Comets are small celestial bodies, consisting mainly of dust and gases that move in an elongated elliptical or nearly parabolic orbit around the sun. When a comet is far from the sun, it consists of a dense solid body or conglomerate of bodies called the nucleus, made up of frozen gases with particles of heavier substances interspersed throughout. This is a few miles in diameter. As it approaches the sun, a luminous "cloud" of dust and gases called the coma envelops the nucleus. The coma's luminosity is caused by the molecules absorbing and reflecting the radiation of the sun.
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