black hole, in astronomy, celestial object of such extremely intense gravity that it attracts everything near it and in some instances prevents everything, including light, from escaping. The term was first used in reference to a star in the last phases of gravitational collapse (the final stage in the life history of certain stars; see stellar evolution), by the American physicist John A. Wheeler. Gravitational collapse begins when a star has depleted its steady sources of nuclear energy and can no longer produce the expansive force, a result of normal gas pressure, that supports the star against the compressive force of its own gravitation. As the star shrinks in size (and increases in density), it may assume one of several forms depending upon its mass. A less massive star may become a white dwarf, while a more massive one would become a supernova.
If the mass is less than three times that of the sun, it will form a neutron star. However, if the final mass of the remaining stellar core is more than three solar masses, as shown by the American physicists J. Robert Oppenheimer and Hartland S. Snyder in 1939, nothing remains to prevent the star from collapsing without limit to an indefinitely small size and infinitely large density, a point called the 'singularity. At the point of singularity the effects of Einstein's general theory of relativity become paramount. According to this theory, space becomes curved in the vicinity of matter; the greater the concentration of matter, the greater the curvature.
When the star (or supernova remnant) shrinks below a certain size determined by its mass, the extreme curvature of space seals off contact with the outside world. The place beyond which no radiation can escape is called the event horizon, and its radius is called the Schwarzschild radius after the German astronomer Karl Schwarzschild, who in 1916 postulated the existence of collapsed celestial objects that emit no radiation. For a star with a mass equal to that of the sun, this limit is a radius of only 0. 9 mi (1.
5 km). Even light cannot escape the black hole but is turned back by the enormous pull of gravitation. It is now believed that the origin of some black holes is non stellar. Some astrophysicists suggest that immense volumes of interstellar matter can collect and collapse into super massive black holes, such as are found at the center of some galaxies.
The British physicist Stephen Hawking has postulated still another kind of non stellar black hole. Called a primordial, or mini, black hole, it would have been created during the 'big bang, in which the universe was created (see cosmology). Unlike stellar black holes, primordial black holes create and emit elementary particles, called Hawking radiation, until they exhaust their energy and expire. It has also been suggested that the formation of black holes may be associated with intense gamma ray bursts.
Beginning with a giant star collapsing on itself or the collision of two neutron stars, waves of radiation and subatomic particles are propelled outward from the nascent black hole and collide with one another, releasing the gamma radiation. Also released is longer-lasting electromagnetic radiation in the form of X rays, radio waves, and visible wavelengths that can be used to pinpoint the location of the disturbance. Because light and other forms of energy and matter are permanently trapped inside a black hole, it can never be observed directly. However, a black hole could be detected by the effect of its gravitational field on nearby objects (e. g.
, if it is orbited by a visible star), during the collapse while it was forming, or by the X rays and radio frequency signals emitted by rapidly swirling matter being pulled into the black hole. A small number of possible black holes have been detected, although none of the discoveries has been conclusive. The first discovered (1971) was Cygnus X-1, an X-ray source in the constellation Cygnus. In the 1980 s, one of the strongest cases for a black hole was identified in the constellation Monoceros and labeled A 0620-00. V 404 Cygne, close to Cygnus X-1, and Nova Muscle 1991 in the constellation Musca were discovered around the turn of the decade. In 1994 a possible black hole at the center of galaxy M 87 was found using the Hubble Space Telescope, and a year later NGC 4261 in the constellation Virgo, QZ Vul in the constellation Vulpecca, and a potential black hole deep within the spiral galaxy NGC 4258 were identified.