Black Holes

The term "black hole" was introduced by American theorist John Archibald Wheeler (1911—) in 1968 and refers to an object that is so dense and massive that no electromagnetic radiation can escape from its gravitational field. Although the idea of a black hole goes back to the eighteenth century, the quantitative aspects of such objects were only analyzed in detail using the general theory of relativity. Black holes may be rotating or nonrotating, charged or uncharged, and may even interact thermodynamically with their environment, losing energy. The key variable associated with a black hole is its Schwarzschild radius, named after the general relativist Karl Schwarzschild (1873-1916), who theorized about gravitationally compact objects in 1916. Assume all the mass of an object is contained within a certain radius. If this radius is less than or equal to the Schwarzschild radius then no radiation can escape from the object to the outside. The surface defined by the limiting radius is called the event horizon. All events within the event horizon remain confined to the black hole, locked inside and forever hidden from the outside universe. The Schwarzschild radius is a function of the mass of the black hole;

for an object the mass of the Sun it is about three kilometers; for an object three million times the mass of the Sun it is around 300 astronomical units. A star that collapses and becomes a black hole will interact gravitationally with the rest of the universe but will interact in no other way. Cosmologist Joseph Silk (1980, 264) has likened this situation to the grin of the Cheshire cat: only the gravitational field is left behind.

Theories of stellar evolution have determined that the final stage in the life of a star is dependent on its mass. A star like the Sun will end its life as a white dwarf, while a star much more massive than the sun will end its life as a supernova, and the central part of the supernova will collapse to form a neutron star. If the core is massive enough (more than about two times the mass of the Sun), it will implode and become a black hole. The most studied candidate for a stellar-sized black hole is the X-ray source Cygnus X-1, first detected by satellite in 1965 and investigated extensively in the 1970s and 1980s. Cygnus X-1 was found to be a binary system consisting of a red giant and an invisible companion. From a study of the small perturbations in the position of the visible star it was inferred that the mass of the companion was over five times the mass of the Sun, implying that it must be a black hole. The X rays are emitted, as matter from the visible star is drawn with ever-increasing velocity into the black hole companion, releasing large amounts of energy as it approaches the event horizon. Several other binary star systems similar to Cygnus X-1 have been found. While not proven with certainty, it is believed that each involves a black hole companion star.

The highly speculative discussion around black holes at times has made them seem almost like things from science fiction. What has emerged as observationally sound and theoretically useful is the concept of a very massive, or supermassive, black hole. The current model of quasars and certain types of galaxies conceives of them as a massive central black hole, around which an accretion disk has formed, consisting of matter falling into the central hole and emitting tremendous amounts of energy in the process. Although active galactic nuclei are the most dramatic evidence of the existence of supermassive black holes, it is now believed that such objects also lie at the centers of most galaxies. Indeed, detailed observation of the Milky Way galaxy has pointed to the presence of a very massive black hole at its center. This center is located within the band of the Milky Way at a point known as Sagittarius A, a fairly weak radio source in the constellation of Sagittarius. The center possesses a mass of about 2.7 million solar masses and radiates X rays, the signature characteristic of a black hole. The study of X-ray emissions was greatly aided by the launch of the Chandra orbiting X-ray telescope, and observations with this instrument have contributed greatly to imaging of the galactic nucleus. Infrared observations of stars very close to the galactic center have enabled astronomers to calculate their orbits and periods, and these values have verified the value for the mass of the black hole at this center. The Schwarzschild radius of this hole is about 300 astronomical units.

The concept of a black hole provides an outstanding example of how a notion rooted in theory can come to play an important role in a science largely governed by observation. Black holes as theoretical entities will be an essential part of any detailed explanation of the universe as a whole, both in terms of its current structure and in terms of what we can conclude about its origin and evolution in time. The finding that galactic nuclei consist of black holes has contributed to a theory of the formation and evolution of galaxies. During the early history of a galaxy, very massive stars tend to congregate in its center, leading to the formation of a supermassive black hole and resulting in an object we observe as a quasar, or a Seyfert galaxy. As the galaxy evolves, the accretion disk dissipates, and the energy emitted by the nucleus decreases, resulting eventually in the relatively quiet black hole nucleus of the sort found in the Milky Way system. Black holes have arisen in an entirely different context in theorizing about the first moments of the universe. It has been shown by Stephen Hawking (1942-) that miniature black holes may have developed in the early universe, only to dissipate during the ensuing expansion.

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