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General relativity
G_{\mu \nu} + \Lambda g_{\mu \nu}= {8\pi G\over c^4} T_{\mu \nu}
Einstein field equations
Kepler problem · Lenses · Waves
Frame-dragging · Geodetic effect
Event horizon · Singularity
Black hole

A black hole is a region of space from which nothing, including light, can escape. According to the general theory of relativity, it is the result of the curving of spacetime caused by a very dense mass.

Around a black hole there is a position of no return, called the event horizon. It is called "black" because it absorbs all the light that hits it, reflecting nothing, just like a perfect black body in thermodynamics. Under the theory of quantum mechanics black holes have a temperature and emit Hawking radiation, which makes them slowly get smaller.

A black hole is found by its interaction with 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 black hole caused by a companion star or nebula, the gas spirals inward, heating to very high temperatures and emitting large amounts of radiation. This radiation 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 every galaxy. After observing the motion of nearby stars for 16 years, in 2008 astronomers found compelling evidence that a supermassive black hole of more than 4 million solar masses is located near the Sagittarius A* region in the center of the Milky Way galaxy.

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


In 1783, an English geologist named John Mitchell wrote that it might be possible for something to be so big and heavy that the escape speed from its gravity is equal to the speed of light. Gravity gets stronger as something gets bigger or more massive. For a small thing, like a rocket, to escape from a larger thing, like Earth, it has to escape the pull of our gravity or it will fall back. The speed that it must travel upward to get away from Earth's gravity is called escape velocity. Bigger planets (like Jupiter) and stars have more mass, so have stronger gravity than Earth, so the escape velocity is much faster. John Mitchell thought it was possible for something to be so big that the escape velocity would be faster than the speed of light, so even light could not escape. In 1796, Pierre-Simon Laplace promoted the same idea in the first and second editions of his book Exposition du système du Monde (it was removed from later editions).

Some scientists thought Mitchell might be right, but others thought that light had no mass and would not be pulled by gravity. His theory was forgotten.

In 1916 Albert Einstein wrote an explanation of gravity called general relativity. It is a very complicated theory, but there are two important things about it:

  • Mass causes space (and spacetime) to bend, or curve. Moving things "fall along" or follow the curves in space. This is what we call gravity.
  • Light always travels at the same speed, and is affected by gravity. If it seems to change speed, it is really traveling along a curve in spacetime.

A few months later, while serving in the war, the German physicist Karl Schwarzschild used Einstein's equations to show that a black hole could exist. In 1930, Subrahmanyan Chandrasekhar predicted that stars heavier than the sun could collapse when they ran out of hydrogen or other nuclear fuels to burn and died. In 1939, Robert Oppenheimer and H. Snyder calculated that a star would have to be at least three times as massive as the sun to form a black hole. In 1967, John Wheeler gave black holes the name "black hole" for the first time. Before that, they were called "dark stars".

In 1970, Stephen Hawking and Roger Penrose proved that black holes must exist. Although the black holes are invisible (they cannot be seen), some of the matter that is falling into them is very bright.

Formation of black holes

Gravitational collapse

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

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


Most black holes are made when a supergiant star dies, and leaves behind a mass that is at least one solar mass. Stars die when they run out of hydrogen or other nuclear fuel to burn and iron is produced. Iron does not give off energy and therefore the star has no fuel and in a short amount of time the star collapses.

A supergiant star's death is called a supernova. Stars are usually in equilibrium, which means they are making enough energy to push their mass outward against the force of gravity. When the star runs out of fuel to make energy, gravity takes over. Gravity pulls the center of the star inward very quickly (so quickly that it would have to be repeated several thousand times before it took up a single second), and it collapses into a little ball. This results in the restart of thermonuclear reactions. The star starts expanding again, but again the nuclear fuel goes out. This continues until the star cannot make any more energy and then comes the final collapse. The collapse is so fast and violent that it makes a shock wave, and that causes the rest of the star to explode outward. As the gravity pushes the star inward, the pressure in the center of star reaches to such an extreme level that it enables heavier molecules like iron and carbon to interact to release nuclear energy. The release of the energy from the star during a very short period of time (about one hour) is with such a high rate that it outshines an entire galaxy.

