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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.
