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A supernova is a STAR that explodes. It suddenly increases in brightness by a factor of many billions, and within a few weeks it slowly fades. In terms of the human lifespan, such explosions are rare occurrences. In our Milky Way galaxy, for example, a supernova may be observed every few hundred years. Three such explosions are recorded in history: in 1054, in 1572, and in 1604. The CRAB NEBULA consists of material ejected by the supernova of 1054. Such materials, known as supernova remnants, are common in the heavens. The supernovas observed in modern times have all occurred in other galaxies, the most distant yet having been detected in 1988 in a galaxy 5 billion light-years away. The most interesting supernova of recent times was detected in the relatively nearby Large MAGELLANIC CLOUD, on Feb. 23, 1987, by an astronomer at Chile's Las Campanas Observatory. It quickly became an object of intense study by all the means available to modern astronomy. A supernova may radiate more energy in a few days than the Sun does in 100 million years, and the energy expended in ejecting material is much greater even than this. In many cases, including the Crab nebula supernova, the stellar remnant left behind after the explosion is a NEUTRON STAR--a star only a few kilometers in diameter having an enormously large density and consisting mainly of neutrons--or a PULSAR, a pulsating neutron star. There are two common types of supernovas, called type I and type II. Type I occurs among old stars of small mass, whereas type II occurs among very young stars of large mass. It is not known how a small-mass star can release the very large amounts of energy needed to explain type I supernovas. Scientists generally believe that this must involve binary systems--two stars revolving around each other. In such a system one of the stars is a WHITE DWARF, a small, dense star that is near the end of its nuclear burning phase. After attracting matter from the companion star for some time, the white dwarf eventually collapses with a great rush, becoming a neutron star, and ejecting matter outward. This rebound of matter is thought to be the supernova. Stars with large masses burn their nuclear fuel very rapidly. Within a million years or less, such stars build cores containing much iron. When the iron eventually burns, energy is quickly drained from the core, and the star cannot continue to support itself against gravity. It suffers a mighty collapse analogous to that of a type I supernova, and the rebound causes matter to be ejected in a type II supernova explosion. Stars ending in this way are typically red SUPERGIANTS, but the one that exploded as 1987A was a blue star, named Sanduleak, with a mass only about 15 times that of the Sun. Its pattern of brightening and fading also varied notably from that of typical type II supernovas, and an as yet unexplained "mystery spot" appeared some time after the explosion, apparently near to Sanduleak's former location. In 1989 astronomers thought that they had detected an extremely fast-spinning pulsar at that location, but much further data is still needed before this finding is confirmed. Cosmologists estimate that the Universe came into existence about 15 billion years ago. This involved the initial creation of hydrogen and helium. Since then nuclear fusion in stars has changed some of the original hydrogen and helium into heavier elements (see STELLAR EVOLUTION). Supernovas have played an important role both in producing the heavy ele

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