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Supermassive black holes
These lie at the centre of quasars and certain active galactic nuclei that appear to be exploding. Millions or billions of stars together with gas, dust and perhaps planets can fall into the black hole and give off enormous amounts of energy. In 1994 the Hubble Space Telescope provided conclusive evidence for the existence of a supermassive black hole at the centre of the M87 galaxy. It has a mass equal to two to three billion stars but is only as big as the solar system.

Stellar black holes
These form at the end of the life of a red giant star more than three times as large as the Sun. Stars with a less mass evolve into white dwarfs or neutron stars. When the star has no more atoms to fuse it collapses into a singularity, perhaps the size of a full stop. This is the black hole. Any object or light photon passing within a few kilometres gets pulled in. This is called the event horizon and its radius is called the Schwarzschild radius. Inside the event horizon the velocity required to escape from exceeds the speed of light, so that not even rays of light can escape into space.

Mini black holes
These might have formed in the big bang. They were proposed by Stephen Hawking. Many tiny primordial black holes, each with a mass of an asteroid, might have been created. Protons and antiprotons may be created very near a mini black hole. One of them may fall into the black hole while the other escapes so appearing to come out of the black hole. This effectively removes energy from the mini black hole and it evaporates over time.

Observation of black holes
Being black, black holes can never be seen directly. Their existence can only be deduced from their gravitational effects and the radiation emitted by material falling into them.

A Supermassive black hole's existence near the center of a galaxy can be deduced from its effects of its enormous gravitational fields on the stars and gas. The stars here are moving so fast that if there was not an enormously heavy small object there they would fly off.

Stellar black holes can be detected in binary stars such as Cygnus X-1. Here a blue supergiant and a stellar black hole orbit each other. Gas is pulled off the supergiant and falls into the black hole, heating and radiating X rays before entering the event horizon. Many galactic centres are also the source of X rays and matter, probably because of other matter falling into a supermassive black hole there.

Mini black holes have long since evaporated, so cannot now be seen.