When stars that are about three-times the mass of the sun "die," they will experience an iron core collapse followed by a supernova. The gravitational pull from the core collapse will eventually form a neutron star. However, when stars that are eight-times or greater the mass of the sun "die" something extraordinary can happen. Scientific models have demonstrated that when these high mass stars experience an iron core collapse and subsequent supernova, the gravitational pull from the iron core collapse may be so strong that it pulls all or most of the matter that it blew out from the supernova back into the collapsed core. The addition of this huge volume of mass sets off a chain reaction that causes heat and pressure to increase the strength of gravity exponentially until gravity's "crush" becomes infinite. This phenomenon is called a "Black Hole". Black Holes are named such because in theory the gravitation force inside the black hole is so strong that not even light can escape from falling into its center.
             Although there is still very much that we do not know about black holes, astronomers are gathering more information about them all the time. Because black holes do not emit any visible light, the only way astronomers can study them is to observe their affects on surrounding matter. Our textbook, The Cosmic Perspective, describes the best conditions for observing Black Holes:
             Black holes in close binaries should be among the easiest to identify. Gas over flowing a black hole's stellar companion will form a hot, x- ray emitting accretion disk similar to the disks that circle accreting neutron stars. The x- rays can escape because the disk emits from well outside the event horizon (Bennett, J., Donahue, M., Schneider, N., Voit, M., 2004, pg 586-587).
             The x-ray binary called Cygnus X-1 may be the most promising observational evidence of a black hole. This binary conta...