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black hole, in space, postulated object whose mass is so intensely concentrated that normal properties of space in its vicinity are altered drastically and even light cannot escape the gravitational attraction within a certain distance from the centre. No black hole has been certainly identified, and the best way of trying to identify one is still uncertain.
The black hole is one of the three postulated final stages of stellar evolution (see also star, neutron; star, white dwarf). It is the theoretical end result for all stars whose mass is much greater than that of the Sun. It is theorized that when the nuclear fuel of such a star is finally exhausted, its core cools and contracts and begins to collapse under the enormous weight of the outer layers. If the mass of the star, or the energy of the inward-falling matter, is too great for the collapse to be halted by the formation of a white dwarf or neutron star structure, the collapse or implosion continues ad infinitum into the black hole stage predicted by the theory of general relativity. Most properties of the black hole are not observable. Space-time around the “object” is so sharply curved that no light escapes, no matter can be ejected, and all details of infalling matter are obliterated. No stationary external observer can see anything of phenomena occurring within a radius at which the escape velocity (i.e., the velocity required for a body to escape the gravitational dominance of the black hole) is close to the speed of light. This radius is also called the event horizon, or Schwarzschild radius, after the German astronomer Karl Schwarzschild, who in 1907 predicted the existence of such invisible stars.
To an observer outside, matter falling into the black hole takes forever to reach the Schwarzschild radius; but relative to an observing station moving in with the collapsing matter, the time of fall to the centre is very short. As the centre is approached, the curvature of the space-time “whirlpool” continues to increase, becoming infinite at the central singularity. Under such extreme conditions an outside observer cannot associate meaningful times with interior events; and hence no communication is possible with an observer inside the Schwarzschild radius. As measured by its Schwarzschild radius, a typical black hole can be expected to be from, say, two kilometres (about one mile) to several tens of kilometres in diameter depending upon its age, mass, and angular momentum. All matter collapsing into the black hole is soon compressed to a near-infinite density, losing in the process virtually every property of its separate identity except mass, electric charge, and angular momentum.
The most likely place to find a black hole seems to be in the company of a normal star, for black holes should be unobservable except in terms of their influence on material that is close by. If a black hole were embedded in or sufficiently near a normal star, matter flowing into it from such a source might become strongly heated, and would then radiate Xrays or gamma rays copiously before entering the Schwarzschild radius of the black hole and disappearing. It has been suggested that a peculiar star, or object, in the binary system Epsilon Aurigae is a black hole. This object seems to have a mass 23 times that of the Sun, yet it emits no visible light.
