..... 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,
..... 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 , 1904–67, American physicist, b. New York City, grad. Harvard (B.A., 1925), Ph.D. Univ. of Göttingen, 1927. He taught at the Univ.
..... 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 Astronomy
black hole [¦blak ′hōl]
| 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. |
Black Hole
a celestial object that is formed as a result of the relativistic gravitational collapse of a massive body. In particular, the evolution of a star whose mass at the moment of collapse exceeds some critical value may terminate in catastrophic gravitational collapse. The value of the critical mass is not precisely determined and, depending on the equation of state of matter used, ranges from 1.5 to 3 solar masses (Mʘ).
For any equation of state of matter, the general theory of relativity predicts that no stable equilibrium exists for cold stars of several solar masses. If, after a star becomes unstable, not enough energy is released to halt the collapse or to cause a partial explosion after which the remaining mass would be less than the critical mass, the central portions of the star collapse and, in a short time, reach the gravitational radius rg. No forces whatsoever can prevent the further collapse of a star if the radius of the star shrinks down to rg, which is also known as the Schwarzschild radius and is the radius of a sphere whose surface is called the event horizon. A fundamental property of the event horizon is that no signals emitted from the surface of the star and reaching the event horizon can escape from the region inside that horizon. Thus, as a result of the gravitational collapse of a massive star, a region in space-time is formed from which no information whatsoever about physical processes occurring within the region can emerge.
A black hole has a gravitational field whose properties are determined by the hole’s mass, angular momentum, and—if the collapsing star was electrically charged—electric charge. At large distances, the gravitational field of a black hole is virtually indistinguishable from the gravitational field of a normal star. In addition, the motion of other objects that interact with a black hole at large distances is governed by the laws of Newtonian mechanics. Calculations show that a region known as the ergosphere, which is bounded by a surface called the static, or stationary, limit, should exist outside the event horizon of a rotating black hole. The attractive force that a black hole exerts on a stationary object situated in the ergosphere tends to infinity. However, the attractive force is finite if the object has an angular momentum whose direction coincides with that of the black hole’s angular momentum. Therefore, any particles that happen to be in the ergosphere will revolve around the black hole.
The presence of an ergosphere may lead to energy losses by a rotating black hole. In particular, energy losses are possible in the case where some object that has entered the ergosphere breaks up (for example, as a result of an explosion) into two fragments near the event horizon of the black hole. In this case, one of the fragments continues to fall into the black hole, but the other fragment escapes from the ergosphere. The parameters of the explosion may be such that the energy of the fragment that escapes from the ergosphere is higher than the energy of the original object. The additional energy in this case is drawn from the rotational energy of the black hole.
As the angular momentum of a rotating black hole decreases, the static limit comes closer to the event horizon; when the angular momentum is zero, the static limit and the event horizon coincide and the ergosphere disappears. Owing to the effects of the centrifugal force of rotation, the rapid rotation of a collapsing object prevents the formation of a black hole. Therefore, a black hole cannot have an angular momentum that is greater than some extreme value.
Quantum-mechanical calculations show that particles—such as photons, neutrinos, gravitons, and electron-positron pairs—may be produced in the strong gravitational field of a black hole. As a result, a black hole radiates like a blackbody with an effective temperature of T = 10–6 (Mʘ/M) °K, where M is the mass of the black hole, even in cases where no matter whatsoever falls into the hole. The energy of the radiation is drawn from the energy of the black hole’s gravitational field; as a result, the mass of the black hole decreases with time. However, owing to their low efficiency, the quantum radiation processes are unimportant for massive black holes, which are formed as a result of stellar collapse.
In the early stages of the evolution of the universe, which were hot and ultradense, black holes with masses ranging from 10–5 g to a solar mass or higher may have been formed as a result of an inhomogeneous distribution of matter. In contrast to the black holes that are formed by collapsed stars, these objects are called primordial black holes. Since quantum radiation processes reduce the mass of a black hole, all primordial black holes with a mass of less than 1015 g should have evaporated by the present time. The intensity and effective temperature of black-hole radiation increase as the mass of a black hole decreases. Therefore, in the final stage, the evaporation of a black hole with a mass of the order of 3 × 109 g would be an explosion accompanied by an energy release of 1030 erg in 0.1 sec. Primordial black holes with a mass of greater than 1015 g have remained virtually unchanged. The detection of primordial black holes on the basis of their radiation would make it possible to draw important conclusions about the physical processes that occurred in the early stages of the evolution of the universe.
The search for black holes in the universe is a task of current interest in modern astronomy. Searches are carried out on the assumption that black holes may be the invisible components of certain binary star systems. However, this inference is not definite, since the normal star in a binary system may be invisible against the higher luminosity of the second component. Another method of identifying black holes in binary systems is based on the radiation emitted by matter flowing from the companion, which is a normal star, to the black hole. In this case, a disk consisting of matter flowing to the black hole is formed near the hole; the layers of the disk move around the hole with various velocities (see Figure 1). Owing to friction between adjacent layers, the matter in the disk is heated to tens of millions of degrees. The inner regions of the disk emit energy in the X-ray region of the electromagnetic spectrum. The same type of radiation is produced in the case where a binary system contains a neutron star rather than a black hole. However, a neutron star cannot have a mass higher than some limiting value.
As a result of space studies, a large number of X-ray sources in binary systems have been discovered. The X-ray source Cygnus X-l is the most likely candidate for a black hole. In this binary system, the mass of the X-ray source, which may be estimated from the observed orbital velocity of the optical star and from Kepler’s laws, exceeds 5 Mʘ, that is, is higher than the limiting mass for a neutron star.
The hypothesis has also been advanced that supermassive black holes—that is, black holes with a mass M ≃ 106–108Mʘ—may be located in the nuclei of active galaxies and in quasars. In this case, the activity of the active galactic nuclei and quasars is attributed to the infall of ambient gas onto the black hole.
REFERENCES
Zel’dovich, Ia. B., and I. D. Novikov. Teoriia tiagoteniia i evoliutsiia zvezd. Moscow, 1971.Penrose, R. “Chernydyry.” Uspekhi fizicheskik nauk, 1973, vol. 109, issue 2.
Shklovskii, I. S. Zvezdy: Ikh rozhdenie, zhizn’ i smert’. Moscow, 1975.
Thorne, K. “Poiski chernykh dyr.” Uspekhi fizicheskikh nauk, 1976, vol. 118, issue 3. (Article translated from English.)
Frolov, V. P. “Chernye dyry i kvantovye protsessy v nikh.” Uspekhi fizicheskik nauk, 1976, vol. 118, issue 3.
Shakura, N. I. Neitronnye zvezdy i “chernye dyry” v dvoinykh zvezdnykh sistemakh. Moscow, 1976.
Novikov, I. D. Chernye dyry vo Vselennoi. Moscow, 1977.
Misner, C, K. Thorne, and J. Wheeler. Gravitatsiia, vols. 1–3. Moscow, 1977. (Translated from English.)
N. I. SHAKURA
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