Black holes are perhaps the strangest objects known. They were first theoretically predicted by Schwarzschild in 1916 and first observed indirectly during the 1960s. By definition, a black hole is a region of space which is so densely packed with matter that nothing, not even light, can escape from its gravitational pull.

Very massive stars end their lives as black holes. Most stars, including the sun, will end their lives as white dwarves once all their nuclear fuel runs out. Stars with masses above about 1.44 solar masses (the Chandrasekhar Limit) cannot support the electron degeneracy pressure necessary to maintain them as white dwarves, so they collapse further. For masses up to about three solar masses, they end their lives as neutron stars. Beyond this mass, the neutron degeneracy pressure can no longer sustain them, so they collapse completely, into black holes. Several black hole candidates are currently known, the most famous in the constellation Cygnus X-1.

In addition to collapsed stars, now every galaxy, including the Milky Way, is believed to have a large black hole at its center. In fact, quasars, very young galaxies in formation, are believed to have black holes which are consuming many stars, causing huge releases of energy.

In 1974, Stephen Hawking discovered that black holes are not as black as was initially thought. According to quantum mechanics, a black hole should in fact emit particles, behaving as a blackbody with a temperature inversely proportional to its Schwarzschild radius. For black holes formed from collapsed stars, this temperature is quite small - of the order of 10^-10 degrees Kelvin. Eventually (after 10^66 years or so!), these black holes will eventually evaporate completely and explode. In fact, after 10^100 years or so, it is believed that the entire universe will have become Hawking radiation once the last black holes have decayed.