 million years -- longer than humans have been standing upright.

*There are actually two major processes that are used by stars. The one presented here is a simplification of one part of the main one. If you are interested, the other cycle, called the "CNO Cycle," is presented in the Advanced version of this web site, as is a more-in-depth version of this, which is called the "pp Chain."

Main Sequence Star Death

After about ten billion years, a main sequence star has converted approximately 10% of its hydrogen to helium. Although this might seem as though it could still undergo hydrogen fusion for another 90 billion years, this is not the case. Remember that there are immense pressures at the core of stars, and it is only because of these pressures that the fusion can occur -- in a fixed volume, increased pressure leads to increased heat. Outside of the range of pressures there is still mostly hydrogen, but it cannot be used because the pressures are not high enough to initiate fusion.

The helium core begins to contract, and the outer layers expand and cool, glowing redder. The star is now called a red giant. At this point, helium fusion begins. The star was previously unable to fuse the helium; however, now that the core has contracted, the added pressure is enough to fuse helium into heavier elements. Simultaneously, hydrogen fusion also occurs at this point in a shell around the helium core, for pressures there have also increased enough for hydrogen to fuse. Life expectancy from here on is about 100,000,000 years.

After this time, the red giant is made of mostly carbon. The next fusion process would be to fuse the carbon into iron. The problem in this star is that there is not enough pressure in the core to do this. Because the outward pressure of energy is no longer maintained, the core collapses and sends a shockwave outwards, and the star's outer layers are cast off in a planetary nebula, with the resulting core becoming a white dwarf. The core is made almost of pure carbon (like coal), and glows white because it still possesses a lot of left-over heat. It now also possess much less mass because it has shed its outer layers, and any planets it has would move to much farther orbits or be completely ejected from the system, if they had not been engulfed by the star in the expanded red giant phase.

The white dwarf is destined to drift in space for millennia as it slowly cools. Most have an approximate size of the Earth (8,000 km (7,500 miles) diameter), and has a density such that a matchbox's worth would weigh about as much as an elephant. It has a maximum weight of 1.4 solar masses. As it cools, it will grow dimmer, and will eventually become a black dwarf - a frozen lump of carbon floating though space.

Supergiant Star Death

A star with a mass much greater than that of the sun will form, live, and die more quickly than a main sequence star. The reason for this is its greater mass, for the resulting gravity squeezes the star's core and creates greater pressures, resulting in a faster fusion rate.

After about 10 to 15 million years (versus over 10 billion for a main sequence star), a supergiant's core has turned to carbon and has swollen into a red supergiant (Betelgeuse, a bright star forming the constellation Orion's shoulder, is an example of a star in this stage). The reason it glows red is that since its outer layers have expanded, it has a much greater volume to heat, yet is only producing the same amount of energy. Thus, it is naturally cooler, and glows red.

The difference at this point between a supergiant and a main sequence star is that a supergiant has the pressures needed to fuse carbon into iron. However, this fusion process takes energy, rather than gives it. So, as energy is lost, the star no longer possesses the balance between outward pressure and gravity pushing in. As a result, gravity wins out, and the core collapses in a violent explosion called a supernova. (One supernova explosion on A.D. July 4, 1054, was so bright that it could be seen in broad daylight for 23 days. The nebula it created is called the Crab Nebula, and is the picture to the left.)

The path here is also divided as to what the final outcome of the star will be. If the star is less than about nine (but more than 1.4) solar masses, the core will collapse into a neutron star - a star made entirely of neutrons. If the star possesses more mass, it will continue to collapse into a black hole - a point of theoretically infinite density that possesses such a strong gravitational pull that not even light can escape its pull.

Dwarf Death

Although the sun is technically considered a main sequence star, it also falls under the classification of a yellow dwarf. On this page as with the rest of the site, however, the term "dwarf" is used to refer to stars smaller than the sun in order to avoid confusion.

Red dwarfs are the only active (undergoing hydrogen fusion) type of dwarf (other types are brown, white, and black). Red dwarfs range between 1/3 and 1/12 the sun's mass, and shine only 1/100 to 1/1,000,000 as brightly. Proxima Centauri, Earth's closest extrasolar star, is a red dwarf 1/5 the size of the Sun, and if it were to trade places with the Sun, it would shine on Earth only 1/10 as much as the sun currently does on Pluto.

Red dwarfs, because of their small size, undergo fusion much less quickly than a solar mass star. Therefore, they use up their supply of hydrogen much less quickly than a main sequence star, and can live for more than 100 trillion years.

When they die, they simply wink out of existence, for they do not have enough pressure to fuse helium. Thus, they simply grow dimmer and cooler as they float through the void of space.

Neutron Star

A neutron star is one of the few possible endpoints of stellar evolution. A neutron star is formed from the collapsed remnant of a massive star after a Type II, Type Ib, or Type Ic supernova.

A typical neutron star has a mass between 1.35 to about 2.1 solar masses, with a corresponding radius between 20 and 10 km (they shrink as their mass increases) — 30,000 to 70,000 times smaller than the Sun. Thus, neutron stars have densities of 8×1013 to 2×1015 g/cm³, about the density of an atomic nucleus.[1] Compact stars of less than 1.44 solar masses, the Chandrasekhar limit, are white dwarfs; above three to five solar masses (the Tolman-Oppenheimer-Volkoff limit), gravitational collapse occurs, inevitably producing a black hole.

Since a neutron star retains most of the angular momentum of its parent star but has only a tiny fraction of its parent's radius, the moment of inertia decreases sharply causing a rotational acceleration to a very high rotation speed, with one revolution taking anywhere from one seven-hundredth of a second to thirty seconds. The neutron star's compactness also gives it high surface gravity, 2×1011 to 3×1012 times stronger than that of Earth. One of the measures for the

gravity is the escape velocity, the velocity needed for an object to escape from the gravitational field to infinite distance. For a neutron star, such velocities are typically 150,000 km/s, about 1/2 of the velocity of light. Conversely, an object falling onto the surface of a neutron star would strike the star also at 150,000 km/s. To put this in perspective, if an average human were to encounter a neutron star, they would impact with roughly the energy yield of a 200 megaton explosion (a power equivalent to four times the Tsar Bomba, the biggest nuclear weapon ever detonated).

Pulsars are rotating neutron stars which emit detectable electromagnetic radiation in the form of radio waves. The radiation intensity varies with a regular period, believed to correspond to the rotation period of the star. Pulsars also exhibit a so-called lighthouse effect, which occurs when the light and other radiation from a pulsar are only seen at specific intervals and not all of the time. Werner Becker of the Max-Planck-Institut für extraterrestrische Physik recently said,

"The theory of how pulsars emit their radiation is still in its infancy, even after nearly forty years of work.. There are many models but no accepted theory."

Three distinct classes of pulsars are currently known to astronomers, according to the source of energy that powers the radiation:

Rotation-powered pulsars, where the loss of rotational energy of the star powers the radiation

Accretion-powered pulsars (accounting for most but not all X-ray pulsars), where the gravitational potential energy of accreted matter is the energy source (producing X-rays that are observable from Earth), and

Magnetars, where the decay of an extremely strong magnetic field powers the radiation.

Although all three classes of objects are neutron stars, their observable behaviour and the underlying physics are quite different. There are, however, connections. For example, X-ray pulsars are probably old rotation-powered pulsars that have already lost most of their energy, and have only become visible again after their binary companions expanded and began transferring matter on to the neutron star. The process of accretion can in turn transfer enough angular momentum to the neutron star to "recycle" it as a rotation-powered millisecond pulsar.

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Glitch prediction

In June 2006, astronomer John Middleditch and his team at LANL announced the first prediction of glitches, with observational data from the Rossi X-ray Timing Explorer. They used observations of the pulsar PSR J0537-6910.

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Application

The study of pulsars has resulted in many applications in physics and astronomy. Striking examples include the confirmation of the existence of gravitational radiation as predicted by general relativity and the first detection of an extra-solar planetary system.

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Significant pulsars

The first radio pulsar, CP 1919 (now known as PSR 1919+21), with a pulse period of 1.337 seconds and a pulse width of 0.04 second, was discovered in 1967 (Nature 217:709-713, 1968). A drawing of this pulsar's radio waves was used as the cover of British rock band Joy Division's debut album, Unknown Pleasures.

The first binary pulsar, PSR 1913+16, confirming general relativity and proving the existence of gravitational waves

The first millisecond pulsar, PSR B1937+21

The first X-ray pulsar, Cen X-3

The first accreting millisecond X-ray pulsar, SAX J1808.4-3658

The first pulsar with planets, PSR B1257+12

The first double pulsar binary system, PSR J0737-3039

The magnetar SGR 1806-20 produced the largest burst of energy in the Galaxy ever experimentally recorded on 27 December 2004

PSR B1931+24 "... appears as a normal pulsar for about a week and then 'switches off' for about one month before emitting pulses again. [..] this pulsar slows down more rapidly when the pulsar is on than when it is off. [.. the] breaking mechanism must be related to the radio emission and the processes creating it and the additional slow-down can be explained by a wind of particles leaving the pulsar's magnetosphere and carrying away rotational energy. [1]

PSR J1748-2446ad, at 716 Hz, the fastest spinning pulsar known.

A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of dust grains that collide and stick to form larger and larger bodies. When the bodies reach sizes of approximately one kilometer,

then they can attract each other directly through their mutual gravity, aiding further growth into moon-sized protoplanets enormously. This is how planetesimals are often defined. Bodies that are smaller than planetesimals must rely on brownian motion or turbulent motions in the gas

to cause the collisions that can lead to sticking. Alternatively planetesimals can form in a very dense layer of dust grains that undergoes a collective gravitational instability in the mid-plane of a protoplanetary disk. Many planetesimals may eventually break apart during violent collisions, but a few of the largest planetesimals can survive such encounters and continue to grow into protoplanets and later planets.

It is generally believed that by about 3.8 billion years ago, after a period known as heavy bombardment, most of the planetesimals within the solar system had either been ejected from the Solar system entirely, into distant eccentric orbits such as the Oort cloud, or had collided with larger objects due to the regular gravitational nudges from the Jovian planets (particularly Jupiter and Neptune). A few planetesimals may have been captured as moons, such as Phobos, Deimos, Triton, and many of the small high-inclination moons of the Jovian planets.

Beta Pictoris (ß Pic / ß Pictoris) is the second brightest star in the constellation Pictor. It is located 64 light years from our solar system and is significantly hotter, more massive and more luminous than our Sun.

The Beta Pictoris system is very young, only 8-20 million years old although it is already in the main sequence stage of its evolution.

Beta Pictoris is the title member of the Beta Pictoris moving group, an association of young stars which share the same motion through space and have the same age.

Beta Pictoris shows an excess of infrared emission compared to normal stars of its type, which is caused by large quantities of dust near the star.

Detailed observations reveal a large disk of dust and gas orbiting the star, which was the first debris disk to be imaged around another star. In addition to the presence of several planetesimal belts and cometary activity, there are indications that planets have formed within this disk and that the processes of planet formation may still be ongoing.

Material from the Beta Pictoris debris disk is thought to be the dominant source of interstellar meteoroids in our solar system.

Planetesimal belts

The dust around Beta Pictoris may be produced by the collisions of large planetesimals.In 2003, imaging of the inner region of the Beta Pictoris system with the Keck II telescope revealed the presence of several features which are interpreted as being belts or rings of material. Belts at approximately 14, 28, 52 and 82 astronomical units from the star were detected, which alternate in inclination with respect to the main disk.

Observations in 2004 revealed the presence of an inner belt containing silicate material at a distance of 6.4 AU from the star. Silicate material was also detected at 16 and 30 AU from the star, with a lack of dust between 6.4 and 16 AU providing evidence that a massive planet may be orbiting in this region.

Modelling of the dust disk at 100 AU from the star suggests the dust in this region may have been produced by a series of collisions initiated by the destruction of planetesimals with radii of about 180 kilometers. After the initial collision, the debris undergoes further collisions in a process called a collisional cascade. Similar processes have been inferred in the debris disks around Fomalhaut and AU Microscopii.

A protoplanetary disk (or proplyd) is a rotating disk of dense gas surrounding a young newly formed star (a T Tauri star).

The protoplanetary disk may be considered an accretion disk because gaseous material may be falling from the inner edge of the disk onto the surface of the star, but this process should not be confused with the accretion process thought to build up the planets themselves.

Protoplanetary disks around T Tauri stars differ from the discs surrounding the primary components of close binary systems in their size and temperature.

Protoplanetary discs have radii up to 1000 astronomical units and are rather cool. Only their innermost parts reach temperatures above 1000 kelvins.

They are very often accompanied by jets. Protostars typically form from molecular clouds consisting primarily of molecular hydrogen. When a portion of a molecular cloud reaches a critical size, mass, or density, it begins to collapse under its own gravity.

As this collapsing cloud, called a solar nebula, becomes more dense, random gas motions originally present in the cloud average out in favor of the direction of the nebula's net angular momentum. Conservation of angular momentum causes the rotation to increase as the nebula becomes smaller.

This rotation causes the cloud to flatten out - much like forming a flat pizza out of dough - and take the form of a disk. The initial collapse takes about 100,000 years.

After that time the star reaches a surface temperature similar to that of a main sequence star of the same mass and becomes visible. It is now a T Tauri star. Accretion of gas onto the star continues for another 10 million years, before the disk disappears, perhaps being blown away by the young star's solar wind, or perhaps simply ceasing to emit radiation after accretion has ended. The oldest protoplanetary disk ever discovered is 25 million years old.

The nebular theory of solar system formation describes how protoplanetary disks are thought to evolve into planetary systems. Electrostatic and gravitational interactions may cause the dust and ice grains in the disk to accrete into planetesimals. This process competes against the stellar wind, which drives the gas out of the system, and accretion, which pulls material into the central T Tauri star.

Protoplanetary disks have been observed around several young stars in our galaxy, the first being found around the star Beta Pictoris in 1984. Recent observations by the Hubble Space Telescope have shown proplyds and planetary discs to be forming within the Orion Nebula.

Astronomers have discovered large discs of material, which may themselves be protoplanetary discs, around the stars Vega, Alphecca and Fomalhaut, all of which are very close to the Sun.

The Castor co-moving group of stars containing Vega and Fomalhaut has recently been isolated. Using data from the Hipparcos satellite telescope the Castor group was found to have an estimated age of 200 ± 100 million years. This indicates that the infrared excesses seen around Vega and Fomalhaut are likely due to a disk of debris from colliding planetesimals rather than a protoplanetary disk. Successful imaging of Fomalhaut's disk by the Hubble Space Telescope confirms this.

Most objects in orbit round the Sun lie within the same shallow plane, called the ecliptic, which is roughly parallel to the Sun's equator. The planets lie very close to the ecliptic, while comets and kuiper belt objects often lie at significant angles to it.

All of the planets, and most other objects, also orbit with the Sun's rotation in a counter-clockwise direction as viewed from a point above

the Sun's north pole. There is a direct relationship between how far away a planet is from the Sun, and how quickly it orbits. Mercury, with the smallest orbital circumference, travels the fastest, while Neptune, being much farther from the Sun, travels more slowly.

