A white Stellar Evolution

neutron star

The imploded core of a massive star produced by a supernova explosion. (typical mass of 1.4 times the mass of the Sun, radius of about 5 miles, density of a neutron.) According to astronomer and author Frank Shu, "A sugar cube of neutron-star stuff on Earth would weigh as much as all of humanity!" Neutron stars can be observed as pulsars.

quasarAn enormously bright object at the edge of our universe which emits massive amounts of energy. In an optical telescope, they appear point-like, similar to stars, from which they derive their name (quasar = quasi-stellar). Current theories hold that quasars are one type of AGN.

active galactic nuclei (AGN)A class of galaxies which spew massive amounts of energy from their centers, far more than ordinary galaxies. Many astronomers believe supermassive black holes may lie at the center of these galaxies and power their explosive energy output.

pulsarA rotating neutron star which generates regular pulses of radiation. Pulsars were discovered by observations at radio wavelengths but have since been observed at optical, X-ray, and gamma-ray energies.

supernova (plural: supernovae)(a)The death explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At peak light output, these type of supernova explosions (called Type II supernovae) can outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud. This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant (SNR).(b) The explosion of a white dwarf which has accumulated enough material from a companion star to achieve a mass equal to

the Chandrasekhar limit. These types of supernovae (called Type Ia) have approximate the same intrinsic brightness, and can be used to determine distances.

black holeAn object whose gravity is so strong that not even light can escape from it.

black-hole dynamic laws; laws of black-hole dynamics

1. First law of black hole dynamics:For interactions between black holes and normal matter, the conservation laws of mass-energy, electric charge, linear momentum, and angular momentum, hold. This is analogous to the first law of thermodynamics.

2. Second law of black hole dynamics:With black-hole interactions, or interactions between black holes and normal matter, the sum of the surface areas of all black holes involved can never decrease. This is analogous to the second law of thermodynamics, with the surface areas of the black holes being a measure of the entropy of the system.’

white dwarfA star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size. Typically, a white dwarf has a radius equal to about 0.01 times that of the Sun, but it has a mass roughly equal to the Sun's. This gives a white dwarf a density about 1 million times that of water!

Chandrasekhar, S. (1910 - 1995)Indian astrophysicist reknowned for creating theoretical models of white dwarf stars, among other achievements. His equations explained the underlying physics behind the creation of white dwarfs, neutron stars and other compact objects.

Chandrasekhar limit A limit which mandates that no white dwarf (a collapsed, degenerate star)

can be more massive than about 1.4 solar masses. Any degenerate object more massive must inevitably collapse into aneutron star

Compton effect (A.H. Compton; 1923)An effect that demonstrates that photons (the quantum of electromagnetic radiation) have momentum. A photon fired at a stationary particle, such as an electron, will impart momentum to the electron and, since its energy has been decreased, will experience a corresponding decrease in frequency.

parsecThe distance to an object which has a parallax of one arc second. It is equal to 3.26 light years, or 3.1 x 1018 cm (see scientific notation). A kiloparsec (kpc) is equal to 1000 parsecs. Amegaparsec (Mpc) is equal to a million (106) parsecs.

parallaxThe apparent motion of a relatively close object compared to a more distant background as the location of the observer changes. Astronomically, it is half the angle which a a star appears to move as the earth moves from one side of the sun to the other.

perigeeThe point in its orbit where an Earth satellite is closest to the Earth. Opposite of apogee.

perihelionThe point in its orbit where a planet is closest to the Sun. Opposite of aphelion.

light yearA unit of length used in astronomy which equals the distance light travels in a year. At the rate of 300,000 kilometers per second (671 million miles per hour),

1 light-year is equivalent to 9.46053 x 1012 km, 5,880,000,000,000 miles or 63,240 AU (see scientific notation).

astronomical unit (AU)149,597,870 km; the average distance from the Earth to the Sun.

galaxyA component of our universe made up of gas and a large number (usually more than a million) of stars held together by gravity. When capitalized, Galaxy refers to our own Milky Way Galaxy.

apogeeThe point in its orbit where an Earth satellite is farthest from the Earth. Opposite of perigee.

aphelionThe point in its orbit where a planet is farthest from the Sun. Opposite of perihelion.