The ball in the center is so dense (a lot of mass in a small space, or volume), that if you could somehow scoop only one teaspoon of material and bring it to Earth, it would sink to the core of the planet. If the remaining mass is of below one solar mass it forms a white dwarf. If it is from 1-3 solar mass it forms a neutron star and if it is above 3 solar mass it forms a black hole.

Even without a supernova, a black hole will form any time there is a lot of matter in a small space, without enough energy to act against gravity and stop it from collapsing.

If supernovas are so bright, why do we not see them often? Actually, there are usually hundreds of years between naked-eye super nova sightings. It is because the period of being a super nova in a star life cycle is only a few hours out of the billions of years in a star's life span. The probability (chance) of looking at a star in sky and that being in super nova state is equal to the ratio of an hour over several billion years.

It is worth mentioning that all of the heavier materials like carbon, oxygen, all the metals, etc., that make the life on the earth possible and are ingredients of all living creatures, can only form in the extreme pressure at the center of a super nova. So we are all a remnant ash from one exploding star several billion years ago. Supernovas replenish the interstellar medium for the next generation of stars also.

Supermassive black holes

Black holes have also been found in the middle of almost every galaxy in the universe. These are called supermassive black holes (SBH), and are the biggest black holes of all. They formed when the Universe was very young, and also helped to form all the galaxies.

Quasars are believed to be powered by gravity collecting material into SBHs in the centers of distant galaxies. Since light cannot escape the SBHs are at the center of quasars, the escaping energy is generated outside the event horizon by gravitational stresses and immense friction on the incoming material.

Huge central masses (106 to 109 solar masses) have been measured in quasars. Several dozen nearby large galaxies, with no sign of a quasar nucleus, contain a similar central black hole in their nuclei. Therefore, it is thought that all large galaxies have one, but only a small fraction are active (with enough accretion to power radiation) and so are seen as quasars.

Effect on light

Accretion disk
Artist's image: a black hole pulling off the outer layer of a nearby star. It is surrounded by an energy disk, which is making a jet of radiation.
Einstein cross
Einstein's Cross: four images from one quasar

At the middle of a black hole, there is a gravitational center called a singularity. It is impossible to see it because the gravity prevents any light escaping. Around the tiny singularity, there is a large area where light which would normally pass by gets sucked in as well. The edge of this area is called the event horizon. The gravity of the black hole gets weaker at a distance. The event horizon is the place farthest away from the middle where the gravity is still strong enough to trap light.

Outside the event horizon, light and matter will still be pulled toward the black hole. If a black hole is surrounded by matter, the matter will form an "accretion disk" (accretion means "gathering") around the black hole. An accretion disk looks something like the rings of Saturn. As it gets sucked in, the matter gets very hot and shoots x-ray radiation into space. Think of this as the water spinning around the hole before it falls in.

Most black holes are too far away for us to see the accretion disk and jet. The only way to know a black hole is there is by seeing how stars, gas and light behave around it. With a black hole nearby, even objects as big as a star move in a different way, usually faster than they would if the black hole was not there.

Since we cannot see black holes, they must be detected by other means. When a black hole passes between us and a source of light, the light bends around the black hole creating a mirror image. That effect is called gravitational lensing.

Hawking radiation

Hawking radiation is black body radiation which is emitted by black holes, due to quantum effects near the event horizon. It is named after the physicist Stephen Hawking, who provided a theoretical argument for its existence in 1974.

Hawking radiation reduces the mass and the energy of the black hole and is therefore also known as black hole evaporation. Because of this, black holes that lose more mass than they gain through other means are expected to shrink and ultimately vanish. Micro black holes (MBHs) are predicted to be larger net emitters of radiation than larger black holes and should shrink and dissipate faster.

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