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The solid, central part of a comet is known as the 'comet nucleus'. The nucleus is a minor planet composed of rock, dust, and frozen gases. When heated by the sun, the gases sublimate or are ignited, and produce an atmosphere surrounding the nucleus known as the coma. The force exerted on the coma by the sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the sun.
The nucleus is generally up to 50km in diameter.