Depending on the star's size, a supernova explosion can leave behind a neutron star, also known as a pulsar, or a black hole. A pulsar is an extremely dense ball of neutrons 10 to 20 miles in diameter that can rotate over 700 times per second. A black hole forms when a supernova remnant is so massive that no force of nature can stop its collapse.
Cosmologists describe a black hole as a singularity, an infinitely small, dense point, surrounded by an event horizon a few miles across. Any object that crosses a black hole's event horizon disappears from the universe. The reason is that at such a close distance, the black hole's gravitational pull is so strong that the fabric of space-time becomes warped and nothing, not even light, can escape.
The minimum size necessary for a star to collapse into a black hole following a supernova is 3-3.2 solar masses. This mass is called the Chandrasekhar limit in honor of the astrophysicist who performed these calculations in the 1930's.
Supergiant stars explode in a supernova when they exhaust their fuel supply and iron forms in their cores. The resulting explosion is so intense that the light from a supernova can briefly outshine all the stars in an entire galaxy.