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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 then 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 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 can collect and collapse into supermassive black holes, such as are found at the center of large galaxies. The British physicist Stephen Hawking 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). 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 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. The first discovery (1971) of a possible black hole was Cygnus X-1, an X-ray source in the constellation Cygnus. In 1994 astronomers employing the Hubble Space Telescope announced that they had found conclusive evidence of a supermassive black hole in the M87 galaxy in the constellation Virgo. Since then others have been found, and in 2011 astronomers announced the discovery of one, in NGC 4889 in the constellation Coma, whose mass may be as great as 21 billion times that of the sun. It is now believed that in most cases supermassive black holes are found at the center of spiral and elliptical galaxies. Sagittarius A*, a compact radio source at the center of the Milky Way is believed to be a supermassive black hole with a mass about 4.6 million times that of the sun. The Chandra observatory has also discovered that massive black holes were associated with galaxies that existed 13 billion years ago.
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 (1,080 million km/hr). The black holes will then collide spectacularly, spewing radiation and gravitational waves across the universe. Subsequently, the Laser Interferometer Gravitational Wave Observatory (since 2015) and the European Gravitational Observatory (since 2017) several times have detected gravitational waves that resulted from the merging of other black hole pairs. In 2019, researchers with the Event Horizon Telescope, a very long baseline array of radio telescopes, imaged the halo of gas and dust outlining the black hole at the heart of the M87 galaxy.
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); M. Bartusiak, Black Hole (2015).
gravitational collapseContraction of a body arising from the mutual gravitational pull of all its constituents. Although there are several examples of such contraction processes in astronomy, ‘gravitational collapse’ usually refers to the sudden collapse of the core of a massive star at the end of nuclear burning, when its internal gas pressure can no longer support its weight. For a massive star this may initially result in a supernova explosion, removing much of the star's mass. The eventual degree of gravitational collapse is determined by the mass that remains after a supernova, or after any other form of mass loss. The three most likely end-products (in order of increasing mass) are white dwarfs, neutron stars, and black holes.
(in astronomy), the catastrophically rapid compression of a star under the action of gravitational attraction.
According to existing astronomical conceptions, gravitational collapse plays a decisive role in the late stages of the evolution of massive stars. During the billions of years of its prior existence, a star is in equilibrium: the forces of gravitational attraction, which tend to compress the star’s material, are balanced by the forces of hot gas pressure, which counteract compression. Thermonuclear reactions proceeding in the star’s central regions at temperatures of tens of millions of degrees are the sources of the star’s radiant energy. After several billion years, the star’s nuclear sources of energy are exhausted. Meanwhile, the star continues to lose energy, radiating light into space from its surface and neutrinos from its interior. This leads to a very slow contraction of the star’s central regions. If the star’s mass is not less than 1.2 solar masses, then the density and pressure in the star’s central regions increase so much that nuclear reactions begin to occur involving the breakdown of complex nuclei, during which an enormous amount of heat is absorbed. This leads to the following: with the increase in the density of the gas the forces of hot gas pressure do not rise as fast as the gravitational forces, the equilibrium between these forces is upset, and under the influence of gravity, now not balanced by the force of gas pressure, the star tends to contract—gravitational collapse occurs.
The process takes a fraction of a second, but in this time the density of the central parts of the star increases to that of the atomic nucleus, about 1014 g/cm3. Now the already powerful repulsive forces of the nuclear particles pressing on each other slow or even halt the compression of matter in the star’s central regions. The falling outer layers of the star encounter the layers that have come to rest, and an outward-traveling shock wave is generated, which is reinforced by neutrinos emanating from the interior and by the detonation of the remnants of the nuclear “fuel” in the star’s envelope. The star’s outer layers are ejected into space. This ejection process is observed as the explosions of supernovas. The core remaining after the ejection of the envelope of a star with a mass not exceeding 2 solar masses is a neutron star. Astronomers observe such stars as sources of pulsating radio emission—pulsars.
If the mass of the star’s core is large (greater than 2 solar masses), then the repulsion of the nuclear particles is not able to withstand the gravity, and the star’s core, after rapid cooling, will continue to contract. In this case, its gravitational field increases so much that the effects of the general theory of relativity begin to play a role, and no force can any longer halt the contraction. This stage of a star’s evolution is called relativistic gravitational collapse. When the star’s radius becomes equal to a critical value (determined by the star’s mass and equal to 3 M0 km, where M0 is the star’s mass expressed in solar masses), the gravitational field no longer releases radiation or particles. Such a celestial object is called a black hole or frozen star.
REFERENCEZel’dovich, Ia. B., and I. D. Novikov. Teoríia tiagoteniia i evoliutsiia zvezd. Moscow, 1971.
I. D. NOVIKOV