Definition : An extremely dense celestial body that has been theorized to exist in the universe. The gravitational field of a black hole 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 the event horizon, through which light can enter but not escape; therefore it appears totally black.
Reason why light cannot escape
The ESCAPE VELOCITY, V, from a body of mass M and radius R is given by V = (2MG/R)^½ where G is the constant of universal gravitation. The larger the mass M and the smaller the radius R, the greater becomes the velocity V needed for any particle to escape from the gravitational attraction of the body. At the limit when V becomes the velocity of light, c, not even light can escape. A black hole of mass M has a maximum radius Rs, given by Rs = 2MG/c², known as the SCHWARZCHILD RADIUS.
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.
Procedure of Formation : A massive star starts to collapse when it exhausts its nuclear fuel and can no longer counteract the inward pull of gravity. The crushing weight of the starís overlying layers implodes the core, and the star digs deeper into the fabric of space-time. Although the star remains barely visible, its light now has a difficult time climbing out of the enormous gravity of the still-collapsing core. The star passes through its event horizon and disappears from our universe, forming a singularity of infinite density.
The English physicist Stephen Hawking has suggested that many black holes may have formed in the early universe. If this were 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 so-called "worm holes" to other universes besides our own.
For black holes of sufficiently small mass it is possible for only 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 thousand million metric tons would have already evaporated, but heavier ones may remain.
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