angular momentumA quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit. According to the conservation laws of physics, the angular momentum of any orbiting body must remain constant at all points in the orbit, i.e., it cannot be created or destroyed. If the orbit is elliptical the radius will vary. Since the mass is constant, the velocity changes. Thus planets in elliptical orbits travel faster at perihelion and more slowly at aphelion. A spinning body also possesses spin angular momentum.

binary starsBinary stars are two stars that orbit around a common center of mass. An X-ray binary is a special case where one of the stars is a collapsed object such as a white dwarf, neutron star, or black hole, and the separation between the stars is small enough so that matter is transferred from the normal star to the compact star star, producing X-rays in the process.

stellar classificationStars are given a designation consisting of a letter and a number according to the nature of theirspectral lines which corresponds roughly to surface temperature. The classes are: O, B, A, F, G, K, and M; O stars are the hottest; M the coolest. The numbers are simply subdivisions of the major classes. The classes are oddly sequenced because they were assigned long ago before we understood their relationship to temperature. O and B stars are rare but very bright; M stars are numerous but dim. The Sun is designated G2.

stellar windThe ejection of gas off the surface of a star. Many different types of stars, including our Sun, have stellar winds; however, a star's wind is strongest near the end of its life when it has consumed most of its fuel.

accretionAccumulation of dust and gas onto larger bodies such as stars, planets and moons.

accretion diskA relatively flat sheet of gas and dust surrounding a newborn star, a black hole, or any massive object growing in size by attracting material.

After stars have exhausted their fuel, gravity pulls them into further collapse leading to a variety of types of peculiar object.

When stars die, a lot of their material is ejected back into the interstellar medium through the following mechanisms:

1) Winds -- all stars lose a small amount of mass due to material which "boils" off the surface in a manner analogous to the solar wind. As a star becomes a red giant, the wind may strengthen. Winds usually remove only a small fraction ( less than a percent) of a star's mass.

2) Planetary Nebula -- the material ejected by a moderate mass red giant as its core collapses into a white dwarf.

3.) Supernova Explosion -- the violent ejection of nearly the entire material in a massive star when its core collapses and the outer layers implode onto the core. The remnant can be a white dwarf, neutron star, or black hole.

All of these events, but mostly supernova explosions, eject material into space that has been processed from hydrogen and helium up to heavier elements. When this material is incorporated into new stars, it also provides the ingredients to make planets like the earth and all the things on their surfaces -- like you!

Deaths of stars can also influence the formation of new stars, such as when shock waves from supernovae cause interstellar clouds to collapse.

M <= 1.4 M  -----> white dwarf/planetary nebulae (PN to the left)

1.4 M  <M < ~3 M  --> neutron stars/pulsars

M > ~ 3 M  ----> supernovae/black holes

All stars form from clouds of gas and dust condensing in deep space.  Only the chemical composition of this cloud, and the amount of material in the cloud that condenses into the actual star, determines what will happen to the star over its entire lifetime.

A supernova (plural supernovae) is a stellar explosion that is more energetic than a nova. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months. During this short interval a supernova can radiate as much energy as theSun is expected to emit over its entire life span.[1] The explosion expels much or all of a star's material[2] at a velocity of up to 30,000 km/s (10% of the speed of light), driving a shock wave[3] into the surrounding interstellar medium. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant

White dwarf,

White Dwarfs and Planetary Nebulae

After a star like the sun exhausts its nuclear fuels, it loses its outer layers as a "planetary nebula" and leaves behind the remnant "white dwarf" core. The white dwarfs are extremely small stars -- they are the bare remnant cores of stars after they have gone through all of their lifetime

also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sunand its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored thermal energy

White dwarfs are thought to be the final evolutionary state of all stars whose mass is not high enough to become supernovae—over 97% of the stars in our galaxy.[5], §1. After thehydrogen-fusing lifetime of a main-sequence star of low or medium mass ends, it will expand to ared giant which fuses helium to carbon and oxygen in its core by the triple-alpha process. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, an inert mass of carbon and oxygen will build up at its center. After shedding its outer layers to form aplanetary nebula, it will leave behind this core, which forms the remnant white dwarf.[6] Usually, therefore, white dwarfs are composed of carbon and oxygen. It

is also possible that core temperatures suffice to fuse carbon but not neon, in which case an oxygen-neon–magnesiumwhite dwarf may be formed.[7] Also, some helium white dwarfs[8][9] appear to have been formed by mass loss in binary systems. The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy, nor is it supported by the heat generated by fusion against gravitational collapse. It is supported only by electron degeneracy pressure, causing it to be extremely dense. The physics of degeneracy yields a maximum mass for a nonrotating white dwarf, the Chandrasekhar limit—approximately 1.4 solar masses—beyond which it cannot be supported by degeneracy pressure. A carbon-oxygen white dwarf that approaches this mass limit, typically by mass transfer from a companion star, may explode as a Type Ia supernova via a process known as carbon detonation.[1][6] (SN 1006 is thought to be a famous example.)

