||These are the remains of a very large stars after they have completed their life cycle. Small stars, about the size of our Sun, will end as white dwarfs: small, dense, cold lumps of matter which take no further part in stellar interactions. Larger stars several times the mass of the Sun have a far more spectacular ending. They die in a massive supernova explosion, losing about half their matter to space, and leave behind a neutron star, a tiny and extraordinarily dense object only a few miles across. But the even larger stars, whose mass after the explosion is still several times that of the Sun, will form black holes.
Some theories speculate that many tiny black holes could have formed in the early universe. None of these have yet been observed.
A black hole is an extraordinary object. Its edge or boundary is defined by what is called the â€˜event horizonâ€™. The radius of this is determined by the mass of the black hole. The event horizon is the closest possible distance that light may approach the black hole and not be pulled into it. So strong is the gravity field of a black hole that, within the event horizon, not even light can escape. (This is the reason for their name.) However, Stephen Hawking has shown that some radiation may be emitted from the black hole, due to the â€˜stressingâ€™ of space and time at the boundary.
The event horizon is so called because we may never have any knowledge of events within it. If light cannot escape, then neither can information about the interior. Another bizarre effect that occurs near the event horizon is time dilation. Time is slowed down by large gravitational fields. This effect is just measurable upon the Earth, but becomes enormous near a black hole. At the event horizon, time stops completely. This means that if one observer sees another fall into a black hole, he will see his friend fall more and more slowly and finally stop. His friend will have no such illusion from her point of view, she will fall faster and faster until she passes through the event horizon. Beyond that point, however, she can never escape, and thus can never return to tell us about it. This is not a very likely scenario, in any case, as the gravitational forces would have stretched her into a long, thin thread long before she reached the event horizon.
Black holes, by their nature, are elusive objects. The most conclusive observations of objects thought to be black holes have come from binary x-ray systems, where a visible star is orbiting a massive, unseen companion. The mass and invisible nature of the companion mean that it is very unlikely to be anything other than a black hole. Three such binaries have been observed. Other possible black holes may be found in the centres of galaxies, such as our own Milky Way. JJ
See also stars.