Black holes are thought to form from stars or other massive objects if and when they collapse from their own gravity to form an object whose density is infinite: in other words, a singularity. During most of a star's lifetime, nuclear fusion in the core generates electromagnetic radiation, including photons, the particles of light. This radiation exerts an outward pressure that exactly balances the inward pull of gravity caused by the star's mass. As the nuclear fuel is exhausted, the outward forces of radiation diminish, allowing the gravitation to compress the star inward...
http://archive.ncsa.uiuc.edu/Cyberia/NumRel/BlackHoleFormation.html