A planet's distance from the Sun varies in the course of its year. Its closest approach to the Sun is known as its perihelion, while its farthest point from the Sun

is called its aphelion. Though planets follow nearly circular orbits, with perihelions roughly equal to their aphelions, many comets, asteroids and objects of the Kuiper belt follow highly elliptical orbits, with large differences between perihelion and aphelion.

Astronomers most often measure distances within the solar system in astronomical units, or AU. One AU is the average distance between the Earth and the Sun, or roughly 149 598 000 km (93,000,000 mi). Pluto is roughly 38 AU from the Sun, while Jupiter lies at roughly 5.2 AU.

Informally, the Solar System is sometimes divided into separate "zones"; the first zone, known as the inner Solar System, comprises the inner planets and the main asteroid belt. The outer solar system is sometimes defined as everything beyond the asteroids; however, it is also the name often given to the region beyond Neptune, with the gas giants as a separate "middle zone."

One common misconception with regards to the Solar System is that the orbits of the major objects (planets, Pluto, and asteroids) are equidistant. Due to the vast distances involved, many representations of the Solar System tend to simplify these orbits, with equal spacing between each object.

However, with certain exceptions, it can generally be stated that the farther a planet or belt is from the Sun, the greater the distance between it and the previous orbit. For example, Venus is approximately 0.33 AU farther out than Mercury, whereas Jupiter lies 1.9 AU from the farthest extent of the asteroid belt, and Neptune's orbit is roughly 20 AU farther out than that of Uranus.

Attempts have been made to determine a correlation between these distances (see Bode's Law) but to date there is no accepted theory that explains the respective orbital distances.

In a decision passed by the International Astronomical Union General Assembly on August 24, 2006, the objects in the Solar System were divided into three separate groups: planets, dwarf planets and small solar system bodies.

Under this classification, a planet is any body in orbit around the Sun that a) has enough mass to form itself into a spherical shape and b) has cleared its immediate neighborhood of all smaller objects. Eight objects in the Solar System currently meet this definition; they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Dwarf planet is a newly defined classification for astronomical objects. The key difference between planets and dwarf planets is that while both are required to orbit the Sun and be of large enough mass that their own gravity pulls them into a nearly round shape, dwarf planets are not required to clear their neighborhood of other celestial bodies.

Three objects in the solar system are currently included in this category; they are Pluto (formerly considered a planet), the asteroid Ceres, and the scattered disc object Eris. The IAU will begin evaluating other known objects to see if they fit within the definition of dwarf planets. The most likely candidates are some of the larger asteroids and several Trans-Neptunian Objects such as Sedna, Orcus, and Quaoar.

The remainder of the objects in the Solar System were classified as small solar system bodies. A small solar system body (SSSB) is a term defined in 2006 by the International Astronomical Union to describe Solar System objects which are neither planets nor dwarf planets.

All other objects ... orbiting the Sun shall be referred to collectively as "Small Solar System Bodies" .... These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

As of 2006, the IAU considers the following bodies to be SSSB's:

all asteroids except Ceres

all centaurs

all trans-Neptunian objects, including Kuiper belt & Scattered disc objects, with the exception of Pluto and Eris

all comets

The Sun is the Solar System's parent star, and far and away its chief component. It is classed as a moderately large yellow dwarf. However, this name is misleading, as on the scale of stars in our galaxy, the Sun is rather large and bright. Stars are classified based on their position on the Hertzsprung-Russell diagram, a graph which plots the brightness of stars against their surface temperature. Generally speaking, the hotter a star is, the brighter it is.

Stars which follow this pattern are said to be on the main sequence, and the Sun lies right in the middle of it. This has led many astronomy textbooks to label the Sun as "average;" however, stars brighter and hotter than it are rare, whereas stars dimmer and cooler than it are common. The vast majority of stars are dim red dwarfs, though they are under-represented in star catalogues as we can observe only those few that are very near the Sun in space.

The Sun's position on the main sequence means, according to current theories of stellar evolution, that it is in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion, and been forced, as older red giants must, to fuse more inefficient elements such as helium and carbon.

The Sun is growing increasingly bright as it ages. Early in its history, it was roughly 75 percent as bright as it is today.[8]Calculations of the ratios of hydrogen and helium within the Sun suggest it is roughly halfway through its life cycle, and will eventually begin moving off the main sequence, becoming larger, brighter and redder, until, about five billion years from now, it too will become a red giant.

The Sun is a population I star, meaning that it is fairly new in galactic terms, having been born in the later stages of the universe's evolution. As such, it contains far more elements heavier than hydrogen and helium ("metals" in astronomical parlance) than older population II stars such as those found in globular clusters.

Since elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, the first generation of stars had to die before the universe could be enriched with them. For this reason, the very oldest stars contain very little "metal", while stars born later have more. This high "metallicity" is thought to have been crucial in the Sun's developing a planetary system, because planets form from accretion of metals

The Sun radiates a continuous stream of charged particles, a plasma known as solar wind, ejecting it outwards at speeds greater than 2 million kilometres per hour, [10]creating a very tenuous "atmosphere" (the heliosphere), that permeates the solar system for at least 100 AU. This environment is known as the interplanetary medium.

Small quantities of cosmic dust (some of it arguably interstellar in origin) are also present in the interplanetary medium and are responsible for the phenomenon of zodiacal light. The influence of the Sun's rotating magnetic field on the interplanetary medium creates the largest structure in the solar system, the heliospheric current sheet.

Earth's magnetic field protects its atmosphere from interacting with the solar wind. However, Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space.

The four inner or terrestrial planets are characterised by their dense, rocky composition, few or no moons, and lack of ring systems. They are composed largely of minerals with high melting points such as silicates to form the planets' solid crusts and semi-liquid mantles, and metallic dust grains such as iron, which forms their cores. Three of the four inner planets have atmospheres.

All have impact craters, and all but one possess tectonic surface features, such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets which are closer to the Sun than the Earth is (i.e. Mercury and Venus).

The four inner planets are:

Mercury

Mercury (0.4 AU), the closest planet to the Sun, is also the least massive of the planets, at only 0.055 Earth masses. Mercury has a very thin atmosphere consisting of atoms blasted off its surface by the solar wind. Because Mercury is so hot, these atoms quickly escape into space.

Thus in contrast to the Earth and Venus whose atmospheres are stable, Mercury's atmosphere is constantly being replenished.[12] Mercury is surrounded by an extremely small amount of helium, hydrogen, oxygen, and sodium. This envelope of gases is so thin that the greatest possible atmospheric pressure (force exerted by the weight of gases) on Mercury would be about 0.000000000002 kgf/cm² (0.00000000003 psi or 0.2 µPa).

The atmospheric pressure on the Earth is about 1.03 kgf/cm² (14.7 psi or 101 kPa).[13] It has no natural satellite, and, to date, no observed geological activity save that produced by impacts. Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the Sun's gravity. The MESSENGER probe should aid in resolving this issue when it arrives in Mercury's orbit in 2011.

Venus

Venus (0.7 AU), the first truly terrestrial planet, is of comparable mass to the Earth (0.815 Earth masses), and, like Earth, possesses a thick silicate mantle around an iron core, as well as a substantial atmosphere and evidence of one-time internal geological activity, such as volcanoes.

However, it is much drier than Earth and its atmosphere is 90 times as dense and is composed overwhelmingly of carbon dioxide and sulfuric acid. Unlike Earth, evidence suggests that Venus's crust is not divided into tectonic plates but instead comprises a single very thick rind.[14] Venus has no natural satellite.

It is the hottest planet, despite being farther from the sun than Mercury, with temperatures reaching more than 400 degrees Celsius. This is most likely due to the amount of greenhouse gases in the atmosphere.

Earth

The largest and densest of the inner planets, Earth (1 AU) is also the only one to demonstrate unequivocal evidence of current geological activity. Earth is the only planet known to have life. Its liquid hydrosphere, unique among the terrestrials, is probably the reason Earth is also the only planet where multi-plate tectonics has been observed, because water acts as a lubricant for subduction.

Its atmosphere is radically different from the other terrestrials, having been altered by the presence of life to contain 21 percent free oxygen. Its satellite, the Moon, is sometimes considered a terrestrial planet in a co-orbit with its partner, because its orbit around the Sun never actually loops back on itself when observed from above.[16] The Moon possesses many features in common with other terrestrial planets, though it lacks an iron core.

Mars

Mars (1.5 AU), at only 0.107 Earth masses, is less massive than either Earth or Venus. It possesses a tenuous atmosphere of carbon dioxide. Its surface, peppered with vast volcanoes and rift valleys such as Valles Marineris, shows that it was once geologically active and recent evidence[17] suggests this may have been true until very recently. Mars possesses two tiny moons (Deimos and Phobos) thought to be captured asteroids.

Asteroid Belt

Asteroids are mostly small solar system bodies that are composed in significant part of rocky, non-volatile minerals.

The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be the remnants of a small terrestrial planet that failed to coalesce due to the gravitational interference of Jupiter. Asteroids range in size from hundreds of kilometers to as small as dust. All asteroids save the largest, Ceres, are classified as small solar system bodies; however, a number of other asteroids, such as Vesta and Hygeia, could potentially be reclassed as dwarf planets if it can be conclusively shown that they are spherical. The asteroid belt contains tens of thousands - and potentially millions - of objects over one kilometre in diameter.[18] However, despite their large numbers, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth.[19] In contrast to its various depictions in science fiction, the main belt is very sparsely populated; spacecraft routinely pass through without incident. Asteroids with a diameter of less than 50 m are called meteoroids.

Outer Planets

The four outer planets, or gas giants, (sometimes called Jovian planets) are so large they collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn are true giants, at 318 and 95 Earth masses, respectively, and composed largely of hydrogen and helium. Uranus and Neptune are both substantially smaller, being only 14 and 17 Earth masses, respectively. Their atmospheres contain a smaller percentage of hydrogen and helium, and a higher percentage of “ices”, such as water, ammonia and methane. For this reason some astronomers suggested that they belong in their own category, “Uranian planets,” or “ice giants.” All four of the gas giants exhibit orbital debris rings, although only the ring system of Saturn is easily observable from Earth. The term outer planet should not be confused with superior planet, which designates those planets which lie outside Earth's orbit (thus consisting of the outer planets plus Mars).

Jupiter

Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. Its composition of largely hydrogen and helium is not very different from that of the Sun, and the planet has been described as a "failed star". Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. Three of its 63 satellites, Ganymede, Io, and Europa share elements in common with the terrestrial planets, such as volcanism and internal heating. Ganymede, the largest satellite in the Solar System, has a diameter larger than Mercury.

Saturn

Saturn (9.5 AU), famous for its extensive ring system, has many qualities in common with Jupiter, including its atmospheric composition, though it is far less massive, being only 95 Earth masses. Two of its 49 moons, Titan and Enceladus, show signs of geological activity, though they are largely made of ice. Titan, like Ganymede, is larger than Mercury; it is also the only satellite in the solar system with a substantial atmosphere, similar in composition to that of the atmosphere of the early Earth.

Uranus

Uranus (19.6 AU) at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt lies at over ninety degrees to the ecliptic. Its core is remarkably cold (compared with the other gas giants; it is still several thousand degrees Celsius) and radiates very little heat into space. Uranus has 27 satellites, the largest being Titania, Oberon, Umbriel, Ariel and Miranda.

Neptune

Neptune (30 AU), though slightly smaller than Uranus, it is denser and slightly more massive, at 17 Earth masses, and radiates more internal heat than Uranus, but not as much as Jupiter or Saturn. Its peculiar ring system is composed of a number of dense "arcs" of material separated by gaps. Neptune has 13 moons. The largest, Triton, is geologically active, with geysers of liquid nitrogen, and is the only large satellite to revolve around its host planet in a prograde (clockwise) motion.

Kuiper Belt

The area beyond Neptune, often referred to as the outer solar system or simply the "trans-Neptunian region", is still largely unexplored.

This region's first formation, which actually begins inside the orbit of Neptune, is the Kuiper belt, a great ring of debris, similar to the asteroid belt but composed mainly of ice and far greater in extent, which lies between 30 and 50 AU from the Sun. This region is thought to be the place of origin for short-period comets, such as Halley's comet. Though it is composed mainly of small solar system bodies, many of the largest Kuiper belt objects could soon be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km; however, the total mass of the Kuiper belt is relatively low, perhaps barely equalling the mass of the Earth.[22] Many Kuiper belt objects have multiple satellites and most have orbits that take them outside the plane of the ecliptic.

The Kuiper belt can be roughly divided into two regions: the "resonant" belt, consisting of objects whose orbits are in some way linked to that of Neptune (orbiting, for instance, three times for every two Neptune orbits, or twice for every one), which actually begins within the orbit of Neptune itself, and the "classical" belt, consisting of objects that don't have any resonance with Neptune, and which extends from roughly 39.4 AU to 47.7 AU.

Pluto and Charon

Pluto (39 AU average), is the largest known object in the Kuiper belt and was previously accepted as the smallest planet in the Solar System. In 2006, it was reclassified as a dwarf planet by the Astronomers Congress organized by the International Astronomers Union (IAU).[24] Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion. Prior to the 2006 redefinitions, Charon was considered a moon of Pluto, but in light of the redefinition it is unclear whether Charon will continue to be classified as a moon of Pluto or as a dwarf planet itself. Charon does not orbit Pluto, but rather both bodies orbit a barycenter of gravity in empty space, making Pluto-Charon a binary system. Two much smaller moons, Nix and Hydra, orbit Pluto and Charon.

Those Kuiper belt objects which, like Pluto, possess a 3:2 orbital resonance with Neptune (ie, they orbit twice for every three Neptunian orbits) are called Plutinos. Other Kuiper belt objects have different resonant orbits (2:1, 4:7, 3:5 etc) and are grouped accordingly. The remaining Kuiper belt objects, in more "classical" orbits, are classified as Cubewanos.

Comets

Comets are small solar system bodies (usually only a few kilometres across) composed largely of volatile ices, which possess highly eccentric orbits, generally having a perihelion within the orbit of the inner planets and an aphelion far beyond Pluto. When a comet approaches the Sun, its icy surface begins to sublimate, or boil away, creating a coma; a long tail of gas and dust which is often visible with the naked eye.

There are two basic types of comet: short-period comets, with orbits less than 200 years, and long-period comets, with orbits lasting thousands of years. Short-period comets are believed to originate in the Kuiper belt, while long period comets, such as Hale-Bopp (pictured), are believed to originate in the Oort Cloud. Some comets with hyperbolic orbits may originate outside the solar system. Old comets that have had most of their volatiles driven out by solar warming are often categorized as asteroids.

Centaurs are icy comet-like bodies that have less-eccentric orbits so that they remain in the region between Jupiter and Neptune. The first centaur to be discovered, 2060 Chiron, has been called a comet since it has been shown to develop a coma just as comets do when they approach the sun.

V838 Monocerotis (V838 Mon for short) is an enigmatic variable star in the constellation Monoceros about 20,000 light years (6 kpc)[1] from the Sun. The star experienced a major outburst in early 2002.

Originally believed to be a typical nova eruption, it was quickly realized to be something completely different. The reason for the outburst is still uncertain, but several theories have been put forward, including an eruption related to stellar death processes and a merger of a binary star or planets.On January 10, 2002, a previously unknown star was

seen to brighten up in Monoceros, the Unicorn.

