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 stellar evolution, life history of a star , beginning with its condensation out of the interstellar gas (see interstellar matter ) and ending, sometimes catastrophically, when the star has exhausted its nuclear fuel or can no longer adjust itself to a stable
..... Click the link for more information. ) by the American physicist John A. Wheeler Wheeler, John, 1911–, American physicist and educator, b. Jacksonville, Fla. Educated at Johns Hopkins University (Ph.D., 1933), he joined the faculty at Princeton in 1938, and after 1976 was director of the Center for Theoretical Physics at the Univ.
..... Click the link for more information. .

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 pressure, in mechanics, ratio of the force acting on a surface to the area of the surface; it is thus distinct from the total force acting on a surface. A force can be applied to and sustained by a single point on a solid.
..... Click the link for more information. , that supports the star against the compressive force of its own gravitation gravitation, the attractive force existing between any two particles of matter .

The Law of Universal Gravitation

Since the gravitational force is experienced by all matter in the universe, from the largest galaxies down to the smallest particles, it
..... Click the link for more information. . 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 white dwarf, in astronomy, a type of star that is abnormally faint for its white-hot temperature (see mass-luminosity relation ). Typically, a white dwarf star has the mass of the sun and the radius of the earth but does not emit enough light or other radiation to be
..... Click the link for more information. , while a more massive one would become a supernova supernova, a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold.
..... Click the link for more information. . If the mass is less than three times that of the sun, it will then form a neutron star neutron star, extremely small, extremely dense star, about double the sun's mass but only a few kilometers in radius, in the final stage of stellar evolution . Astronomers Baade and Zwicky predicted the existence of neutron stars in 1933.
..... Click the link for more information. . 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 Oppenheimer, J. Robert (ŏp`ənhī'mər), 1904–67, American physicist, b. New York City, grad. Harvard (B.A., 1925), Ph.
..... Click the link for more information. 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 relativity, physical theory, introduced by Albert Einstein, that discards the concept of absolute motion and instead treats only relative motion between two systems or frames of reference.
..... Click the link for more information. 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 1.86 mi (3.0 km). Even light cannot escape a black hole, but is turned back by the enormous pull of gravitation.

It is now believed that the origin of some black holes is nonstellar. Some astrophysicists suggest that immense volumes of interstellar matter interstellar matter, matter in a galaxy between the stars, known also as the interstellar medium.

Distribution of Interstellar Matter

Compared to the size of an entire galaxy, stars are virtually points, so that the region occupied by the
..... Click the link for more information. can collect and collapse into supermassive black holes, such as are found at the center of some galaxies. The British physicist Stephen Hawking Hawking, Stephen William, 1942–, British theoretical physicist, b. Oxford, England, grad. University College, Oxford, 1962, Ph.D. Trinity Hall, Cambridge, 1966.
..... Click the link for more information. has postulated still another kind of nonstellar 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 cosmology, area of science that aims at a comprehensive theory of the structure and evolution of the entire physical universe .

Modern Cosmological Theories

..... Click the link for more information. ). Unlike stellar black holes, primordial black holes create and emit elementary particles elementary particles, the most basic physical constituents of the universe.

Basic Constituents of Matter

Molecules are built up from the atom , which is the basic unit of any chemical element .
..... Click the link for more information. , 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 gamma radiation, high-energy photons emitted as one of the three types of radiation resulting from natural radioactivity . It is the most energetic form of electromagnetic radiation , with a very short wavelength (high frequency).
..... Click the link for more information. . Also released is longer-lasting electromagnetic radiation electromagnetic radiation, energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an
..... Click the link for more information. in the form of X rays X ray, invisible, highly penetrating electromagnetic radiation of much shorter wavelength (higher frequency) than visible light. The wavelength range for X rays is from about 10⁻⁸ m to about 10⁻¹¹
..... Click the link for more information. , 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 can 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. The first discovered (1971) was Cygnus X-1, an X-ray source in the constellation Cygnus. In 1994 astronomers employing the Hubble Space Telescope Hubble Space Telescope (HST), the first large optical orbiting observatory . Built from 1978 to 1990 at a cost of $1.5 billion, the HST (named for astronomer E. P. Hubble ) was expected to provide the clearest view yet obtained of the universe.
..... Click the link for more information. announced that they had found conclusive evidence of a supermassive black hole in the M87 galaxy in the constellation Virgo. The first evidence (2002) of a binary black hole, two supermassive black holes circling one another, was detected in images from the orbiting Chandra X-ray Observatory. Located in the galaxy NGC6240, the pair are 3,000 light years apart, travel around each other at a speed of about 22,000 mph (35,415 km/hr), and have the mass of 100 million suns each. As the distance between them shrinks over 100 million years, the circling speed will increase until it approaches the speed of light, about 671 million mph (1080 million km/hr). The black holes will then collide spectacularly, spewing radiation and gravitational waves across the universe.

Bibliography

See S. W. Hawking, Black Holes and Baby Universes and Other Essays (1994); P. Strathern, The Big Idea: Hawking and Black Holes (1998); J. A. Wheeler, Geons, Black Holes, and Quantum Foam: A Life in Physics (1998); H. Falcke and F. W. Hehl, The Galactic Black Hole: Studies in High Energy Physics, Cosmology and Gravitation (2002).

black hole

Cosmic body with gravity (see gravitation) so intense that nothing, not even light, can escape. It is suspected to form in the death and collapse of a star that has retained at least three times the Sun's mass. Stars with less mass evolve into white dwarf stars or neutron stars. Details of a black hole's structure are calculated from Albert Einstein's general theory of relativity: a “singularity” of zero volume and infinite density pulls in all matter and energy that comes within an event horizon, defined by the Schwarzschild radius, around it. Black holes cannot be observed directly because they are small and emit no light. However, their enormous gravitational fields affect nearby matter, which is drawn in and emits X rays as it collides at high speed outside the event horizon. Some black holes may have nonstellar origins. Astronomers speculate that supermassive black holes at the centres of quasars and many galaxies are the source of energetic activity that is observed. Stephen W. Hawking theorized the creation of numerous tiny black holes, possibly no more massive than an asteroid, during the big bang. These primordial “mini black holes” lose mass over time and disappear as a result of Hawking radiation. Although black holes remain theoretical, the case for their existence is supported by many observations of phenomena that match their predicted effects.

black hole

See blackholing, spam filter and Blacklist of Internet Advertisers.

black hole Astronomy

an object in space so dense that its escape velocity exceeds the speed of light

1.		black hole - An expression which depends on its own value or a technique to detect such expressions. In graph reduction, when the reduction of an expression is begun, the root of the expression can be overwritten with a black hole. If the expression depends on its own value, e.g. x = x + 1 then it will try to evaluate the black hole which will usually print an error message and abort the program. A secondary effect is that, once the root of the expression has been black-holed, parts of the expression which are no longer required may be freed for garbage collection. Without black holes the usual result of attempting to evaluate an expression which depends on itself would be a stack overflow. If the expression is evaluated successfully then the black hole will be updated with the value. Expressions such as ones = 1 : ones are not black holes because the list constructor, : is lazy so the reference to ones is not evaluated when evaluating ones to WHNF.
2.		black hole - Where an electronic mail message or news aritcle has gone if it disappears mysteriously between its origin and destination sites without returning a bounce message. Compare bit bucket.