BLACKHOLE

BLACK HOLE

BLACK HOLE, concept in the study of gravitation. The gravitational field of a black hole, which is an extremely dense body, is so strong that, if the body is large enough, nothing, including electromagnetic radiation, can escape from its vicinity. The body is surrounded by a spherical boundary, called a horizon, through which light can enter but not escape; it therefore appears totally black.

Properties

The black hole concept was developed by the German astronomer Karl Schwarzschild (1873-1916) in 1916 on the basis of Albert Einstein’s theory of general relativity. The radius of the horizon of a Schwarzschild black hole depends only on the mass of the body, being 2.95 km (1.83 mi) times the mass of the body in solar units (the mass of the body divided by the mass of the sun). If a body is electrically charged or rotating, Schwarzschild’s results are modified. An "ergosphere" forms outside the horizon, within which matter is forced to rotate with the black hole; in principle, energy can be emitted from the ergosphere.

According to general relativity, gravitation severely modifies space and time near a black hole. As the horizon is approached from outside, time slows down relative to that of distant observers, stopping completely on the horizon. On this horizon, a celestial object being sucked into a black hole would retain most of its energy, causing the object to reach extremely high temperatures. As it passed through the horizon, both the object and its energy would disappea-a hypothetical phenomenon referred to as advection-dominated accretion flow (ADAF). Once a body has contracted within its Schwarzschild radius, it would theoretically collapse to a singularity, that is, a dimensionless object of infinite density.

Formation

Black holes may form during the course of stellar evolution. As nuclear fuels are exhausted in the core of a star, the pressure associated with their heat is no longer available to resist contraction of the core to ever higher densities. Two new types of pressure arise at densities a million and a million billion times that of water, respectively, and a compact white dwarf or a neutron star may form. If the core mass exceeds about 1.7 solar masses, however, neither electron nor neutron pressure is sufficient to prevent collapse to a black hole.

Astronomers have discovered X-ray emissions from a binary star system, Cygnus X-1, in which the primary is a normal star of about 30 solar masses. Doppler shifts in its spectrum show that a companion object of 10 to 15 solar masses must be in orbit around it; evidence exists that the X rays originate near the companion. Normally such X rays are produced by an "accretion disk", a dense, hot disk of gas that forms as the gas from a normal star spirals into a compact object. The companion in Cygnus X-1, because of its massiveness, is thought likely to be a black hole rather than a white dwarf or neutron star. By the late 1980s, other potential black holes had been found, and many astronomers conjectured that many, if not all, galaxies of substantial size contained a black hole in their center. In 1994, astronomers, with the help of the Hubble Space Telescope, found evidence of a supermassive black hole at the center of M87, a galaxy in the constellation Virgo. Observations of M87 show a spiraling accretion disk 500 light-years across, rotating at about 1.2 million miles per hour, and with a core weighing an estimated 2 billion solar masses.

In 1997, scientists found evidence, again using the Hubble Space Telescope, of three new black holes within 50 million light years of earth; they estimated that the mass of these black holes ranged from 50 to 500 million solar masses. This discovery, as well as data gathered on other possible black holes, has provided stimuli not only for the belief that supermassive black holes exist at the core of galaxies, but for understanding other aspects of these entities, including the thesis that each has a mass proportional to the mass of its host galaxy, though there can be notable differences between aspects of otherwise similarly situated black holes.

Data from several X-ray satellites provided the first direct evidence of the physical forces at work at the edges of black holes; pictures from the satellites allowed scientists to observe horizons of supposed black holes. On the basis of the ADAF theory, scientists observed stars from which energy was emanating toward a dense object. In four cases, the energy disappeared when it collided with the dense object, evidence that the star had passed through the horizon of a black hole.

The English physicist Stephen Hawking has suggested that many black holes may have formed in the early universe. If this is so, many of these black holes could be too far from other matter to form detectable accretion disks, and they could even compose a significant fraction of the total mass of the universe. In reaction to the concept of singularities, Hawking has also proposed that black holes do not collapse in such a manner but instead form "worm holes" to other universes besides our own.

In early 2000 the Chandra X-ray Observatory, a satellite launched in mid-1999, detected a surprisingly low temperature from accretion-disk gases entering a black hole at the center of the Andromeda galaxy (Andromeda is similar in size and shape to the Milky Way as well as relatively close to it). The temperature of the gases entering its black hole is less than that of gases entering the Milky Way’s black hole. One theory to explain the cooler temperature is that much less hot gas than relatively cool gas is entering Andromed’s black hole, resulting in an overall lowering of the temperature of the gases. Chandra also found that Andromed’s black hole emits a much lower ratio of X rays to radio waves than does the Milky Way’s black hole, and that the Milky Way’s black hole is not as bright as astronomers had predicted.

For black holes of sufficiently small mass it is possible for one member of an electron-positron pair near the horizon to fall into the black hole, the other escaping The resulting radiation carries off energy, in a sense evaporating the black hole. Any primordial black holes weighing less than a few billion metric tons would have already evaporated, but heavier ones may remain.