Being a new variable star, it was designated V838 Monocerotis, the 838th variable star of Monoceros. The initial light curve resembled that of a nova, an eruption that occurs when enough hydrogen gas has accumulated on the surface of a white dwarf from its close binary companion. Therefore it was also designated Nova Monocerotis 2002.

V838 Monocerotis reached maximum visual magnitude of 6.75 on February 6, 2002 after which it started to dim rapidly, as expected. However, in early March the star started to brighten again, this time mostly in infrared wavelengths.

Yet another brightening in infrared occurred in early April after which the star returned to near its original brightness before the eruption, magnitude 15.6. The lightcurve produced by the eruption is unlike anything previously seen.

The star brightened to about a million times solar luminosity[4] ensuring that at the time of of maximum V838 Monocerotis was one of the most luminous star in the Milky Way galaxy. The brightening was caused by a rapid expansion of the outer layers of the star. The star was observed using the Palomar Testbed Interferometer which provided a radius of 1,570 ± 400 times solar (comparable to Jupiter's orbital radius), confirming the earlier indirect calculations.

The expansion took only a couple of months meaning the expansion speed was phenomenal. The laws of thermodynamics dictate that expanding gas cools. Therefore the star became extremely cool and deep red. In fact, some astronomers argue that the spectra of the star resembled that of L-type brown dwarfs. If that is the case, V838 Monocerotis would be the first known L-type supergiant

Supergiants

Supergiants are the most massive stars. In the Hertzsprung-Russel diagram they occupy the top region of the diagram. In the Yerkes spectral classification supergiants are class Ia (most luminous supergiants) or Ib (less luminous supergiants). The most luminous supergiants are often classified as hypergiants of class 0.

Supergiants can have masses from 10 to 70 solar masses and brightness from 30,000 up to hundreds of thousands times the solar luminosity. They vary greatly in radii, usually from 30 to 500, or even in excess of 1000 solar radii. The Stefan-Boltzmann law dictates that the relatively cool surfaces of red supergiants radiate much less energy per unit area than those of blue supergiants; thus, for a given luminosity red supergiants are larger than their blue counterparts

Because of their extreme masses they have short lifespans of only 10 to 50 million years and are only observed in young cosmic structures such as open clusters, the arms of spiral galaxies, and in irregular galaxies. They are less abundant in spiral galaxy bulges, and are not observed in elliptical galaxies, or globular clusters, all of which are believed to be composed of old stars.

Supergiants occur in every spectral class from young blue class O supergiants stars to highly evolved red class M supergiants. Rigel, the brightest star in the constellation Orion is a typical blue-white supergiant, whereas Betelgeuse and Antares are red supergiants.

The modelling of supergiants is still an active area of research and is made more difficult by issues such as stellar mass loss. Rather than modelling individual stars, the latest trend has been to model clusters of stars and then compare the distribution of the resulting models with the observed supergiant distributions in galaxies like the Magellanic Clouds.

The first stars in the universe are thought to have been considerably brighter and more massive than the stars in modern universe. These stars were part of the theorized population III of stars. Their existence is necessary to explain observations of elements other than hydrogen and helium in quasars.

Most type II supernova progenitors are thought to be red supergiants. However, the progenitor for Supernova 1987A was a blue supergiant. It is believed that it was a red supergiant before losing its outer layers to its strong stellar wind.

Currently, the largest known stars in terms of physical size, not mass or luminosity, are the supergiants VV Cephei, V354 Cephei, KW Sagitarii, KY Cygni, and µ Cephei (the Garnet Star).

A planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives.

They are in fact unrelated to planets; the name originates from a

supposed similarity in appearance to giant planets. They are a short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years. About 1,500 are known to exist in the Milky Way Galaxy.

Planetary nebulae are important objects in astronomy because they play a crucial role in the chemical evolution of the galaxy, returning material to the interstellar medium which has been enriched in heavy elements and other products of nucleosynthesis (such as carbon, nitrogen, oxygen and calcium).

In other galaxies, planetary nebulae may be the only objects observable enough to yield useful information about chemical abundances.

In recent years, Hubble Space Telescope images have revealed many planetary nebulae to have extremely complex and varied morphologies. About a fifth are roughly spherical, but the majority are not spherically symmetric.

The mechanisms which produce such a wide variety of shapes and features are not yet well understood, but binary central stars, stellar winds and magnetic fields may all play a role

The Pleiades (also known as M45 or the Seven Sisters) is an open cluster in the constellation of Taurus.

It is among the nearest to the Earth of all open clusters, probably the best known and certainly the most striking to the naked eye. Accurate knowledge of the distance to the cluster is very important in astronomy as it is a crucial first step on the cosmic distance ladder, the calibration of the distance scale of the whole universe.

The Hipparcos satellite caused consternation when it measured a distance to the cluster which was 10% smaller than most previous measurements, but it was later found to have suffered from a systematic error when observing the Pleiades which led to the discrepancy.

The cluster is now known to lie at a distance of about 135 parsecs (440 light years).

The cluster is dominated by hot blue stars, which have formed within the last 100 million years. Dust that forms faint reflection nebulosity around the brightest stars was thought at first to be left over from the formation of the cluster but is now known to be an unrelated dust cloud that the stars are currently passing through.

Astronomers estimate that the cluster will survive for about another 250 million years, after which time it will have dispersed due to gravitational interactions with the spiral arms of the galaxy and giant molecular clouds.

The Pleiades are a prominent sight in the Northern Hemisphere in winter and in the Southern Hemisphere in summer, and have been known since antiquity to cultures all around the world, including the Maori and Australian Aborigines, the Japanese, the Aztec and the Sioux of North America. Some Greek astronomers considered them to be a distinct constellation, and they are mentioned by Hesiod, and in Homer's Iliad and Odyssey.

They are also mentioned three times in the Bible (Job 9:9, 38:31; Amos 5:8). The Pleiades (Kartika) are particularly revered in Hindu mythology as the seven mothers of the war god Skanda.

They have long been known to be a physically related group of stars rather than any chance alignment. The Reverend John Michell calculated in 1767 that the probability of a chance alignment of so many bright stars was only 1 in 500,000, and so correctly surmised that the Pleiades and many other clusters of stars must be physically related .

When studies were first made of the stars' proper motions, it was found that they are all moving in the same direction across the sky, at the same rate, further demonstrating that they were related.

Charles Messier measured the position of the cluster and included it as M45 in his catalogue of comet-like objects, published in 1771. Along with the Orion Nebula and the Praesepe cluster, Messier's inclusion of the Pleiades has been noted as curious, as most of Messier's objects were much fainter and more easily confused with comets—something which seems scarcely possible for the Pleiades. One possibility is that Messier simply wanted to have a larger catalogue than his scientific rival Lacaille, whose 1755 catalogue contained 42 objects, and so he added some bright, well-known objects to boost his list.

Distance

The distance to the Pleiades is an important step in calibrating distance scales for the whole universe, and has been estimated by many methods. As the cluster is so close to the Earth, its distance is relatively easy to measure. Accurate knowledge of the distance allows astronomers to plot a Hertzsprung-Russell Diagram for the cluster which, when compared to those plotted for clusters whose distance is not known, allows their distances to be estimated.

Other methods can then extend the distance scale from open clusters to galaxies and clusters of galaxies, and a cosmic distance ladder can be constructed. Ultimately astronomers' understanding of the age and future evolution of the universe is influenced by their knowledge of the distance to the Pleiades.

Results prior to the launch of the Hipparcos satellite generally found that the Pleiades were about 135 parsecs away from Earth. Hipparcos caused consternation among astronomers by finding a distance of only 118 parsecs by measuring the parallax of stars in the cluster—a technique which should yield the most direct and accurate results.

Later work has consistently found that the Hipparcos distance measurement for the Pleiades was in error, but it is not yet known why the error occurred . The distance to the Pleiades is currently thought to be the higher value of about 135 parsecs

The cluster is about 12 light years in diameter and contains approximately 500 stars in total. It is dominated by young, hot blue stars, up to 14 of which can be seen with the naked eye depending on local observing conditions. The arrangement of the brightest stars is somewhat similar to Ursa Major and Ursa Minor. The total mass contained in the cluster is estimated to be about 800 solar masses.

The cluster contains many brown dwarfs — objects with less than about 8% of the Sun's mass, which are not heavy enough for nuclear fusion reactions to start in their cores and become proper stars. They may constitute up to 25% of the total population of the cluster, although they contribute less than 2% of the total mass.

Astronomers have made great efforts to find and analyse brown dwarfs in the Pleiades and other young clusters, because they are still relatively bright and observable, while brown dwarfs in older clusters have faded and are much more difficult to study.

Also present in the cluster are several white dwarfs. Given the young age of the cluster normal stars are not expected to have had time to evolve into white dwarfs, a process which normally takes several billion years. It is believed that, rather than being individual low- to intermediate-mass stars, the progenitors of the white dwarfs must have been high-mass stars in binary systems.

Transfer of mass from the higher-mass star to its companion during its rapid evolution would result in a much quicker route to the formation of a white dwarf.

Age and future evolution

Ages for star clusters can be estimated by comparing the H-R diagram for the cluster with theoretical models of stellar evolution, and using this technique, ages for the Pleiades of between 75 and 150 million years have been estimated.

The spread in estimated ages is a result of uncertainties in stellar evolution models. In particular, models including a phenomenon known as convective overshoot, in which a convective zone within a star penetrates an otherwise non-convective zone, result in higher apparent ages.

Another way of estimating the age of the cluster is by looking at the lowest-mass objects. In normal main sequence stars, lithium is rapidly destroyed in nuclear fusion reactions, but brown dwarfs can retain their lithium.

Due to its very low ignition temperature of 2.5 million kelvins, the highest-mass brown dwarfs will burn lithium eventually, and so determining the highest mass of brown dwarfs still containing lithium in the cluster can give an idea of its age. Applying this technique to the Pleiades gives an age of about 115 million years.

Like most open clusters, the Pleiades will not stay gravitationally bound forever, as some component stars will be ejected after close encounters and others will be stripped by tidal gravitational fields. Calculations suggest that the cluster will take about 250 million years to disperse, with gravitational interactions with giant molecular clouds and the spiral arms of the galaxy also hastening its demise.

Under ideal observing conditions, some hint of nebulosity may be seen around the cluster, and this shows up in long-exposure photographs. It is a reflection nebula, caused by dust reflecting the blue light of the hot, young stars.

It was formerly thought that the dust was left over from the formation of the cluster, but at the age of about 100 million years generally accepted for the cluster, almost all the dust originally present would have been dispersed by radiation pressure. Instead, it seems that the cluster is simply passing through a particularly dusty region of the interstellar medium.

Studies show that the dust responsible for the nebulosity is not uniformly distributed, but is concentrated mainly in two layers along the line of sight to the cluster. These layers may have been formed by deceleration due to radiation pressure as the dust has moved towards the stars

In general relativity, a naked singularity is a gravitational singularity without an event horizon. The singularities inside black holes are always surrounded by an area which does not allow light to escape, and therefore cannot be directly observed. A naked singularity, by contrast, is a point in space where the density is infinite and which is observable from the outside.

The theoretical existence of naked singularities is important because their existence would mean that it would be possible to observe the collapse of an object to infinite density.

Computer simulations of the collapse of a disk of dust have indicated that these objects can exist, and thus the cosmic censorship hypothesis (stating that singularities are always hidden) does not hold. Stephen Hawking lost a bet about this question.

This is, of course, an example of a mathematical difficulty (divergence to infinity of the density) which reveals a more profound problem in our

understanding of the relevant physics involved in the process. A workable theory of quantum gravity should be able to solve problems such as this.

Fate of the universe

Main article: Ultimate fate of the universe

Depending on the average density of matter and energy in the universe, it will either keep on expanding forever or it will be gravitationally slowed down and will eventually collapse back on itself in a "Big Crunch".

Currently the evidence suggests not only that there is insufficient mass/energy to cause a recollapse, but that the expansion of the universe seems to be accelerating and will accelerate for eternity (see accelerating universe). Other ideas of the fate of our universe include the Big Rip, the Big Freeze, and Heat death of the universe theory. For a more detailed discussion of other theories, see the ultimate fate of the universe.

Big Crunch

In physical cosmology, the Big Crunch is a hypothesized collapse of the universe upon itself after its expansion eventually stops — a counterpart to the Big Bang.

If the gravitational attraction of all the matter within the observable horizon is high enough, it could slow the expansion of the universe, and then reverse it. The universe would then contract, with about the same duration as the expansion. Eventually, all matter and energy would be compressed back into a gravitational singularity. It is meaningless to ask what would happen after this, because time, as we know it, would end in this singularity.

For this to occur, the average density of matter in the universe has to be sufficient so that the overall spatial curvature of the universe is positive, like the surface of a sphere. If the matter density is less than a certain value, called the critical density, the curvature is negative (like a hyperbolic surface, which is a mathematical manifold often compared to the form of a saddle) and gravitation will be too feeble to completely counter inertia, so that expansion will continue to slow down but never come to an end.

These two cases, and the limiting case in between in which space is flat, are called the three Friedmann models. They assume the cosmological constant to be zero.

However, recent experimental evidence (namely the observation of distant supernovae as standard candles, and the well-resolved mapping of the cosmic microwave background) have—to most scientists' considerable surprise—shown that the expansion of the universe is not being slowed down by gravity, but instead, accelerating, suggesting that the universe will not end with a Big Crunch, but will instead expand forever, though some scientists have contested this theory. (The evidence of an accelerating universe has been considered conclusive by most cosmologists since 2002.)

In the framework of the field equations of the General Theory of Relativity, the simplest model of an accelerating expansion corresponds to a positive value of the cosmological constant, which can be attributed to the quantum vacuum itself exerting a force that repels gravitationally on large scales.

More generally, the accelerating expansion is attributed to dark energy, which could be the cosmological constant, or a dynamical field with negative "pressure", leading to an effective cosmological constant that could be time-varying. In such cases, it is theoretically possible that the cosmological "constant" need not remain positive, leaving open the possibility of a Big Crunch as the ultimate fate of the universe.

A Big Crunch is also still theoretically possible if Einstein's theory of general relativity were found not to apply on large scales. The current evidence neither favors nor rules out dark energy, or modifications of general relativity, of a form that could halt or reverse an eternal expansion; it does, however set lower bounds on the duration collapse (approximately 42 billion years from now, or more than 24 billion years at the 95% confidence level, according to one group led by Andrei Linde).

Accelerating Universe

The deceleration parameter q in cosmology is a dimensionless measure of the cosmic acceleration of the expansion of the universe.

Any expanding universe should have a nonincreasing Hubble parameter and the local expansion of space is always slowing (or, in the case of a cosmological constant, proceeds at a constant rate, as in de Sitter space).

Observations of the cosmic microwave background demonstrate that the universe is very nearly flat.

This implies that the universe is decelerating for any cosmic fluid with equation of state w greater than - 1 / 3 (any fluid satisfying the strong energy condition does so, as does any form of matter present in the Standard Model, but excluding inflation).

However, observations of distant type Ia supernovae indicate that q is negative; the expansion of the universe is accelerating. This is an indication that the gravitational attraction of matter, on the cosmological scale, is more than counteracted by negative pressure dark energy, in the form of either quintessence or a positive cosmological constant.