A white dwarf is very hot when it is formed but since it has no source of energy, it will gradually radiate away its energy and cool down. This means that its radiation, which initially has a high color temperature, will lessen and redden with time. Over a very long time, a white dwarf will cool to temperatures at which it will no longer be visible, and become a cold black dwarf.[6] However, since no white dwarf can be older than the age of the Universe (approximately 13.7 billion years),[10] even the oldest white dwarfs still radiate at temperatures of a few thousandkelvins, and no black dwarfs are thought to exist yet.

Image of Sirius A and Sirius B taken by the Hubble Space Telescope. Sirius B, which is a white dwarf, can be seen as a faint pinprick of light to the lower left of the much brighter Sirius A.

The Nature of White Dwarfs

Once nuclear reactions cease, the stellar remnant has no means of counteracting the force of gravity, and the interior of the star collapses. It doesn't collapse forever because a new force develops which can resist gravity. This force is electron pressure. The material in a white dwarf has been compressed so much by gravity that all the electrons have been stripped away from all of the atomic nuclei. The electrons form a gas. The electrons are squeezed together by gravity, but the electrons become degenerate and resist being squeezed together any further.

Degenerate matter: No two electrons can have exactly the same energy, spin, position so when electrons are compressed enough, they fill up all of the available energy states. Such dense matter is called degenerate.

A white dwarf has a diameter similar to the Earth's and a density such that a teaspoonful weighs a ton!

Models of white dwarfs can be calculated using the laws of quantum mechanics --

The radii of white dwarfs DECREASE with INCREASING mass because of the increasing strength of gravity.

When the mass exceeds 1.4 M , electron degeneracy is no longer strong enough to resist the pull of gravity and the white dwarf abruptly collapses into a neutron star.(animation by G. Rieke)

1.4 M  is called the Chandrasekar limit in honor of the astronomer who first explained the nature of white dwarfs (and won the Nobel prize for his work).

Neutron StarsA neutron star is about 20 km in diameter and has the mass of about 1.4 times that of our Sun. This means that a neutron star is so dense that on Earth, one teaspoonful would weigh a billion tons! Because of its small size and high density, a neutron star possesses a surface gravitational field about 2 x 1011times that of Earth. Neutron stars can also have magnetic fields a million times stronger than the strongest magnetic fields produced on Earth.

Neutron stars are one of the possible ends for a star. They result from massive stars which have mass greater than 4 to 8 times that of our Sun. After these stars have finished burning their nuclear fuel, they undergo a supernova explosion. This explosion blows off the outer layers of a star into a beautiful supernova remnant. The central region of the star collapses under gravity. It collapses so much that protons and electrons combine to form neutrons. Hence the name "neutron star".

Neutron stars may appear in supernova remnants, as isolated objects, or inbinary systems. Four known neutron stars are thought to have planets. When a neutron star is in a binary system, astronomers are able to measure its mass. From a number of such binaries seen with radio or X-ray telescopes, neutron star masses has been found to be about 1.4 times the mass of the Sun. For binary systems containing an unknown object, this information helps distinguish whether the object is a neutron star or a black hole, since black holes are more massive than neutron stars.

Neutron stars are compact objects that are created in the cores of massive starsduring supernova explosions. The core of the star collapses, and crushes together every proton with a corresponding electron turning each electron-proton pair into a neutron. The neutrons, however, can often stop the collapse and remain as a neutron star.

Neutron stars are fascinating objects because they are the most dense objects known. They are only about 10 miles in diameter, yet they are more massive than the Sun. One sugar cube of neutron star material weighs about 100 million tons, which is about as much as a mountain.

Like their less massive counterparts, white dwarfs, the heavier a neutron star gets the smaller it gets. Imagine if a 10 pound bag of flour was smaller than a 5 pound bag!

