A black hole is a theorized celestial body whose surface gravity is so strong that no light can escape from within it. If a star more massive that the sun undergoes gravitational collapse at the end of its evolution, it will form a white dwarf, a neutron star, or (according to theory) a black hole, depending primarily on its mass. The only other way to identify a black hole would be though its interaction with other matter. For example, if the black hole is formed in a binary star system, gas form the normal star may later flow toward the black hole.
Black holes, if they exist, could come in a great range of sizes. The english physicist Stephen Hawking has speculated that tiny black holes with masses no larger than that of a large mountain are possible. Such black holes, in the size range of elementary particles, would have been formed only under the extreme conditions that cosmology theories indicate existed in the very first moments of the universe. On the other hand, gigantic black holes may lie at the center of galaxies. Some astronomy experts suggest that such black holes may be liked to the differences that exist between galaxies.
 A popular misconception is that black holes act like cosmic vacuum cleaners, sucking up everything within reach. In fact the gravitational attraction of a black hole would be stronger that that of a noral star of the same mass. Thus if the Sun could collapse to form a black hole, and of course the Earth would contiue to orbit just asit does now.
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