Before the first indications of an accelerating universe, in 1998, it was thought that the universe was dominated by dust with negligible equation of state, . This had suggested that the deceleration parameter was equal to one half; the experimental effort to confirm this prediction led to the discovery of acceleration.

Big Rip

The Big Rip is a cosmological hypothesis about the ultimate fate of the Universe, in which the elements of the universe, from galaxies to atoms, are progressively torn apart by the expansion of the universe.

The hypothesis relies crucially on the type of dark energy in the universe. The key value is the equation of state w, the ratio between the dark energy pressure and its energy density. At w < - 1, the universe will eventually be pulled apart. Such energy is called phantom energy, a more extreme form of quintessence.

In a phantom energy dominated universe the "fabric" of the universe expands at an ever increasing rate. However, this implies that the size of the observable universe is continually shrinking; the distance to the edge of the observable universe which is moving away at the speed of light from any point gets ever closer.

When the size of the observable universe is smaller than any particular structure, then no interaction between the furthest parts of the structure can occur, neither gravitational nor electromagnetic (nor weak or strong), and they will be ripped apart.

First, the galaxies would be separated from each other. Arguably, this is what is happening right now, with galaxies that move outside the observable universe (approximately 13.7 billion light years away). About 60 million years before the end, gravity would be too weak to hold the Milky Way and other individual galaxies together.

Approximately three months before the end, the Solar system will be gravitationally unbound. In the last minutes, stars and planets will be torn apart, and an instant before the end, atoms will be destroyed.

The authors of this hypothesis calculate that the end of the universe as we now know it would be approximately 35 billion years after the Big Bang, or 20 billion years from now.

Big Freeze

The Big Freeze is a scenario in which the universe simply becomes too cold to sustain life due to continued expansion. The Big Freeze is a theory of a possible fate of the universe. For other theories, see Ultimate fate of the universe. The Big Freeze could occur if the universe's geometry is either flat or hyperbolic, because either of those would mean that the universe would expand infinitely, causing a permanent fate.

Geometry

For the universe to expand indefinitely, the shape of the universe as a whole needs to be flat or hyperbolic; this requirement stems from the density of the universe in those particular geometric shapes. There are three likely possibilities for the universe's shape.

Possible Universe Shapes

Spherical, a curved, ball shape.

Hyperbolic, a "saddle shape".

Flat, two-dimensional shape, like a piece of paper.

Density

For the Big Freeze theory to happen, the shape of the Universe must be either hyperbolic or flat. If it is hyperbolic, which is usually pictured as a saddle shape, then the density must be lower than the critical density, which means that the universe isn't heavy enough to collapse under gravity.

If our Universe is flat, then the density will be exactly at the critical point, preventing the universe from collapsing as well. If the universe is heavier than the critical point, it will result in a shrinking universe, which leads to another theory, called the Big Crunch.

For the Big Crunch to occur, the shape of the Universe would have to be spherical. The problem with measuring the current density of our Universe is that we cannot see most of the matter in it, and it is theorized that most of the universe may be made up of dark matter, a hypothetical, invisible form of matter in space.

Expansion

Expansion of the Universe is the entire basis of the Big Freeze theory. If the shape of the Universe is hyperbolic, then the Universe will reach a fixed expansion rate and expand forever, which means that the universe will never really "die". If the universe is flat, then the Universe will continue to expand until it reaches an expansion rate of zero; a Universe in this scenario wouldn't ever "die" either.

Heat Death of the Universe

The heat death is a possible final state of the universe, in which it has "run down" to a state of no free energy to sustain motion or life. In physical terms, it has reached maximum entropy.Origins of the idea

The idea of heat death stems from the second law of thermodynamics, which states that entropy tends to increase in an isolated system.

If the universe lasts for a sufficient time, it will asymptotically approach a state where all energy is evenly distributed. Hermann von Helmholtz is thought to be the first to propose the idea of heat death in 1854, 11 years before Clausius's definitive formulation of the Second law of thermodynamics in terms of entropy (1865). However, observations about the loss of available energy as heat had been formulated by Sadi Carnot as early as 1824.

Temperature of the universe

Despite the term "heat death", the temperature of the entire universe would be very close to absolute zero in this scenario. Heat death is however not quite the same as "cold death" or the "Big Freeze" in which the universe simply becomes too cold to sustain life due to continued expansion, though the result is quite similar .

Current status

Inflationary cosmology suggests that in the early universe (or, more accurately, the small part of it from which the currently observed universe stemmed) before cosmic expansion the energy was uniformly distributed[1] and thus it was in a state superficially similar to heat death.

However, the two states are in fact very different: in the early universe gravity was a very important force, and in a gravitational system if the energy is uniformly distributed the entropy is quite low, compared to a state in which most matter has collapsed into black holes.

Thus it is not in thermal equilibrium, and in fact there is no thermal equilibrium for such a system (it is thermodynamically unstable)[2][3]. However, in the heat death scenario the energy density is so low that the system can be thought of as non-gravitational, and a state in which the energy is uniformly distributed is a thermal equilibrium state, i.e. the state of maximal entropy.

Meanwhile, in an expanding universe, some believe the maximum possible entropy increases far more quickly than the actual entropy with each time increment, pushing the universe continually further away from an equilibrium state despite increasing entropy. Furthermore, the very notion of thermodynamic modelling of the universe has been questioned, since the effects of such factors as gravity and quantum phenomena are very difficult to reconcile with simple thermodynamic models, rendering the utility of such models as predictive systems highly doubtful according to some.

Nonetheless, assuming that the second law of thermodynamics is an appropriate model and the Universe is a closed system, the scientific evidence overwhelmingly points to an eventual heat death.

However, all models of the universe assume its approximate homogeneity in large scales. Advanced living beings in the far future may in principle be able to change this (presumably by dragging galaxies from one place to another), thus changing the fate of at least a small part of the universe. See Final anthropic principle for a discussion of another perspective of the idea that the second law does not imply life's eventual extinction.

Timeline for heat death

The Degenerate Age - from 1014 to 1040 years

Galaxy and star formation ceases: 1014 years

Stellar formation stops, leaving matter to decay over a very long period of time. The hydrogen fuel used for fusion by stars will be eventually depleted, leaving all matter in the Universe in a compact state populated by the following objects after all stars burn out:

Planets and planetoids (this category includes asteroids, comets, brown dwarfs, etc.)

White dwarfs

Neutron stars

Quark stars

Black Holes

Formerly luminous bodies like stars cool and dim, eventually reaching the same temperature as the Universe's microwave background radiation.

Planets are flung from orbits: 1015 years

Over time, the orbits of planets are kicked into other masses (see above) or scattered throughout the Universe due to gravitational perturbations.

Stars are flung from orbits: 1016 years

The same scattering effect happens to stars and their remnants within galaxies, leaving mostly scattered stellar debris and supermassive black holes.

An estimated 1/2 of protons decay: 1036 years

If estimates on the half-life of protons are correct, then one-half of all the free-floating matter in the Universe has been converted into gamma radiation and leptons through proton decay.

All protons decay: 1040 years

If estimates on the half-life of protons are correct, then these particles (and nucleonic neutrons as well) have now undergone roughly 10,000 half-lives. To put this into perspective: There are an estimated 1080 protons in the Universe, and the estimated half-life for protons is 1036 years. That means the proton's numbers have been slashed in half 10,000 times. If one does the math, there are now roughly 10-3,000 as many protons as there were at the beginning of the Universe.

So that means the total number of remaining protons in the Universe at the end of the Degenerate Age would be far less than one (a very tiny fraction something like 3,000 zeroes after the decimal place before the first significant digit). Effectively, all matter is now contained in the only bodies in the Universe immune to proton decay: black holes.

Note: This number is based on loose estimates as the exact value for the half-life of protons is an unknown quantity with only a known lower-bound. The end of the Degenerate Era is meant to mark the end of baryonic matter's influence on the Universe, so the estimate for how long this era will last may change if and when the exact value for proton decay is pinned down. The specific numerical values are not meant to be taken literally, and are provided only for demonstration purposes.

The Black Hole Age - from 1040 years to 10100 years

Black holes dominate: 1040 years

Black holes continue to evaporate via Hawking radiation, but this process is very slow.

Black holes disintegrate: 10100 years

Few if any black holes remain; virtually all matter is now converted into photons.

See also 1019 seconds for times further than 3 billion years into the future.

Ultimate fate

The Dark Age - from 10100 years until 10150 years

All Black Holes now Disintegrated: 10150 years

The remaining black holes evaporate: first the small ones, and then the supermassive black holes. All matter that used to make up the stars and galaxies has now degenerated into photons and leptons.

The Photon Age - from 10150 years until Distant Time

The Universe Achieves Low-Energy State: 101000 years and beyond

The Universe now reaches extreme low-energy state. What happens after this is speculative. It's possible a Big Rip event may occur far off into the future, or the Universe may settle into this state forever, achieving true heat death. Extreme low-energy states imply that localized quantum events become major macro-scale phenomenon rather than micro-scale non-events because the smallest perturbations make the biggest difference in this era, so there is no telling what may happen to space or time during this era.

A black hole is an object predicted by general relativity with a gravitational field so strong that nothing can escape it — not even light.

A black hole is defined to be a region of space-time where escape to the outside universe is impossible. The boundary of this region is a surface called the event horizon. This surface is not a physically tangible one, but merely a figurative concept of an imaginary boundary. Nothing can move from inside the event horizon to the outside, even briefly.

The existence of black holes in the universe is well supported by astronomical observation, particularly from studying X-ray emission from X-ray binaries and active galactic nuclei. It has also been

hypothesized that black holes radiate energy due to quantum mechanical effects known as Hawking radiation.

The concept of a body so massive that even light could not escape was put forward by the English geologist John Michell in a 1784 paper sent to the Royal Society. At that time, the Newtonian theory of gravity and the concept of escape velocity were well known. Michell computed that a body with 500 times the radius of the Sun and of the same density would have, at its surface, an escape velocity equal to the speed of light, and therefore would be invisible. In his words: 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 (inertial mass), with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity.

Although he thought it unlikely, Michell considered the possibility that many such objects that cannot be seen might be present in the cosmos.

In 1796, the French mathematician Pierre-Simon Laplace promoted the same idea in the first and second edition of his book Exposition du système du Monde. It disappeared in later editions. The whole idea gained little attention in the 19th century, since light was thought to be a massless wave, not influenced by gravity.

In 1915, Albert Einstein developed the theory of gravity called General Relativity. Earlier he had shown that gravity does influence light. A few months later, Karl Schwarzschild gave the solution for the gravitational field of a point mass, showing that something we now call a black hole could theoretically exist. The Schwarzschild radius is now known to be the radius of the event horizon of a non-rotating black hole, but this was not well understood at that time. Schwarzschild himself thought it was not physical. In a remarkable coincidence, the name Schwarzschild actually translates into black shield. In another coincidence, only a few months after Schwarzschild, a student of Lorentz, Johannes Droste, independently gave the same solution for the point mass as Schwarzschild had and wrote even more extensively about its properties.

In the 1920s, Subrahmanyan Chandrasekhar argued that special relativity demonstrated that a non-radiating body above 1.44 solar masses, now known as the Chandrasekhar limit, would collapse since there was nothing known at that time that could stop it from doing so. His arguments were opposed by Arthur Eddington, who believed that something would inevitably stop the collapse. Both were correct, since a white dwarf more massive than the Chandrasekhar limit will collapse into a neutron star. However, a neutron star above about three solar masses (the Tolman-Oppenheimer-Volkoff limit) will itself become unstable against collapse due to similar physics.

In 1939, Robert Oppenheimer and H. Snyder predicted that massive stars could undergo a dramatic gravitational collapse. Black holes could, in principle, be formed in nature. Such objects for a while were called frozen stars since the collapse would be observed to rapidly slow down and become heavily redshifted near the Schwarzschild radius. The mathematics showed that an outside observer would see the surface of the star frozen in time at the instant where it crosses that radius. However, these hypothetical objects were not the topic of much interest until the late 1960s. Most physicists believed that they were a peculiar feature of the highly symmetric solution found by Schwarzschild, and that objects collapsing in nature would not form black holes.

Interest in black holes was rekindled in 1967 because of theoretical and experimental progress. Stephen Hawking and Roger Penrose proved that black holes are a generic feature in Einstein's theory of gravity, and cannot be avoided in some collapsing objects. Interest was renewed in the astronomical community with the discovery of pulsars. Shortly thereafter, the use of the expression "black hole" was coined by theoretical physicist John Wheeler,being first used in his public lecture Our Universe: the Known and Unknown on 29 December, 1967. The older Newtonian objects of Michell and Laplace are often referred to as "dark stars" to distinguish them from the "black holes" of general relativity.

Formation

General relativity (as well as most other metric theories of gravity) not only says that black holes can exist, but in fact predicts that they will be formed in nature whenever a sufficient amount of mass gets packed in a given region of space, through a process called gravitational collapse; as the mass inside the given region of space increases, its gravity becomes stronger and (in the language of relativity) increasingly deforms the space around it, ultimately until nothing (not even light) can escape the gravity; at this point an event horizon is formed, and matter and energy must inevitably collapse to a density beyond the limits of known physics. For example, if you compressed the Sun to a radius of three kilometers (about four millionths of its present size), the resulting high density would create an event horizon around it, and thus a black hole.

A quantitative analysis of this idea led to the prediction that a stellar remnant above about three to five times the mass of the Sun (the Tolman-Oppenheimer-Volkoff limit) would be unable to support itself as a neutron star via degeneracy pressure, and would inevitably collapse into a black hole. Stellar remnants with this mass are expected to be produced immediately at the end of the lives of stars that are more than 25 to 50 times the mass of the Sun, or by accretion of matter onto an existing neutron star.

Stellar collapse will generate black holes containing at least three solar masses. Black holes smaller than this limit can only be created if their matter is subjected to sufficient pressure from some source other than self-gravitation. The enormous pressures needed for this are thought to have existed in the very early stages of the universe, possibly creating primordial black holes which could have masses smaller than that of the Sun.

Supermassive black holes are believed to exist in the center of most galaxies, including our own Milky Way. This type of black hole contains millions to billions of solar masses, and there are several models of how they might have been formed. The first is via gravitational collapse of a dense cluster of stars. A second is by large amounts of mass accreting onto a "seed" black hole of stellar mass. A third is by repeated fusion of smaller black holes.

Intermediate-mass black holes have a mass between that of stellar and supermassive black holes, typically in the range of thousands of solar masses. Intermediate-mass black holes have been proposed as a possible power source for ultra-luminous X ray sources, and in 2004 detection was claimed of an intermediate-mass black hole orbiting the Sagittarius A* supermassive black hole candidate at the core of the Milky Way galaxy. This detection is disputed.

Certain models of unification of the four fundamental forces allow the formation of micro black holes under laboratory conditions. These postulate that the energy at which gravity is unified with the other forces is comparable to the energy at which the other three are unified, as opposed to being the Planck energy (which is much higher). This would allow production of extremely short-lived black holes in terrestrial particle accelerators. No conclusive evidence of this type of black hole production has been presented, though even a negative result improves constraints on compactification of extra dimensions from string theory or other models of physics.

In theory, no object beyond the event horizon of a black hole can ever escape, including light. However, black holes can be inductively detected from observation of phenomena near them, such as gravitational lensing, galactic jets, and stars that appear to be in orbit around space where there is no visible matter.

