The supermassive black hole inside the core of the supergiant elliptical galaxy Messier 87 in the constellation Virgo. The black hole was the first to be directly imaged (Event Horizon Telescope, released April 10, 2019).
A **black hole** is a region of space-time from which nothing, not even light, can escape. According to the general theory of relativity, it starts existing when spacetime gets curved by a huge mass. There is a sphere around the black hole. If something goes inside the sphere, it cannot leave. This sphere is called the event horizon. A black hole is black because it absorbs all the light that hits it. It reflects nothing, just like a perfect black body in thermodynamics. Under quantum mechanics, black holes have a temperature and emit Hawking radiation, which makes them slowly get smaller.

Because black holes are very hard to see, people find them by the way they affect other things near them. The place where there is a black hole can be found by tracking the movement of stars that orbit somewhere in space. Or people can find it when gas falls into a black hole, because the gas heats up and is very bright. This can be found by telescopes on Earth or by Earth-orbiting telescopes such as Chandra or XMM Newton. Inside a black hole the rules of physics are very different.

Astronomers have found evidence of supermassive black holes at the center of almost all galaxies. In 2008 astronomers found evidence that a supermassive black hole of more than four million solar masses is near the Sagittarius A\* part of the Milky Way galaxy.

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

History
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In 1783, an English clergyman named John Michell wrote that it might be possible for something to be so heavy you would have to go at the speed of light to get away from its gravity. Gravity gets stronger as something gets more massive. For a small thing, like a rocket, to escape from a larger thing, like Earth, it has to escape the pull of the Earth's gravity or it will fall back. The speed that it must travel to get away is called escape velocity. Bigger planets (like Jupiter) and stars have more mass, and have stronger gravity than Earth. Therefore, the escape velocity would need to be much faster. John Michell thought it was possible for something to be so big that the escape velocity would need to be faster than the speed of light, so even light could not escape. In 1796, Pierre-Simon Laplace wrote about 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 Michell 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.

* 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 World War I, the German physicist Karl Schwarzschild used Einstein's equations to show that a black hole could exist. The Schwarzschild radius is the size of the event horizon of a non-rotating black hole. This radius was the measurement where the escape velocity was equal to the speed of light. If the radius of a star is smaller, then light is unable to escape, and it would be a dark star, or black hole.

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. 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 invented the name "black hole" for the first time. Before that, they were called "dark stars".

In 1970, Stephen Hawking and Roger Penrose showed 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.

As of spring 2019, there was an image of a black hole, or rather, the things orbiting the black hole. The image required many photos from different locations. One of the team members (Katie Bouman) made a compilation of all the images into one singular image.

In 2020, Roger Penrose, Reinhard Genzel and Andrea Ghez were awarded the Nobel Prize in Physics for their work on the theory of black holes.

Formation of black holes
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The gravitational collapse of huge (high-mass) stars cause "stellar mass" black holes. This happens when really big stars run out of fuel.

Star formation in the early universe may have created very big stars. These stars were so large that their cores collapsed into black holes. The black hole at the center of these stars slowly consumed the star from within making black holes of up to 103 solar masses. These black holes may be the seeds of the supermassive black holes found in the centers of most galaxies. If these stars really existed, they may even explain where supermassive black holes come from.

Most of the energy released in gravitational collapse is given off very quickly. A distant observer sees the material falling in slowly and then stop just above the event horizon because of gravitational time dilation. The light given off just before the event horizon is delayed an infinite amount of time. So the observer never sees the formation of the event horizon. Instead, the collapsing material seems to become dimmer and increasingly red-shifted, eventually fading away.

Supermassive black holes
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See the main article: Supermassive black hole
Black holes have also been found in the middle of almost every galaxy in the known 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 supermassive black holes in the centers of distant galaxies. Light cannot escape the SBHs at the center of quasars, so the escaping energy is made 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, have 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.

Parts of a Black Hole
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### The Event Horizon

See the main article: Event horizon
A black hole is separated from the rest of the Universe by its event horizon. Anything which crosses it cannot escape, not even light. Anything that happens inside this boundary can't be seen from the outside.

Near a black hole, time appears to slow down. This is known as gravitational time dilation. This happens because gravity affects time. The stronger the gravity, the slower time passes. Objects seem to turn red and fade away when they reach the event horizon. This is known as gravitational redshift. 

The shape of the event horizon for non-rotating black holes is spherical and slightly flattened for spinning ones.

The photon sphere is a spherical boundary of zero thickness in which photons that move on tangents to that sphere would be trapped in a circular orbit about the black hole.

Effect on light
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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'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 into it because the gravity prevents any light escaping.

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" 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. This effect is called gravitational lensing.

Hawking radiation
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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*. This happens because of the virtual particle-antiparticle pairs. Due to quantum fluctuations, this is when one of the particles falls in and the other gets away with the energy/mass. 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.

Properties of black holes
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See the main article: No-hair theorem
The no hair theorem basically says that once a black hole has formed, it only has three physical properties that we can measure: mass, charge, and spin. If this is true, then if two black holes had the same mass, charge and spin then they will look the same. As of 2020, it is unclear if the no hair theorem is true for real black holes.

The properties are special, because all of them can be measured from outside the black hole. For example, a charged black hole repels other like charges just like any other charged object. Similarly, the total mass inside a sphere containing a black hole can be found by using the gravitational analog of Gauss's law, far away from the black hole. The angular momentum or spin can also be measured from far away.

When anything falls into a black hole, any property that isn't part of the three properties of a black hole, like shape or charge distribution, gets erased and can't be seen from outside. This is a very big problem because we lose a lot of information about what went into the black hole, including certain quantum properties and physics tells us that information cannot be destroyed. This puzzle is called the black hole information loss paradox.