in astronomy, theoretically predicted final stage in the life history of certain stars (see stellar evolution). A star in the last phases of gravitational collapse is often referred to as a “black hole.” The 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 3 times that of the sun, it will form a neutron star. However, if the final mass of the remaining stellar core is more than 3 solar masses, nothing remains to prevent the star from collapsing without limit to an indefinitely small size. At this point, 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. For a star with a mass equal to that of the sun, this limit is a diameter of only 1.8 mi (3 km). Even light cannot escape the black hole but is turned back by the enormous pull of gravitation. Because light and other forms of energy and matter are permanently trapped inside the black hole, it can never be observed directly. However, a black hole could be detected if it is orbited by a visible star or during the collapse while it was forming. Four possible black holes have been detected in our galaxy, the Milky Way. The first discovered (1971) is Cygnus X-1, an X-ray source in the constellation Cygnus. In the 1980s, one of the strongest cases for a black hole was identified in the constellation Monoceros and labeled A0620–00. V404 Cygni, close to Cygnus X-1, and Nova Muscae 1991 in the constellation Musca were discovered more recently.
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