The most conspicuous effects are believed to come from matter accreting onto a black hole, which is predicted to collect into an extremely hot and fast-spinning accretion disk. The internal viscosity of the disk causes it to become extremely hot, and emit large amounts of X-ray and ultraviolet radiation. This process is extremely efficient and can convert about 10% of the rest mass energy of an object into radiation, as opposed to nuclear fusion which can only convert a few percent of the mass to energy. Other observed effects are narrow jets of particles at relativistic speeds heading along the disk's axis.

However, accretion disks, jets, and orbiting objects are found not only around black holes, but also around other objects such as neutron stars and white dwarfs; and the dynamics of bodies near these non-black hole attractors is largely similar to that of bodies around black holes. It is currently a very complex and active field of research involving magnetic fields and plasma physics to disentangle what is going on. Hence, for the most part, observations of accretion disks and orbital motions merely indicate that there is a compact object of a certain mass, and says very little about the nature of that object.

The identification of an object as a black hole requires the further assumption that no other object (or bound system of objects) could be so massive and compact. Most astrophysicists accept that this is the case, since according to general relativity, any concentration of matter of sufficient density must necessarily collapse into a black hole.

One important observable difference between black holes and other compact massive objects is that any infalling matter will eventually collide with the latter at relativistic speeds, leading to emission as the kinetic energy of the matter is thermalised. In addition thermonuclear "burning" may occur on the surface as material builds up. These processes produce irregular intense flares of X-rays and other hard radiation. Thus the lack of such flare-ups around a compact concentration of mass is taken as evidence that the object is a black hole, with no surface onto which matter can collect.

There is now a great deal of indirect astronomical observational evidence for black holes in two mass ranges:

stellar mass black holes with masses of a typical star (4–15 times the mass of our Sun), and

supermassive black holes with masses ranging from on the order of 105 to 1010 solar masses.

Additionally, there is some evidence for intermediate-mass black holes (IMBHs), those with masses of a few hundred to a few thousand times that of the Sun. These black holes may be responsible for the emission from ultraluminous X-ray sources (ULXs).

Candidates for stellar-mass black holes were identified mainly by the presence of accretion disks of the right size and speed, without the irregular flare-ups that are expected from disks around other compact objects. Stellar-mass black holes may be involved in gamma ray bursts (GRBs); short duration GRBs are believed to be caused by colliding neutron stars, which form a black hole on merging.

Observations of long GRBs in association with supernovae suggest that long GRBs are caused by collapsars; a massive star whose core collapses to form a black hole, drawing in the surrounding material. Therefore, a GRB could possibly signal the birth of a new black hole, aiding efforts to search for them.

Candidates for more massive black holes were first provided by the active galactic nuclei and quasars, discovered by radioastronomers in the 1960s. The efficient conversion of mass into energy by friction in the accretion disk of a black hole seems to be the only explanation for the copious amounts of energy generated by such objects. Indeed the introduction of this theory in the 1970s removed a major objection to the belief that quasars were distant galaxies — namely, that no physical mechanism could generate that much energy.

From observations in the 1980s of motions of stars around the galactic centre, it is now believed that such supermassive black holes exist in the centre of most galaxies, including our own Milky Way. Sagittarius A* is now generally agreed to be the location of a supermassive black hole at the centre of the Milky Way galaxy. The orbits of stars within a few AU of Sagittarius A* rule out any object other than a black hole at the centre of the Milky Way assuming the current standard laws of physics are correct.

The current picture is that all galaxies may have a supermassive black hole in their centre, and that this black hole accretes gas and dust in the middle of the galaxies generating huge amounts of radiation — until all the nearby mass has been swallowed and the process shuts off. This picture may also explain why there are no nearby quasars.

Although the details are still not clear, it seems that the growth of the black hole is intimately related to the growth of the spheroidal component — an elliptical galaxy, or the bulge of a spiral galaxy — in which it lives.

In 2002, the Hubble Telescope identified evidence indicating that intermediate size black holes exist in globular clusters named M15 and G1. The evidence for the black holes stemmed from the orbital velocity of the stars in the globular clusters; however, a group of neutron stars could cause similar observations.

Recent discoveries

In 2004, astronomers found 31 candidate supermassive black holes from searching obscured quasars. The lead scientist said that there are from two to five times as many supermassive black holes as previously predicted.

In June 2004 astronomers found a super-massive black hole, Q0906+6930, at the centre of a distant galaxy about 12.7 billion light years away. This observation indicated rapid creation of super-massive black holes in the early universe.

In November 2004 a team of astronomers reported the discovery of the first intermediate-mass black hole in our Galaxy, orbiting three light-years from Sagittarius A*. This medium black hole of 1,300 solar masses is within a cluster of seven stars, possibly the remnant of a massive star cluster that has been stripped down by the Galactic Centre. This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars.

In February 2005, a blue giant star SDSS J090745.0+24507 was found to be leaving the Milky Way at twice the escape velocity (0.0022 of the speed of light), having been catapulted out of the galactic core which its path can be traced back to. The high velocity of this star supports the hypothesis of a super-massive black hole in the centre of the galaxy.

The formation of micro black holes on Earth in particle accelerators has been tentatively reported,but not yet confirmed. So far there are no observed candidates for primordial black holes.

Features and theories

Black holes require the general relativistic concept of a curved spacetime: their most striking properties rely on a distortion of the geometry of the space surrounding them.

Gravitational field

The gravitational field outside a black hole is identical to the field produced by any other spherically symmetric object of the same mass. The popular conception of black holes as "sucking" things in is false: objects can orbit around black holes indefinitely without getting any closer. The strange properties of spacetime only become noticeable closer to the black hole.

Event horizon

The "surface" of a black hole is the so-called event horizon, an imaginary surface surrounding the mass of the black hole. Stephen Hawking proved that the topology of the event horizon of a non-spinning black hole is a sphere. At the event horizon, the escape velocity is equal to the speed of light. Anything inside the event horizon, including a photon, is prevented from escaping across the event horizon by the extremely strong gravitational field. Particles from outside this region can fall in, cross the event horizon, and will never be able to leave.

Since external observers cannot probe the interior of a black hole, according to classical general relativity, black holes can be entirely characterised according to three parameters: mass, angular momentum, and electric charge. This principle is summarised by the saying, coined by John Wheeler, "black holes have no hair" meaning that there are no features that distinguish one black hole from another, other than mass, charge, and angular momentum.

Space-time distortion and frame of reference

Objects in a gravitational field experience a slowing down of time, called time dilation. This phenomenon has been verified experimentally in the Scout rocket experiment of 1976, and is, for example, taken into account in the Global Positioning System (GPS). Near the event horizon, the time dilation increases rapidly.

From the viewpoint of a distant observer, an object falling into a black hole appears to slow down, approaching but never quite reaching the event horizon. As the object falls into the black hole, it appears redder and dimmer to the distant observer, due to the extreme gravitational red shift caused by the gravity of the black hole. Eventually, the falling object becomes so dim that it can no longer be seen, at a point just before it reaches the event horizon.

From the viewpoint of the falling object, nothing particularly special happens at the event horizon. The object crosses the event horizon and reaches the singularity at the center within a finite amount of proper time, as measured by a watch carried with the falling observer.

From the viewpoint of the falling observer distant objects may appear either blue-shifted or red-shifted, depending on his exact trajectory. Light is blue-shifted by the gravity of the black hole, but is red-shifted by the velocity of the falling object.

Inside the event horizon

Spacetime inside the event horizon of an uncharged non-rotating black hole is peculiar in that the singularity is in every observer's future, so all particles within the event horizon move inexorably towards it (Penrose and Hawking). This means that there is a conceptual inaccuracy in the non-relativistic concept of a black hole as originally proposed by John Michell in 1783. In Michell's theory, the escape velocity equals the speed of light, but it would still, for example, be theoretically possible to hoist an object out of a black hole using a rope.

General relativity eliminates such loopholes, because once an object is inside the event horizon, its time-line contains an end-point to time itself, and no possible world-lines come back out through the event horizon. A consequence of this is that a pilot in a powerful rocket ship that had just crossed the event horizon who tried to accelerate away from the singularity would reach it sooner in his frame, since geodesics (unaccelerated paths) are paths that maximise proper time.

As the object continues to approach the singularity, it will be stretched radially with respect to the black hole and compressed in directions perpendicular to this axis. This phenomenon, called spaghettification, occurs as a result of tidal forces: the parts of the object closer to the singularity feel a stronger pull towards it (causing stretching along the axis), and all parts are pulled in the direction of the singularity, which is only aligned with the object's average motion along the axis of the object (causing compression towards the axis).

Singularity

At the center of the black hole, well inside the event horizon, general relativity predicts a singularity, a place where the curvature of spacetime becomes infinite and gravitational forces become infinitely strong.

It is expected that future refinements or generalisations of general relativity (in particular quantum gravity) will change what is thought about the nature of black hole interiors. Most theorists interpret the mathematical singularity of the equations as indicating that the current theory is not complete, and that new phenomena must come into play as one approaches the singularity.

The cosmic censorship hypothesis asserts that there are no naked singularities in general relativity. This hypothesis is that every singularity is hidden behind an event horizon and cannot be probed. Whether this hypothesis is true remains an active area of theoretical research.

Rotating black holes

According to theory, the event horizon of a black hole that is not spinning is spherical, and its singularity is (informally speaking) a single point. If the black hole carries angular momentum (inherited from a star that is spinning at the time of its collapse), it begins to drag space-time surrounding the event horizon in an effect known as frame-dragging. This spinning area surrounding the event horizon is called the ergosphere and has an ellipsoidal shape.

Since the ergosphere is located outside the event horizon, objects can exist within the ergosphere without falling into the hole. However, because space-time itself is moving in the ergosphere, it is impossible for objects to remain in a fixed position. Objects grazing the ergosphere could in some circumstances be catapulted outwards at great speed, extracting energy (and angular momentum) from the hole, hence the Greek name ergosphere ("sphere of work") because it is capable of doing work.

The singularity inside a rotating black hole is a ring. It is possible for an observer to avoid hitting this singularity, for example, by proceeding along the black hole spin axis; however, it is still not possible to escape the black hole's event horizon.

Entropy and Hawking radiation

In 1971, Stephen Hawking showed that the total area of the event horizons of any collection of classical black holes can never decrease. This sounded remarkably similar to the Second Law of Thermodynamics, with area playing the role of entropy. Classically, one could violate the second law of thermodynamics by material entering a black hole disappearing from our universe and resulting in a decrease of the total entropy of the universe. Therefore, Jacob Bekenstein proposed that a black hole should have an entropy and that it should be proportional to its horizon area. Since black holes do not classically emit radiation, the thermodynamic viewpoint was simply an analogy.

However, in 1974, Hawking applied quantum field theory to the curved spacetime around the event horizon and discovered that black holes can emit Hawking radiation, a form of thermal radiation. Using the first law of black hole mechanics, it follows that the entropy of a black hole is one quarter of the area of the horizon. This is a universal result and can be extended to apply to cosmological horizons such as in de Sitter space. It was later suggested that black holes are maximum-entropy objects, meaning that the maximum entropy of a region of space is the entropy of the largest black hole that can fit into it. This led to the holographic principle.

The Hawking radiation reflects a characteristic temperature of the black hole, which can be calculated from its entropy. This temperature in fact falls the more massive a black hole becomes: the more energy a black hole absorbs, the colder it gets. A black hole with roughly the mass of the planet Mercury would have a temperature in equilibrium with the cosmic microwave background radiation (about 2.73 K).

More massive than this, a black hole will be colder than the background radiation, and it will gain energy from the background faster than it gives energy up through Hawking radiation, becoming even colder still. However, for a less massive black hole the effect implies that the mass of the black hole will slowly evaporate with time, with the black hole becoming hotter and hotter as it does so.

Although these effects are negligible for black holes massive enough to have been formed astronomically, they would rapidly become significant for hypothetical smaller black holes, where quantum-mechanical effects dominate. Indeed, small black holes are predicted to undergo runaway evaporation and eventually vanish in a burst of radiation.

Black hole unitarity

An open question in fundamental physics is the so-called information loss paradox, or black hole unitarity paradox. Classically, the laws of physics are the same run forward or in reverse. That is, if the position and velocity of every particle in the universe were measured, we could (disregarding chaos) work backwards to discover the history of the universe arbitrarily far in the past. In quantum mechanics, this corresponds to a vital property called unitarity which has to do with the conservation of probability.

Black holes, however, might violate this rule. The position under classical general relativity is subtle but straightforward: because of the classical no hair theorem, we can never determine what went into the black hole. However, as seen from the outside, information is never actually destroyed, as matter falling into the black hole appears from the outside to become more and more red-shifted as it approaches (but never ultimately appears to reach) the event horizon.

Ideas of quantum gravity, on the other hand, suggest that there can only be a limited finite entropy (ie a maximum finite amount of information) associated with the space near the horizon; but the change in the entropy of the horizon plus the entropy of the Hawking radiation is always sufficient to take up all of the entropy of matter and energy falling into the black hole.

Many physicists are concerned however that this is still not sufficiently well understood. In particular, at a quantum level, is the quantum state of the Hawking radiation uniquely determined by the history of what has fallen into the black hole; and is the history of what has fallen into the black hole uniquely determined by the quantum state of the black hole and the radiation? This is what determinism, and unitarity, would require.

For a long time Stephen Hawking had opposed such ideas, holding to his original 1975 position that the Hawking radiation is entirely thermal and therefore entirely random, representing new nondeterministically created information. However, on 21 July 2004 he presented a new argument, reversing his previous position.

On this new calculation, the entropy associated with the black hole itself would still be inaccessible to external observers; and in the absence of this information, it is impossible to relate in a 1:1 way the information in the Hawking radiation (embodied in its detailed internal correlations) to the initial state of the system. However, if the black hole evaporates completely, then such an identification can be made, and unitarity is preserved.

It is not clear how far even the specialist scientific community is yet persuaded by the mathematical machinery Hawking has used (indeed many regard all work on quantum gravity so far as highly speculative); but Hawking himself found it sufficiently convincing to pay out on a bet he had made in 1997 with Caltech physicist John Preskill, to considerable media interest.

Alternative models

Several alternative models, which behave like a black hole but avoid the singularity, have been proposed. But most researchers judge these concepts artificial, as they are more complicated but do not give near term observable differences from black holes (see Occam's razor). The most prominent alternative theory is the Gravastar.

In March 2005, physicist George Chapline at the Lawrence Livermore National Laboratory in California proposed that black holes do not exist, and that objects currently thought to be black holes are actually dark-energy stars. He draws this conclusion from some quantum mechanical analyses. Although his proposal currently has little support in the physics community, it was widely reported by the media.

Among the alternate models are clusters of elementary particles (e.g., boson stars), fermion balls, self-gravitating, degenerate heavy neutrinos and even clusters of very low mass (~0.04 Msolar) black holes.

An object with mean density greater or equal to the critical density and with a radius equal to that of the observable universe is a black hole. Our visible universe does not have a singularity like the one associated with this kind of black hole.

