A comet consists of a tiny, irregularly shaped nucleus, an extended atmosphere known as the coma, and often along tail of gas and dust. A passing star or molecular cloud can disturb the orbit of a comet nucleus, bringing it near the sun. The solid nucleus of ice and dust hen begins to vaporize, forming first a fuzzy, spherical coma and, as temperatures increase, a tail that may stretch hundreds of milions of kilometers. The dual pressures of sunlight and solar wind push the tail so that it always points away from the sun. The elongated nucleus of comet Halley measures 16 by 8 km and has a surface as black as coal.
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