Only stars that are at least 3 solar masses (3 times the mass of our Sun) can become black holes at the end of their lifespan; stars smaller than this simply become brown dwarves.
The intense gravitational fields generated by black holes will convert any matter into x-ray bursts, as in the case of Cygnus-X1; in fact, only by observing such secondary effects is it possible to locate suspected black holes.
Why are black holes "black"? Gravity bends light; a black hole's gravity is so strong that any light that might be emitted from the collapsed star is bent to the point that it never "escapes". Any object that neither emits nor reflects light is invisible.
It is suspected that there are "super-massive" black holes at the center of all galaxies, but we cannot yet filter out all the interference from the dense star fields near galactic centers to make any reasonable observations.
All known laws of physics break down at the event horizon, or Swarzchild radius, making it impossible to know what is going on inside a black hole.
Stephen Hawking hypothesizes that black holes do emit a form of radiation and eventually disappear. The smaller the hole the faster it emits and disppears. Also, his hypotheses allow for the existence of miniature black holes that exist for only milliseconds.
That's just some starter information; a reasonable Internet search will turn up much more information. Good luck! [img]graemlins/petard.gif[/img]
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