Another alternative concept to black holes, known as Magnetospheric Eternally Collapsing Objects or MECOs has been put forward, and was featured in a July 2006 New Scientist article. MECOs are distinguished from black holes in that they do not possess an event horizon, but do possess a strong intrinsic magnetic field (which a black hole would not have). It is not possible for both MECOs and black holes to exist in the universe, so a confirmed discovery of one would probably disprove the existence of the other. However, like the other alternative models discussed here, MECOs have not gained general acceptance in the scientific community at the present time.

General Relativity by Albert Einstein

General relativity (GR) is the geometrical theory of gravitation published by Albert Einstein in 1915.It unifies special relativity and Sir Isaac Newton's law of universal gravitation with the insight that gravitation is not due to a force but rather is a manifestation of curved space and time, this curvature being produced by the mass-energy and momentum content of the spacetime. General relativity is distinguished from other metric theories of gravitation by its use of the Einstein field equations to relate spacetime content and spacetime curvature.

In this theory, spacetime is treated as a 4-dimensional Lorentzian manifold which is curved by the presence of mass, energy and momentum (or stress-energy) within it. The relationship between stress-energy and the curvature of spacetime is described by the Einstein field equations. The motion of objects being influenced solely by the geometry of spacetime (inertial motion) occurs along special paths called timelike and null geodesics of spacetime.

One of the defining features of general relativity is the idea that gravitational 'force' is replaced by geometry. In general relativity, phenomena that in classical mechanics are ascribed to the action of the force of gravity (such as free-fall, orbital motion, and spacecraft trajectories) are taken in general relativity to represent inertial motion in a curved spacetime. So what people standing on the surface of the Earth perceive as the 'force of gravity' is a result of their undergoing a continuous physical acceleration caused by the mechanical resistance of the surface on which they are standing.

Justification

The justification for creating general relativity came from the equivalence principle, which dictates that freefalling observers are the ones in inertial motion. A consequence of this insight is that inertial observers can accelerate with respect to each other. (Think of two balls falling on opposite sides of the Earth, for example.) This redefinition is incompatible with Newton's first law of motion, and cannot be accounted for in the Euclidean geometry of special relativity. To quote Einstein himself:

"If all accelerated systems are equivalent, then Euclidean geometry cannot hold in all of them."

Thus the equivalence principle led Einstein to search for a gravitational theory which involves curved spacetimes.

Another motivating factor was the realization that relativity calls for gravitation to be expressed as a rank-two tensor, and not just a vector as was the case in Newtonian physics (An analogy is the electromagnetic field tensor of special relativity). Thus, Einstein sought a rank-two tensor means of describing curved spacetimes surrounding massive objects. This effort came to fruition with the discovery of the Einstein field equations in 1915.

Fundamental principles

General relativity is based on the following set of fundamental principles which guided its development:

The general principle of relativity: The laws of physics must be the same for all observers (accelerated or not).

The principle of general covariance: The laws of physics must take the same form in all coordinate systems.

The principle that inertial motion is geodesic motion: The world lines of particles unaffected by physical forces are timelike or null geodesics of spacetime.

The principle of local Lorentz invariance: The laws of special relativity apply locally for all inertial observers.

Spacetime is curved: This permits gravitational effects such as freefall to be described as a form of inertial motion. (See the discussion below of a person standing on Earth, under "Coordinate vs. physical acceleration.")

Spacetime curvature is created by stress-energy within the spacetime: This is described in general relativity by the Einstein field equations.

(The equivalence principle,[5] which was the starting point for the development of general relativity, ended up being a consequence of the general principle of relativity and the principle that inertial motion is geodesic motion.)

Spacetime as a curved Lorentzian manifold

In general relativity, the spacetime concept introduced by Hermann Minkowski for special relativity is modified. More specifically, general relativity stipulates that spacetime is:

curved: Spacetime has a non-Euclidean geometry. In special relativity, spacetime is flat.

Lorentzian: The metrics of spacetime must have a mixed metric signature. This is inherited from special relativity.

four dimensional: to cover the three spatial dimensions and time. This is also inherited from special relativity.

The curvature of spacetime (caused by the presence of stress-energy) can be viewed intuitively in the following way. Placing a heavy object such as a bowling ball on a trampoline will produce a 'dent' in the trampoline. This is analogous to a large mass such as the Earth causing the local spacetime geometry to curve. This is represented by the image at the top of this article. The larger the mass, the bigger the amount of curvature. A relatively light object placed in the vicinity of the 'dent', such as a ping-pong ball, will accelerate towards the bowling ball in a manner governed by the 'dent'. Firing the ping-pong ball at just the right speed towards the 'dent' will result in the ping-pong ball 'orbiting' the bowling ball. This is analogous to the Moon orbiting the Earth, for example.

Similarly, in general relativity massive objects do not directly impart a force on other massive objects as hypothesized in Newton's action at a distance idea. Instead (in a manner analogous to the ping-pong ball's response to the bowling ball's dent rather than the bowling ball itself), other massive objects respond to how the first massive object curves spacetime.

The mathematics of general relativity

Main article: Mathematics of general relativity

Due to the expectation that spacetime is curved, Riemannian geometry (a type of non-Euclidean geometry) must be used. In essence, spacetime does not adhere to the "common sense" rules of Euclidean geometry, but instead objects that were initially traveling in parallel paths through spacetime (meaning that their velocities do not differ to first order in their separation) come to travel in a non-parallel fashion. This effect is called geodesic deviation, and it is used in general relativity as an alternative to gravity.

For example, two people on the Earth heading due north from different positions on the equator are initially traveling on parallel paths, yet at the north pole those paths will cross. Similarly, two balls initially at rest with respect to and above the surface of the Earth (which are parallel paths by virtue of being at rest with respect to each other) come to have a converging component of relative velocity as both accelerate towards the center of the Earth due to their subsequent freefall. (Another way of looking at this is how a single ball moving in a purely timelike fashion parallel to the center of the Earth comes through geodesic motion to be moving towards the center of the Earth.)

The requirements of the mathematics of general relativity are further modified by the other principles. Local Lorentz Invariance requires that the manifolds described in GR be 4-dimensional and Lorentzian instead of Riemannian. In addition, the principle of general covariance forces that mathematics to be expressed using tensor calculus. Tensor calculus permits a manifold as mapped with a coordinate system to be equipped with a metric tensor of spacetime which describes the incremental (spacetime) intervals between coordinates from which both the geodesic equations of motion and the curvature tensor of the spacetime can be ascertained.

Coordinate vs. physical acceleration

One of the greatest sources of confusion about general relativity comes from the need to distinguish between coordinate and physical accelerations.

In classical mechanics, space is preferentially mapped with a Cartesian coordinate system. Inertial motion then occurs as one moves through this space at a consistent coordinate rate with respect to time. Any change in this rate of progression must be due to a force, and therefore a physical and coordinate acceleration were in classical mechanics one and the same. It is important to note that in special relativity that same kind of Cartesian coordinate system was used, with time being added as a fourth dimension and defined for an observer using the Einstein synchronization procedure. As a result, physical and coordinate acceleration correspond in special relativity too, although their magnitudes may vary.

In general relativity, the elegance of a flat spacetime and the ability to use a preferred coordinate system are lost (due to stress-energy curving spacetime and the principle of general covariance). Consequently, coordinate and physical accelerations become sundered. For example: Try using a radial coordinate system in classical mechanics. In this system, an inertially moving object which passes by (instead of through) the origin point is found to first be moving mostly inwards, then to be moving tangentially with respect to the origin, and finally to be moving outwards, yet is moving in a straight line. This is an example of an inertially moving object undergoing a coordinate acceleration, and the way this coordinate acceleration changes as the object travels is given by the geodesic equations for the manifold and coordinate system in use.

Another more direct example is the case of someone standing on the Earth, where they are at rest with respect to the surface coordinates for the Earth (latitude, longitude, and elevation) but are undergoing a continuous physical acceleration because the mechanical resistance of the Earth's surface keeps them from free falling.

Predictions of general relativity

(For more detailed information about tests and predictions of general relativity, see tests of general relativity).

Gravitational effects

Acceleration effects

These effects occur in any accelerated frame of reference, and are therefore independent of the curvature of spacetime. (Note however that spacetime curvature usually is the source of the causative acceleration when these effects are being observed.)

Gravitational redshifting of light: The frequency of light will decrease (shifting visible light towards the red end of the spectrum) as it moves to higher gravitational potentials (out of a gravity well). Confirmed by the Pound-Rebka experiment.[6][7][8]

Gravitational time dilation: Clocks will run slower at lower gravitational potentials (deeper within a gravity well). Confirmed by the Hafele-Keating experiment[9][10] and GPS.[citations needed]

Shapiro effect (also known as gravitational time delay): Signals will take longer than expected to move through a gravitational field. Confirmed through observations of signals from spacecraft and pulsars passing behind the Sun as seen from the Earth.

Bending of light

This bending also occurs in any accelerated frame of reference. However, the details of the bending and therefore the gravitational lensing effects are governed by spacetime curvature.

The magnitude of this effect is twice the Newtonian prediction. It was confirmed by astronomical observations during eclipses of the Sun and observations of pulsars passing behind the Sun.

Gravitational lensing: One distant object in front of or close to being in front of another much more distant object can change how the more distant object is seen. These effects include

Multiple views of the same object: Observations of quasars whose light passes close to an intervening galaxy.

Brightening of a star due to the focusing effects of a planet or another star passing in front of it: Such "microlensing" events are now regularly observed.

Einstein rings and arcs: One object directly behind another can make the more distant object's light appear as a ring. When almost directly behind, the result is an arc. Observed for distant galaxies.

Orbital effects

These are ways in which the celestial mechanics of general relativity differs from that of classical mechanics.

Non-Newtonian periapsis precession: The apsides of orbits precess more than expected under Newton's theory of gravity. This has been confirmed for Mercury and observed in several binary pulsars.

Orbital decay due to the emission of gravitational radiation: This has been observed in binary pulsars.

Geodetic precession: Because of the curvature of spacetime, the orientation of an orbiting gyroscope will change over time. This is being tested by Gravity Probe B.

Rotational effects

These involve the behavior of spacetime around a rotating massive object.

Frame dragging: A rotating object will drag the spacetime along with it. This will cause the orientation of a gyroscope to change over time. For a spacecraft in a polar orbit, the direction of this effect is perpendicular to the geodetic precession mentioned above. This prediction is also being tested by Gravity Probe B.

Black holes

Black holes are objects which have gravitationally collapsed behind an event horizon. In a "classical" black hole, nothing that enters can ever escape. The disappearance of light and matter within a black hole may be thought of as their entering a region where all null and timelike geodesic paths are warped so that they point inwards. Stephen Hawking has predicted that black holes can "leak" mass,[13] a phenomenon called Hawking radiation, a quantum effect not in violation of general relativity.

Cosmological effects

Expansion of the universe: This is predicted by cosmological solutions of the Einstein field equations. Its existence was confirmed by Edwin Hubble in 1929.[14]

Cosmological redshift: Light from distant galaxies will be redshifted due to their movement away from the observer according to Hubble's law.

Big Bang: The arising of the universe from a primordial singularity.

Cosmic microwave background radiation: The remnants of the primordial fireball. Discovered by Arno Penzias and Robert Wilson in 1965.[15]

Dark energy: This is an energy field of unknown composition that may exist throughout the universe. Recent observations of distant supernovae indicate that the expansion of the universe is currently "accelerating" [citations needed]. The solutions of the Einstein field equations that call for this behavior for the current universe, which may require the reintroduction of the cosmological constant, are for a stress-energy which is at least 70% dark energy.

Other predictions

The equivalence of inertial mass and gravitational mass: This follows naturally from freefall being inertial motion.

The strong equivalence principle: Even a self-gravitating object will respond to an external gravitational field in the same manner as a test particle would. (This is often violated by alternative theories.)

Gravitational radiation: Orbiting objects and merging neutron stars and/or black holes are expected to emit gravitational radiation.

Orbital decay (described above).

Binary pulsar mergers: May create gravitational waves strong enough to be observed here on Earth. Several gravitational wave observatories are (or will soon be) in operation. However, there are no confirmed observations of gravitational radiation at this time.

Gravitons: According to quantum mechanics, gravitational radiation must be composed of quanta called gravitons. General relativity predicts that these will be spin-2 particles. They have not been observed.

Only quadrupole (and higher order multipole) moments create gravitational radiation.

Dipole gravitational radiation (prohibited by this prediction) is predicted by some alternative theories. It has not been observed.

Relationship to other physical theories

Classical mechanics and special relativity

Classical mechanics and special relativity are lumped together here because special relativity is in many ways intermediate between general relativity and classical mechanics, and shares many attributes with classical mechanics.

In the following discussion, the mathematics of general relativity is used heavily. Also, under the principle of minimal coupling, the physical equations of special relativity can be turned into their general relativity counterparts by replacing the Minkowski metric (?ab) with the relevant metric of spacetime (gab) and by replacing any partial derivatives with covariant derivatives. In the discussions that follow, the change of metrics is implied.

Einstein gravity is non renormalizable

It is often said that general relativity is incompatible with quantum mechanics. This means that if one attempts to treat the gravitational field using the ordinary rules of quantum field theory, one finds that physical quantities are divergent. Such divergences are common in quantum field theories, and can be cured by adding parameters to the theory known as counterterms. These counterterms are infinities which are equal in magnitude and opposite in sign to the divergent terms. When they are added, the infinities cancel, leaving only finite terms, but modifying the meaning of terms in the equation such as "mass" and "charge" .

Many of the best understood quantum field theories, such as quantum electrodynamics, contain divergences which are canceled by counterterms that have been effectively measured. One needs to say effectively because the counterterms are formally infinite, however it suffices to measure observable quantities, such as physical particle masses and coupling constants, which depend on the counterterms in such a way that the various infinities cancel.

A problem arises, however, when the cancellation of all infinities requires the inclusion of an infinite number of counterterms. In this case the theory is said to be nonrenormalizable. While nonrenormalizable theories are sometimes seen as problematic, the framework of effective field theories presents a way to get low-energy predictions out of non-renormalizable theories. The result is a theory that works correctly at low energies, though such a theory cannot be considered to be a theory of everything because it cannot be self-consistently extended to the high-energy realm.

Proposed quantum gravity theories

General relativity fits nicely into the effective field theory formalism and makes sensible predictions at low energies (Donoghue, 1995). However, high enough energies will "break" the theory.

It is generally held that one of the most important unsolved problems in modern physics is the problem of obtaining the true quantum theory of gravitation, that is, the theory chosen by nature, one that will work at all energies. Discarded attempts at obtaining such theories include supergravity, a field theory which unifies general relativity with supersymmetry. In the second superstring revolution, supergravity has come back into fashion, with its quantum completion rebranded with a new name: M-theory.

A very different approach to that described above is employed by loop quantum gravity. In this approach, one does not try to quantize the gravitational field as one quantizes other fields in quantum field theories. Thus the theory is not plagued with divergences and one does not need counterterms. However it has not been demonstrated that the classical limit of loop quantum gravity does in fact contain flat space Einsteinian gravity. This being said, the universe has only one spacetime and it is not flat.

Of these two proposals, M-theory is significantly more ambitious in that it also attempts to incorporate the other known fundamental forces of Nature, whereas loop quantum gravity "merely" attempts to provide a viable quantum theory of gravitation with a well-defined classical limit which agrees with general relativity.

