Formation of black holes:

Black holes are thought to form during the course of stellar evolution. As nuclear fuels are exhausted in the core of a star, the pressure associated with their energy production is no longer available to resist contraction of the core to ever-higher densities. Two new types of pressure, electron and neutron pressure, arise at densities a million and a million billion times that of water, respectively, and a compact white dwarf or a neutron star may form. If the star is more than about five times as massive as the Sun, however, neither electron nor neutron pressure is sufficient to prevent collapse to a black hole.

	Stephen William Hawking Author of the best-selling book A Brief History of Time, physicist Stephen Hawking has strived to make difficult concepts in physics more accessible to the public. His discoveries about gravitation are regarded as some of the most important contributions to that area of physics since Albert Einstein introduced the general theory of relativity in 1915. Encarta Encyclopedia Globe Photos, Inc.
	Full Size

In 1994 astronomers used the Hubble Space Telescope (HST) to uncover the first convincing evidence that a black hole exists. They detected an accretion disk (disk of hot, gaseous material) circling the center of the galaxy M87 with an acceleration that indicated the presence of an object 2.5 to 3.5 billion times the mass of the Sun. By 2000, astronomers had detected supermassive black holes in the centers of dozens of galaxies and had found that the masses of the black holes were correlated with the masses of the parent galaxies. More massive galaxies tend to have more massive black holes at their centers. Learning more about galactic black holes will help astronomers learn about the evolution of galaxies and the relationship between galaxies, black holes, and quasars.

The English physicist Stephen Hawking has suggested that many black holes may have formed in the early universe. If this were so, many of these black holes could be too far from other matter to form detectable accretion disks, and they could even compose a significant fraction of the total mass of the universe. For black holes of sufficiently small mass it is possible for only one member of an electron-positron pair near the horizon to fall into the black hole, the other escaping (see X Ray: Pair Production). The resulting radiation carries off energy, in a sense evaporating the black hole. Any primordial black holes weighing less than a few thousand million metric tons would have already evaporated, but heavier ones may remain.

The American astronomer Kip Thorne of California Institute of Technology in Pasadena, California, has evaluated the chance that black holes can collapse to form 'wormholes,' connections between otherwise distant parts of the universe. He concludes that an unknown form of 'exotic matter' would be necessary for such wormholes to survive.