Neutron stars can be observed occasionally, as with Puppis A above, as an extremely small and hot star within a supernova remnant. However, they are more likely to be seen when they are a pulsar or part of an X-ray binary.

Under the effect of the gravitational collapse of a core heavier than 1.4 solar masses, the matter is forced into a degenerate state : electrons are unable to remain in their orbits around the nuclei (they would have to traver faster than light in order to obey

the Pauli exclusion principle) and they are forced to penetrate the atomic nuclei. So they fuse with protons, and form neutrons.

Pauli's principle, that we've seen before, forbids two neutrons having the same state to stay in the same place . This principle creates a degeneracy pressure fighting against gravity, and so allows the remnant of the star to find an equilibrium state.

The transformation of the atoms of the initial star into neutrons releases a tremendous amount of energy, corresponding to the binding energy of theses atoms. This energy is mainly radiated in the form of neutrinos, over a timescale of several tens of seconds only.The neutrino luminosity of such a supernova is typically 100 times its optical luminosity.

Interaction between neutrinos and the matter is very weak, so they can emerge promptly from the core of the collapsing star, whereas the photons may take hours or days to emerge.Looking at the neutrinos can therefore give informations about the very early stages of core collapse.

Following the explosion of a supernova, a neutron star is created with a temperature probably over 1000 billion degrees.It will rapidly cool in less than 1000 years, to 1 million degrees. After that, its temperature will decrease much more slowly.

At its birth, this neutron star is recovering the rotation of the previous star, following the conservation of angular momentum . It will rotate at a very high speed. The Crab pulsar inside the nebula, for example, spins 30 times a second.

Until recently, one supposed that a neutron star began by rotating at a very high speed, and slowed down with time.This scenario seems satisfactory for a lone star, but in the case of a binary system, where the companion is a small sized star, magnetic coupling effects with the forming accretion disk seems to cause a later acceleration of the spinning speed.

What is a Pulsar and What Makes it Pulse?Simply put, pulsars are rotating neutron stars. And pulsars appear to pulse because they rotate!

A diagram of a pulsar, showing its rotation axisand its magnetic axis

Pulsars were discovered in late 1967 by graduate student Jocelyn Bell Burnell as radio sources that blink on and off at a constant frequency. Now we observe the brightest ones at almost every wavelength of light. Pulsars are spinning neutron stars that have jets of particles moving almost at the speed of lightstreaming out above their magnetic poles. These jets

produce very powerful beams of light. For a similar reason that "true north" and "magnetic north" are different on Earth, the magnetic and rotational axes of a pulsar are also misaligned. Therefore, the beams of light from the jets sweep around as the pulsar rotates, just as the spotlight in a lighthouse does. Like a ship in the ocean that sees only regular flashes of light, we see pulsars "turn on and off" as the beam sweeps over the Earth. Neutron stars for which we see such pulses are called "pulsars", or sometimes "spin-powered pulsars," indicating that the source of energy is the rotation of the neutron star.

X-ray Observations of PulsarsSome pulsars emit X-rays.

Below, we see the famous Crab Nebula, an undisputed example of a neutron star formed during a supernova explosion. The supernova itself was observed in 1054 A.D. These images are from the Einstein X-ray observatory. They show the diffuse emission of the Crab Nebula surrounding the bright pulsar in both the "on" and "off" states, i.e. when the magnetic pole is "in" and "out" of the line-of-sight from Earth.

Crab Pulsar "On" Crab Pulsar "Off"

A very different type of pulsar is seen by X-ray telescopes in some X-ray binaries. In these cases, a neutron star and a normal star form the binary system. The strong gravitational force from the neutron star pulls material from the normal star. The material is funneled onto the neutron star at its magnetic poles. In this process, called accretion, the material becomes so hot that it produces X-rays. The pulses of X-rays are seen when the hot spots on the spinning neutron star rotate through our line of sight from Earth. These pulsars are sometimes called "accretion-powered pulsars" to distinguish them from the spin-powered pulsars.

Pulsars

The powerful magnetic and electrical field which surrounds the star will generate a thin beam of light, in the radio-wave frequencies. This beam sweeps across the sky, in the same way a lighthouse beam sweeps across the sea. These stars are called pulsars

Some pulsars rotate a few hundred times per second. The rotation of a pulsar is exceedingly precise, and can be used as a cosmic clock.

