National Aeronautics and Space Administration

Black Holes

A black hole is a region of space from which neither radiation (light) nor matter can escape. We can most easily detect them when they are in a binary star system. This means the black hole has a "normal" star in orbit around it. The black hole has a gravitational field that is so strong that matter is pulled off the companion star into a disk that rotates around the black hole. This rotating disk of matter is called an accretion disk. As the matter from the accretion disk comes close to the event horizon, or outer limit, of the black hole, it heats up to millions of degrees in temperature, releasing X-rays.

A black hole is an endpoint of stellar evolution. When a massive star dies, it will explode violently. This explosion is called a supernova. If the star had enough mass left before it died, it will be collapsed by its own gravity and become a black hole. A black hole resulting from a supernova explosion will have a mass greater than 3 times the mass of the Sun. If the object created by the explosion has between 1.4 and 3 solar masses, it will actually be a neutron star. But if the object has more than 3 solar masses, not even the tightly packed neutrons that make up the neutron star can withstand the star's gravitational pull, and a black hole will be created.

For more information about black holes read:
http://imagine.gsfc.nasa.gov/docs/introduction/black_holes.html
and
http://imagine.gsfc.nasa.gov/docs/teachers/blackholes/blackholes.html