Alternative theories

Well known classical theories of gravitation other than general relativity include:

Nordström's theory of gravitation (1913) was one of the earliest metric theories (an aspect brought out by Einstein and Fokker in 1914). Nordström soon abandoned his theory in favor of general relativity on theoretical grounds, but this theory, which is a scalar theory, and which features a notion of prior geometry, does not predict any light bending, so it is solidly incompatible with observation.

Alfred North Whitehead formulated an alternative theory of gravity that was regarded as a viable contender for several decades, until Clifford Will noticed in 1971 that it predicts grossly incorrect behavior for the ocean tides!

George David Birkhoff's (1943) yields the same predictions for the classical four solar system tests as general relativity, but unfortunately requires sound waves to travel at the speed of light! Thus, like Whitehead's theory, it was never a viable theory after all, despite making an initially good impression on many experts.

Like Nordström's theory, the gravitation theory of Wei-Tou Ni (1971) features a notion of prior geometry, but Will soon showed that it is not fully compatible with observation and experiment.

The Brans-Dicke theory and the Rosen bi-metric theory are two alternatives to general relativity which have been around for a very long time and which have also withstood many tests. However, they are less elegant and more complicated than general relativity, in several senses.

There have been many attempts to formulate consistent theories which combine gravity and electromagnetism. The first of these, Weyl's gauge theory of gravitation, was immediately shot down (on a postcard!) by Einstein himself,[citation needed] who pointed out to Hermann Weyl that in his theory, hydrogen atoms would have variable size, which they do not. Another early attempt, the original Kaluza-Klein theory, at first seemed to unify general relativity with classical electromagnetism, but is no longer regarded as successful for that purpose. Both these theories have turned out to be historically important for other reasons: Weyl's idea of gauge invariance survived and in fact is omnipresent in modern physics, while Kaluza's idea of compact extra dimensions has been resurrected in the modern notion of a braneworld.

The Fierz-Pauli spin-two theory was an optimistic attempt to quantize general relativity, but it turns out to be internally inconsistent. Pascual Jordan's work toward fixing these problems eventually motivated the Brans-Dicke theory, and also influenced Richard Feynman's unsuccessful attempts to quantize gravity.

Einstein-Cartan theory includes torsion terms, so it is not a metric theory in the strict sense.

Teleparallel gravity goes further and replaces connections with nonzero curvature (but vanishing torsion) by ones with nonzero torsion (but vanishing curvature).

The Nonsymmetric Gravitational Theory (NGT) of John W. Moffat is a dark horse in the race.

Even for "weak field" observations confined to our Solar system, various alternative theories of gravity predict quantitatively distinct deviations from Newtonian gravity. In the weak-field, slow-motion limit, it is possible to define 10 experimentally measurable parameters which completely characterize predictions of any such theory. This system of these parameters, which can be roughly thought of as describing a kind of ten dimensional "superspace" made from a certain class of classical gravitation theories, is known as PPN formalism (Parametric Post-Newtonian formalism). Current bounds on the PPN parameters are compatible with GR.

See in particular confrontation between Theory and Experiment in Gravitational Physics, a review paper by Clifford Will.

History

General relativity was developed by Einstein in a process that began in 1907 with the publication of an article on the influence of gravity and acceleration on the behavior of light in special relativity. Most of this work was done in the years 1911–1915, beginning with the publication of a second article on the effect of gravitation on light. By 1912, Einstein was actively seeking a theory in which gravitation was explained as a geometric phenomenon. In December of 1915, these efforts culminated in Einstein's submission of a paper presenting the Einstein field equations, which are a set of differential equations . This paper was subsequently published in 1916.[2] Since 1915, the development of general relativity has focused on solving the field equations for various cases. This generally means finding metrics which correspond to realistic physical scenarios. The interpretation of the solutions and their possible experimental and observational testing also constitutes a large part of research in GR.

The expansion of the universe created an interesting episode for general relativity. Starting in 1922, researchers found that cosmological solutions of the Einstein field equations call for an expanding universe. Einstein did not believe in an expanding universe, and so he added a cosmological constant to the field equations to permit the creation of static universe solutions. In 1929, Edwin Hubble found evidence that the universe is expanding. This resulted in Einstein dropping the cosmological constant, referring to it as "the biggest blunder in my career".

Progress in solving the field equations and understanding the solutions has been ongoing. Notable solutions have included the Schwarzschild solution (1916), the Reissner-Nordström solution, the Friedmann-Robertson-Walker solution and the Kerr solution.

Observationally, general relativity has a history too. The perihelion precession of Mercury was the first evidence that general relativity is correct. Eddington's 1919 expedition in which he confirmed Einstein's prediction for the deflection of light by the Sun helped to cement the status of general relativity as a likely true theory. Since then, many observations have confirmed the predictions of general relativity. These include studies of binary pulsars, observations of radio signals passing the limb of the Sun, and even the GPS system.

Status

The status of general relativity is decidedly mixed.

On the one hand, general relativity is a highly successful model of gravitation and cosmology. It has passed every unambiguous test that it has been subjected to so far, both observationally and experimentally. It is therefore almost universally accepted by the scientific community.

On the other hand, general relativity is inconsistent with quantum mechanics, and the singularities of black holes also raise some disconcerting issues. So while it is accepted, there is also a sense that something beyond general relativity may yet be found.

Currently, better tests of general relativity are needed. Even the most recent binary pulsar discoveries only test general relativity to the first order of deviation from Newtonian projections in the post-Newtonian parameterizations. Some way of testing second and higher order terms is needed, and may shed light on how reality differs from general relativity (if it does).

In general relativity, event horizon is a general term for a boundary in spacetime, defined with respect to an observer, beyond which events cannot affect the observer. Light emitted beyond the horizon can never reach the observer, and anything that passes through the horizon from the observer's side is never seen again.

A black hole is surrounded by an event horizon, for example. This means that an outside observer cannot be affected by anything inside the black hole. More specific types of horizons include the related but distinct absolute and apparent horizons found around a black hole. Still other distinct notions include the Cauchy and Killing horizon; particle

and cosmological horizons relevant to cosmology; and isolated and dynamical horizons important in current black hole research.

Event horizons around black holes: The most commonly known example of an event horizon is defined around general relativity's description of a black hole, a celestial object compact enough that no matter or radiation can escape. This is sometimes described as the boundary within which the black hole's escape velocity is greater than the speed of light. While this definition can be made to work, it only does so if the effects of special and general relativity are taken into account. A more accurate description is to

note that within this horizon, all lightlike paths (paths light could take), and hence all paths in the forward light cones of particles within the horizon, are warped so as to fall further into the hole. Once a particle is inside the horizon, moving into the hole is as inevitable as moving forward in time (and can actually be thought of as equivalent to doing so, depending on the spacetime coordinate system used).

Black hole event horizons are especially noteworthy for three reasons. First, there are many examples near enough to study. Second, black holes tend to pull in matter from their environment, which provides examples where matter passing through an event horizon is expected to be observable. Third, the description of black holes given by general relativity is known to be an approximation, with quantum gravity effects expected to become significant near the vicinity of the event horizon. This allows observations of matter in the vicinity of a black hole's event horizon to be used to indirectly study general relativity and proposed extensions to it.

The definition of "event horizon" given by Hawking & Ellis[1], Misner, Thorne & Wheeler[2], and Wald[3] differs from the one presented here. Their definition of an event horizon rules out the cosmological and particle horizons presented below (as well as the apparent horizon). However, modern usage has brought those ideas under the umbrella of the term "event horizon".

To make the distinction clearer, some authors refer to their more specific notion of an horizon as an "absolute horizon". In the context of black holes, event horizon almost always refers to the absolute horizon, as distinct from the apparent horizon.

Event horizon of the observable universe

The particle horizon of the observable universe is the boundary that represents the maximum distance at which events can presently be observed. For events beyond that distance, light hasn't had time to reach our location, even if it was emitted at the time the universe began. How the particle horizon changes with time depends on the nature of the expansion of the universe. If the expansion has appropriate characteristics, there are parts of the universe that will never be observable, no matter how long the observer waits for light from those regions to arrive. The boundary past which events can't ever be observed is an event horizon, and represents the maximum extent of the particle horizon.

Examples of cosmological models without an event horizon are universes dominated by matter or by radiation. An example of a cosmological model with an event horizon is a universe dominated by the cosmological constant (a de Sitter universe).

Event horizon of an accelerated particle

If a particle is moving at a constant velocity in a non-expanding universe free of gravitational fields, any event that occurs in that universe will eventually be observable by the particle, because the forward light cones from these events intersect the particle's world line. On the other hand, if the particle is accelerating, it's possible to construct situations where light cones from some events never intersect the particle's world line. Under these conditions, an event horizon is present in the particle's (accelerating) reference frame, representing a boundary beyond which events are unobservable.

One situation where this occurs is the case of a uniformly accelerated particle. A spacetime diagram of this situation is shown in the figure to the right. As the particle accelerates, it approaches, but never reaches, the speed of light with respect to its original reference frame. On the spacetime diagram, its path is a hyperbola, which asymptotically approaches a 45 degree line (the path of a light ray). An event whose light cone's edge is this asymptote or is farther away than this asymptote can never be observed by the accelerating particle. In the particle's reference frame, there appears to be a boundary behind it from which no signals can escape (an event horizon).

While approximations of this type of situation can occur in the real world (in particle accelerators, for example), a true event horizon is never present, as the particle must be accelerated indefinitely (requiring arbitrarily large amounts of energy and an arbitrarily large apparatus).

Interacting with an event horizon

A misconception concerning event horizons, especially black hole event horizons, is that they represent an immutable surface that destroys objects that approach them. In practice, several features are common to all event horizons: they appear to be some distance away from any observer, and objects sent towards an event horizon never appear to cross it from the sending observer's point of view (as the horizon-crossing event's light cone never intersects the observer's world line). Attempting to make an object approaching the horizon remain stationary with respect to an observer requires applying a force whose magnitude becomes unbounded (becoming infinite) the closer it gets.

For the case of a horizon perceived by a uniformly accelerating observer in empty space, the horizon seems to remain a fixed distance from the observer no matter how its surroundings move. Varying the observer's acceleration may cause the horizon to appear to move over time, or may prevent an event horizon from existing, depending on the acceleration function chosen. The observer never touches the horizon, and never passes a location where it appeared to be.

For the case of a horizon perceived by an occupant of a de Sitter universe, the horizon always appears to be a fixed distance away for a non-accelerating observer. It is never contacted, even by an accelerating observer.

For the case of the horizon around a black hole, observers stationary with respect to a distant object will all agree on where the horizon is. While this seems to allow an observer lowered towards the hole on a rope to contact the horizon, in practice this cannot be done. If the observer is lowered very slowly, then, in the observer's frame of reference, the horizon appears to be very far away, and ever more rope needs to be paid out to reach the horizon. If the observer is lowered quickly, then indeed the observer, and some of the rope can touch and even cross the (distant lowerer's) event horizon. If the rope is pulled taut to fish the observer back out, then the forces along the rope increase without bound as they approach the event horizon, and at some point the rope must break. Furthermore, the break must occur not at the event horizon, but at a point where the lowerer can observe it.

Attempting to stick a rigid rod through the hole's horizon cannot be done: if the rod is lowered extremely slowly, then it is always too short to touch the event horizon, as the coordinate frames near the tip of the rod are extremely compressed. From the point of view of an observer at the end of the rod, the event horizon remains hopelessly out of reach. If the rod is lowered quickly, then the same problems as with the rope are encountered: the rod must break and the broken off pieces inevitably fall in.

These peculiarities only occur because of the supposition that the observers be stationary with respect to some other distant observer. Observers that fall into the hole are moving with respect to the distant observer, and so perceive the horizon as being in a different location, seeming to recede in front of them so that they never contact it. Increasing tidal forces (and eventual impact with the hole's gravitational singularity) are the only locally noticeable effects. While this seems to allow an infalling observer to relay information from objects outside their perceived horizon but inside the distant observer's perceived horizon, in practice the horizon recedes by an amount small enough that by the time the infalling observer receives any signal from farther into the hole, they've already crossed what the distant observer perceived to be the horizon, and this reception event (and any retransmission) can't be seen by the distant observer.

Event horizons beyond general relativity

The description of event horizons given by general relativity is thought to be incomplete. When the conditions under which event horizons occur are modelled using a more complete picture of the way the universe works, that includes both relativity and quantum mechanics, event horizons are expected to have properties that are different from those predicted using general relativity alone.

At present, the primary expected impact of quantum effects is for event horizons to possess a temperature, and emit radiation as a result. For black holes, this manifests as Hawking radiation, and the larger question of how the black hole possesses a temperature is part of the topic of black hole thermodynamics. For accelerating particles, this manifests as the Unruh effect, which causes space around the particle to appear to be filled with matter and radiation.

A complete description of event horizons is expected to at minimum require a theory of quantum gravity. As of 2006, the most promising candidate theory is M-theory.

Astronomy (Greek: astronomia = astron + nomos, literally, "law of the stars") is the science of celestial objects (e.g., stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (e.g., auroras and cosmic background radiation). It is concerned with the evolution, physics, chemistry, and motion of celestial objects, as well as the formation and development of the universe.

Astronomy is one of the oldest sciences. Astronomers of early civilizations performed methodical observations of the night sky, and

astronomical artifacts have been found from much earlier periods. However, it required the invention of the telescope before astronomy developed into a modern science.

Since the 20th century, the field of professional astronomy has split into observational astronomy and theoretical astrophysics. Observational astronomy is concerned with acquiring data, which involves building and maintaining instruments, as well as processing the results. Theoretical astrophysics is concerned with ascertaining the observational implications of computer or analytic models. The two fields complement each other, with theoretical astronomy seeking to explain the observational results. Astronomical observations can be used to test fundamental theories in physics, such as general

relativity.

Historically, amateur astronomers have contributed to many important astronomical discoveries, and astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient phenomena.

Modern astronomy is not to be confused with astrology, the belief system that claims human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin, most thinkers in both fields believe they are now distinct.

History

Main articles: history of astronomy and archaeoastronomy

In early times, astronomy only comprised the observation and predictions of the motions of the naked-eye objects. In some locations, such as Stonehenge, early cultures assembled massive artifacts that likely had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops, as well as the length of the year.

As civilizations developed, most notably Babylonia, Egypt, ancient Greece, India, and China, astronomical observatories were assembled and ideas on the nature of the universe began to be explored. Early ideas on the motions of the planets were developed, and the nature of the Sun, Moon and the Earth in the universe were explored philosophically. These included speculations on the spherical nature of the Earth and Moon, and the rotation and movement of the Earth through the heavens.

A few notable astronomical discoveries were made prior to the application of the telescope. For example, the obliquity of the ecliptic was estimated as early as 1,000 B.C by the Chinese. The Chaldeans discovered that eclipses recurred in a repeating cycle known as a saros. In the second century B.C., the size and distance of the Moon were estimated by Hipparchus.

During the Middle Ages, observational astronomy was mostly stagnant in medieval Europe until the 13th century. However, observational astronomy flourished in the Persian Empire and other parts of the Islamic world. Islamic astronomers introduced many names that are now used for individual stars.[

During the Renaissance, Nicolaus Copernicus proposed a heliocentric model of the Solar System. His work was defended, expanded upon, and corrected by Galileo Galilei and Johannes Kepler. Galileo added the innovation of using telescopes to enhance his observations.

Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not succeed in formulating a theory behind the laws he wrote down. It was left to Newton's invention of celestial dynamics and his law of gravitation to finally explain the motions of the planets. Newton also developed the reflecting telescope.

Further discoveries paralleled the improvements in size and quality of the telescope. More extensive star calatogues were produced by Lacaille. The astronomer William Herschel made an extensive catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found. The distance to a star was first announced in 1838 when the parallax of 61 Cygni was measured by Friedrich Bessel.

During the nineteenth century, attention to the three body problem by Euler, Clairaut and D'Alembert led to more accurate predictions about the motions of the Moon and planets. This work was further refined by Lagrance and Laplace, allowing the masses of the planets and moons to be estimated from their perturbations.

Significant advances in astronomy came about with the introduction of new technology, including the spectroscope and photography. Fraunhofer discovered about 600 bands in the spectrum of the Sun in 1814-15, which, in 1859, Kirchhoff ascribed to the presence of different elements. Stars were proven to be similar to Earth's own sun, but with a wide range of temperatures, masses, and sizes.

The existence of Earth's galaxy, the Milky Way, as a separate group of stars was only proved in the 20th century, along with the existence of "external" galaxies, and soon after, the expansion of the universe, seen in the recession of most galaxies from us. Modern astronomy has also discovered many exotic objects such as quasars, pulsars, blazars and radio galaxies, and has used these observations to develop physical theories which describe some of these objects in terms of equally exotic objects such as black holes and neutron stars. Physical cosmology made huge advances during the 20th century, with the model of the Big Bang heavily supported by the evidence provided by astronomy and physics, such as the cosmic microwave background radiation, Hubble's law, and cosmological abundances of elements.

In Babylon and ancient Greece, astronomy consisted largely of astrometry, measuring the positions of stars and planets in the sky. Later, the work of astronomers Kepler and Newton led to the development of celestial mechanics, and astronomy focused on mathematically predicting the motions of gravitationally interacting celestial bodies. This was applied to solar system objects in particular. Today, the motions and positions of objects are more easily determined, and modern astronomy concentrates on observing and understanding the physical nature of celestial objects.

Methods of data collection

Main article: Observational astronomy

In astronomy, information is mainly received from the detection and analysis of light and other forms of electromagnetic radiation. Other cosmic rays are also observed, and several experiments are designed to detect gravitational waves in the near future. Neutrino detectors have been used to observe solar neutrinos, and neutrino emissions from supernovae have also been detected.

A traditional division of astronomy is given by the region of the electromagnetic spectrum observed. At the low frequency end of the spectrum, radio astronomy detects radiation of millimeter to dekameter wavelength. The radio telescope receivers are similar to those used in radio broadcast transmission but much more sensitive. Microwaves form the millimeter end of the radio spectrum and are important for studies of the cosmic microwave background radiation.

Infrared astronomy and far infrared astronomy deal with the detection and analysis of infrared radiation (wavelengths longer than red light). The most common tool is the telescope but using a detector which is sensitive to the infrared. Infrared radiation is heavily absorbed by atmospheric water vapor, so infrared observatories have to be located in high, dry places or in outer space. Space telescopes in particular avoid atmospheric thermal emission, atmospheric opacity, and the negative effects of astronomical seeing at infrared and other wavelengths. Infrared is particularly useful for observation of galactic regions cloaked by dust.

Historically, most astronomical data has been collected through optical astronomy. This is the portion of the spectrum that uses optical components (mirrors, lenses, CCD detectors and photographic films) to observe light from near infrared to near ultraviolet wavelengths. Visible light astronomy (using wavelengths that can be detected with the eyes, about 400 - 700 nm) falls in the middle of this range. The most common tool is the telescope, with electronic imagers and spectrographs.

More energetic sources are observed and studied in high-energy astronomy, which includes X-ray astronomy, gamma ray astronomy, and extreme UV (ultraviolet) astronomy, as well as studies of neutrinos and cosmic rays.

Optical and radio astronomy can be performed with ground-based observatories, because the Earth's atmosphere is transparent at the wavelengths being detected. The atmosphere is opaque at the wavelengths of X-ray astronomy, gamma-ray astronomy, UV astronomy and (except for a few wavelength "windows") far infrared astronomy, so observations must be carried out mostly from balloons or space observatories. Powerful gamma rays can, however be detected by the large air showers they produce, and the study of cosmic rays can also be regarded as a branch of astronomy.

Planetary astronomy has benefited from direct observation in the form of spacecraft and sample return missions. These include fly-by missions with remote sensors, landing vehicles that can perform experiments on the surface materials, impactors that allow remote sensing of buried materials, and sample return missions that allow direct laboratory examination.

Astrometry and celestial mechanics

Main articles: Astrometry and Celestial mechanics

One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects in the sky. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in celestial navigation.

Careful measurement of the positions of the planets has led to a solid understanding of gravitational perturbations and an ability to determine past and future positions of the planets with great accuracy, a field known as celestial mechanics. More recently the tracking of near-Earth objects will allow for predictions of close encounters, and potential collisions, with the Earth.

The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the universe. Parallax measurements of nearby stars provides an absolute baseline for the properties of more distant stars, because their properties can be compared. Measurements of radial velocity and proper motion show the kinematics of these systems through the Milky Way galaxy. Astrometric results are also used to measure the distribution of dark matter in the galaxy.

During the 1990s, the astrometric technique of measuring the stellar wobble was used to detect large extrasolar planets orbiting nearby stars.

Interdisciplinary studies

Astronomy has developed significant interdisciplinary links with other major scientific fields. These include:

Astrophysics: the study of the physics of the universe, including the physical properties (luminosity, density, temperature, chemical composition) of astronomical objects.

Astrobiology: the study of the advent and evolution of biological systems in the universe.

Archaeoastronomy: the study of ancient or traditional astronomies in their cultural context, utilising archaeological and anthropological evidence.

Astrochemistry: the study of the chemicals found in outer space, usually in molecular gas clouds, and their formation, interaction and destruction. As such, it represents an overlap of the disciplines of astronomy and chemistry.

Astronomical objects

Solar astronomy

Main article: Sun

The most frequently studied star is the Sun, a typical main-sequence dwarf star of stellar class G2 V, and about 4.6 Gyr in age. The Sun is not considered a variable star, but it does undergo periodic changes in activity known as the sunspot cycle. This is an 11-year fluctuation in sunspot numbers. Sunspots are regions of lower than average temperature that are associated with intense magnetic activity.

The Sun has steadily increased in luminosity over the course of its life, increasing by 40% since it first became a main-sequence star. The Sun has also undergone periodic changes in luminosity that can have a significant impact on the Earth. The Maunder minimum, for example, is believed to have caused the Little Ice Age phenomenon during the Middle Ages.

The visible outer surface of the Sun is called the photosphere. Above this layer is a thin region known as the chromosphere. This is surrounded by a transition region of rapidly increasing temperatures, then by the super-heated corona.

At the center of the Sun is the core region, a volume of sufficient temperature and pressure for nuclear fusion to occur. Above the core is the radiation zone, where the plasma conveys the energy flux by means of radiation. The outer layers form a convection zone where the gas material transports energy primarily through physical displacement of the gas. It is believed that this convection zone creates the magnetic activity that generates sun spots.

A solar wind of plasma particles constantly streams outward from the Sun until it reaches the heliopause. This solar wind interacts with the magnetosphere of the Earth to create the Van Allen radiation belts, as well as the aurora where the lines of the Earth's magnetic field descend into the atmosphere.

Planetary science

Main articles: planetary science and planetary geology

This astronomical field examines the assemblage of planets, moons, dwarf planets, comets, asteroids, and other bodies orbiting the Sun, as well as extrasolar planets. The solar system has been relatively well-studied, initially through telescopes and then later by spacecraft. This has provided a good overall understanding of the formation and evolution of this planetary system, although many new discoveries are still being made.

The solar system is subdivided into the inner planets, the asteroid belt, and the outer planets. The inner terrestrial planets consist of Mercury, Venus, Earth, and Mars. The outer gas giant planets are Jupiter, Saturn, Uranus and Neptune.

The planets formed from a protoplanetary disk that surrounded the early Sun. Through a process that included gravitational attraction, collision, and accretion, the disk formed clumps of matter that with time became protoplanets. The radiation pressure of the solar wind then expelled most of the unaccreted matter, and only those planets with sufficient mass retained their gaseous atmosphere. The planets continued to sweep up or eject the remaining matter during a period of intense bombardment evidenced by the many impact craters on the Moon. During this period some protoplanets may have collided, the leading hypothesis for how the Moon was formed.

Once a planet reaches sufficient mass, the materials with different densities segregate within its interior during planetary differentiation. This process can form a stony or metallic core surrounded by a mantle and outer surface. The core may include solid and liquid regions, and some planetary cores generate their own magnetic field, which can protect its atmosphere from solar wind stripping.

A planet or moon's interior heat is produced from the collisions that created the body, radioactive materials (e.g. uranium, thorium, and 26Al), or tidal heating. Some planets and moons accumulate enough heat to drive geologic processes such as volcanism and tectonics. Those that accumulate or retain an atmosphere can also undergo surface erosion due to wind or water. Smaller bodies without tidal heating cool more quickly and their geological activity ceases with the exception of impact cratering.

The study of stars and stellar evolution is fundamental to our understanding of the universe. The astrophysics of stars has been determined through observation, theoretical understanding and from computer simulations of the interior.

Star formation occurs in dense regions of dust and gas, known as giant molecular clouds. When destabilized, cloud fragments can collapse under the influence of gravity to form a protostar. A sufficiently dense and hot core region will trigger nuclear fusion and it becomes a main-sequence star.

The characteristics of the resulting star depend primarily on its starting mass. The more massive the star, the greater its luminosity and the more rapidly it expends the hydrogen fuel in its core. Over time this hydrogen fuel is completely converted into helium and the star begins to evolve. Fusion of helium requires a higher core temperature, so the star both expands in size and increases in density at the core. The resulting red giant enjoys a brief life span before the helium fuel is in turn consumed. Very massive stars can also undergo a series of shorter and shorter evolutionary phases as they fuse increasingly heavier elements.

The final fate of the star depends on its mass, with stars of mass greater than 1.4 times the Sun becoming supernovae, while smaller stars will form planetary nebulae and evolve into white dwarfs. The remnant of a supernova is a dense neutron star, or, if the stellar mass was at least three times that of the Sun, a black hole.

Our solar system orbits within the Milky Way, a barred spiral galaxy that is a prominent member of the Local Group of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.

In the center of the Milky Way is the core region, a bar-shaped bulge with what is believed to be a supermassive black hole at the center. This is surrounded by four primary arms that spiral out from the core. This is a region of active star formation that contains many younger, population II stars. The disk is surrounded by a spheroid halo of older, population I stars, as well as relatively dense concentrations of stars known as globular clusters.

Between the stars lies the interstellar medium, a region of sparse matter. In the densest regions, molecular clouds of molecular hydrogen and other elements create star-forming regions. These begin as irregular dark nebulae, which concentrate and collapse (in volumes determined by the Jeans length) to form compact protostars.

As the more massive stars appear, they transform the cloud into an H II region of glowing gas and plasma. The stellar wind and supernova explosions from these stars eventually serve to disperse the cloud, often leaving behind one or more young open clusters of stars. These gradually disperse to join the population of stars in the Milky Way.

Kinematic studies of matter in the Milky Way and other galaxies have demonstrated that there is more mass than can be accounted for by visible matter. A dark matter halo appears to dominate the mass, although the nature of this dark matter remains undetermined

The study of objects outside our galaxy is a branch of astronomy concerned with the formation and evolution of Galaxies, their morphology and classification, the examination of active galaxies and the groups and clusters of galaxies. The later is important for the understanding of the large-scale structure of the cosmos.

Most galaxies are organized into distinct shapes that allow for classification schemes. They are commonly divided into spiral, elliptical and Irregular galaxies.

As the name suggests, an elliptical galaxy has the cross-sectional shape of an ellipse. The stars move along random orbits with no preferred direction. These galaxies contains little or no interstellar dust, few star-forming regions and generally older stars. Elliptical galaxies are more commonly found at the core of galactic clusters and may be formed through mergers of large galaxies.

A spiral galaxy is organized into a flat, rotating disk, usually with a prominent bulge or bar at the center, and trailing bright arms that spiral outward. The arms are dusty regions of star formation where massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the Milky Way and the Andromeda Galaxy are spiral galaxies.

Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical in form. About a quarter of all galaxies are irregular, and their peculiar shape may be the result of gravitational interaction.

An active galaxy is a formation that is emitting a significant amount of its energy from a source other than stars, dust and gas. They are powered by a compact region at the core, usually thought to be a supermassive black hole that is emitting radiation due to infalling material.

A radio galaxy is an active galaxy that is very luminous in the radio portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit high-energy radiation include Seyfert galaxies, Quasars, and Blazars. Quasars are believed to be the most consistently luminous objects in the known universe.

The large-scale structure of the cosmos is represented by groups and clusters of galaxies. This structure is organized in a hierarchy of groupings, with the largest being the superclusters. The collective matter is formed into filaments and walls, leaving large voids in between.

Cosmology

Main articles: physical cosmology and timeline of the Big Bang

Observations of the large-scale structure of the universe, a branch known as physical cosmology, have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the big bang, wherein our universe began at a single point in time and thereafter expanded over the course of 13.7 Gyr to its present condition. The concept of the big bang can be traced back to the discovery of the microwave background radiation in 1965.

In the course of this expansion, the universe underwent several evolutionary stages. In the very early moments, it is theorized that the universe underwent a very rapid cosmic inflation, which homogenized the starting conditions. Thereafter nucleosynthesis produced the elemental abundance of the early universe.

When the first atoms formed space became transparent to radiation; releasing the energy viewed today as the microwave background radiation. The expanding universe then underwent a dark age due to the lack of stellar energy sources.

A hierarchical structure of matter began to form from minute variations in the mass density. Matter accumulated in the densest regions, forming clouds of gas and the earliest stars. These massive stars triggered the reionization process and are believed to have created many of the heavy elements in the early universe.

Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually organizations of gas and dust merged to form the first primitive galaxies. Over time these pulled in more matter, and were often organized into groups and clusters of galaxies, then into larger-scale superclusters.

Fundamental to the structure of the universe is the existence of dark matter and dark energy. These are now thought to be the dominant components, forming 96% of the density of the universe. So much effort is being spent to try and understand the physics of these components.

Major questions in astronomy

Although the scientific discipline of astronomy has made tremendous strides in understanding the nature of the universe and its contents, there remain some important unanswered questions. Answers to these may require the construction of new ground and space-based instruments, and possibly new developments in theoretical and experimental physics.

Are there Earth-like planets around other stars? Astronomers have found massive stars and disks of debris around other stars. So the existence of smaller, terrestrial planets seems likely.

Is there other life in the Universe? Especially, is there other intelligent life? If so, what is the explanation for the Fermi paradox? The existence of life elsewhere has important scientific and philosophical implications.

What is the nature of dark matter and dark energy? These dominate the evolution and fate of the cosmos, yet we are still uncertain about their true nature.

Why did the universe come to be? Why, for example, are the physical constants so finely tuned that they permit the existence of life? What caused the cosmic inflation that produced our homogeneous universe?