In particular, the system known as PSR 1913+16, made up of two pulsars, allowed the scientists to measure the very small effects of gravitational waves, predicted by general relativity

Because the magnetic axis of the pulsar is not aligned with its rotational axis, the radio emission, generated by particles trapped in the magnetic field lines, will sweep across the sky, like a lighthouse beam sweeps across the sea.

Pulsars:

1) typically do not have visible counterparts

2) their radio output varies in a precise, repetitive pattern

3) they are found in or near supernova remnants

In the late 1960s, radio astronomers discovered a type of radio source that they

dubbed "pulsars"

Pulsars appear to be spinning neutron stars with rotation axes tilted to their magnetic fields. Energetic electrons and light pour out the magnetic poles, and as the star spins the beam of light is swept across the sky like a lighthouse beaconTheir behavior is like a spinning flashlight in a darkened room

Pulsars were considered extremely remarkable when first discovered because of the extremely short periods of their variations (ranging from a fractions of a second to a few seconds; vastly different from the 10-1000 day periods for red giant/supergiant variables), and because of the extreme constancy of their periods --- they are extremely accurate clocks. A normal star could not vary so quickly and regularly, because even at the speed of light it could not communicate across its diameter fast enough! Neutron stars were the only objects predicted to exist that could have the behavior of pulsars.The concept is strengthened by noting that if you imagine shrinking a normal star with its ~20 day rotation period, it would speed up to pulsar-like time scale if shrunk to a size only 10 km in diameter.

Later it was discovered that all pulsars are slowing down, but slowing down so gradually that the change can be detected only be using the most accurate atomic

clocks. Further confirmation comes from the equivalence of the amount of energy in the escaping  beam of particles and light and the energy loss indicated by the rate of slowing down.

Black Holes: What Are They?Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun. If a star that massive or larger undergoes a supernovaexplosion, it may leave behind a fairly massive burned out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinitedensity, creating what is known as a " singularity ". Around the singularity is a region where the force of gravity is so strong that not even light can escape. Thus, no information can reach us from this region. It is therefore called a black hole, and its surface is called the " event horizon ".

But contrary to popular myth, a black hole is not a cosmic vacuum cleaner. If our Sun was suddenly replaced with a black hole of the same mass, the Earth's orbit around the Sun would be unchanged. (Of course the Earth's temperature would change, and there would be no solar wind or solar magnetic storms affecting us.) To be "sucked" into a black hole, one has to cross inside the Schwarzschild radius. At this radius, the escape speed is equal to the speed of light, and once light passes through, even it cannot escape.

The Schwarzschild radius can be calculated using the equation for escape speed:

vesc = (2GM/R)1/2

For photons, or objects with no mass, we can substitute c (the speed of light) for Vesc and find the Schwarzschild radius, R, to be

R = 2GM/c2

If the Sun was replaced with a black hole that had the same mass as the Sun, the Schwarzschild radius would be 3 km (compared to the Sun's radius of nearly 700,000 km). Hence the Earth would have to get very close to get sucked into a black hole at the center of our Solar System.

If We Can't See Them, How Do We Know They're There?

if a black hole passes through a cloud of interstellar matter, or is close to another "normal" star, the black hole can accrete matter into itself. As the matter falls or is pulled towards the black hole, it gains kinetic energy, heats up and is squeezed by tidal forces. The heating ionizes the atoms, and when the atoms reach a few million Kelvin, they emit X-rays. The X-rays are sent off into space before the matter crosses the Schwarzschild radius and crashes into thesingularity. Thus we can see this X-ray emission.

Cygnus X-1 (Cyg X-1) is the longest known of the black hole candidates

What About All the Wormhole Stuff?

Unfortunately, wormholes are more science fiction than they are science fact. A wormhole is a theoretical opening in space-time that one could use to travel to far away places very quickly. The wormhole itself is two copies of the black hole geometry connected by a throat. The throat, or passageway, is called an Einstein-Rosen bridge. It has never been proven that wormholes exist, and there is no experimental evidence for them, but it is fun to think about the possibilities their existence might create

Black Hole A black hole is a region of space whose gravitational force is so strong that nothing can escape from it. A black hole is invisible because it even traps light. The fundamental descriptions of black holes are based on equations in the theory of general relativity developed by the German-born physicist Albert Einstein. The theory was published in 1916.

