Black holes are fascinating, so letís pause a moment to learn a little more about them. A black hole is a small region of space which contains so much and has such a strong that nothing, not even , can escape. The region therefore looks dark, hence the name. There are thought to be three kinds of black hole:
These lie at the centre of quasars and certain active nuclei that appear to be exploding. of stars together with , and perhaps 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 . There is almost certainly one in the nucleus of our Galaxy.
Stellar black holes
These form at the end of the life of a star more than three times as large as the Sun. Stars with a less mass evolve into or . When the star has no more to 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 kilometers 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 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 . They were proposed by Stephen Hawking. Many tiny primordial black holes, each with a mass of an , might have been created. and 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 effects and the emitted by material falling into them.
A Supermassive black holeís 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 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 s before entering the event horizon. Many centers 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.
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