Characteristics of black holes

The gravitational force is strong near a black hole because all the black hole's matter is concentrated at a single point in its center. Physicists call this point a singularity. It is believed to be much smaller than an atom's nucleus.

The surface of a black hole is known as the event horizon. This is not a normal surface that you could see or touch. At the event horizon, the pull of gravity becomes infinitely strong. Thus, an

object can exist there for only an instant as it plunges inward at the speed of light.

Astronomers use the radius of the event horizon to specify the size of a black hole. The radius of a black hole measured in kilometers equals three times the number of solar masses of material in the black hole. One solar mass is the mass (amount of matter) of the sun.

No one has yet discovered a black hole for certain. To prove that a compact object is a black hole, scientists would have to measure effects that only a black hole could produce. Two such effects would be a severe bending of a light beam and an extreme slowing of time. But astronomers have found compact objects that are almost certainly black holes. The astronomers refer to these objects simply as "black holes" in spite of the small amount of uncertainty. The remainder of this article follows that practice.

Formation of black holes

According to general relativity, a black hole can form when a massive star runs out of nuclear fuel and is crushed by its own gravitational force. While a star burns fuel, it creates an outward push that counters the inward pull of gravity. When no fuel remains, the star can no longer support its own weight. As a result, the core of the star collapses. If the mass of the core is three or more solar masses, the core collapses into a singularity in a fraction of a second.

Galactic black holes

Most astronomers believe that the Milky Way Galaxy -- the galaxy in which our solar system is located -- contains millions of black holes. Scientists have found a number of black holes in the Milky Way. These objects are in binary stars that give off X rays. A binary star is a pair of stars that orbit each other.

In a binary system containing a black hole, that object and a normal, visible star orbit one another closely. As a result, the black hole strips gas from the normal star, and the gas falls violently toward the black hole. Friction between the gas atoms heats the gas near the event horizon to several million degrees.

Consequently, energy radiates from the gas as X rays. Astronomers have detected this radiation with X-ray telescopes.

Astronomers believe that a number of binary star systems contain black holes for two reasons: (1) Each system is a source of intense and variable X rays. The existence of these rays proves that the system contains a compact star -- either a black hole or a less compact object called a neutron star. (2) The visible star orbits the compact object at such a high velocity that the object must be more massive than three solar masses.

Supermassive black holes

Scientists believe that most galaxies have a supermassive black hole at the center. The mass of each of those objects is thought to be between 1 million and 1 billion solar masses. Astronomers suspect that supermassive black holes formed several billion years ago from gas that accumulated in the centers of the galaxies.

There is strong evidence that a supermassive black hole lies at the center of the Milky Way. Astronomers believe this black hole is a radio-wave source known as Sagittarius A* (SgrA*). The clearest indication that SgrA* is a supermassive black hole is the rapid movement of stars around it. The fastest of these stars appears to orbit SgrA* every 15.2 years at speeds that reach about 3,100 miles (5,000 kilometers) per second. The star's motion has led astronomers to conclude that an object several million times as massive as the sun must lie inside the star's orbit. The only known object that could be that massive and fit inside the star's orbit is a black hole.

A black hole is a region of space from which nothing, not even light, can escape. It is the result of the deformation of spacetime caused by a very compact mass. Around a black hole there is an undetectable surface which marks the point of no return. This surface is called an event horizon. It is called "black" because it absorbs all the light that hits it, reflecting nothing, just like a perfect black body inthermodynamics.[1] Quantum mechanics predicts that black holes also emitradiation like a black body with a finite temperature. This temperature decreases with the mass of the

black hole, making it difficult to observe this radiation for black holes of stellar mass.

Despite its invisible interior, a black hole can be observed through its interaction with other matter. A black hole can be inferred by tracking the movement of a group of stars that orbit a region in space. Alternatively, when gas falls into a stellar black hole from a companion star, the gas spirals inward, heating to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and Earth-orbiting telescopes.

Astronomers have identified numerous stellar black hole candidates, and have also found evidence of supermassive black holes at the center of galaxies. In 1998, astronomers found compelling evidence that a supermassive black hole of more than 2 million solar masses is located near the Sagittarius A* region in the center of the Milky Way galaxy. More recent results using additional data indicate that the supermassive black hole is more than 4 million solar masses.