Comets are frozen balls of gas and dust that orbit the Sun in a
"cloud" of about 105 Astronomical Units (A.U.)
in radius. Comets consist of two main parts: the head and
the tail. The head is also believed to be made up of two parts: the
nucleus, composed of dust and frozen compounds such as methane,
ammonia, and water; and the coma, which is the part of the comet
that becomes visible from the Earth when the comet approaches the Sun.
When this happens, the ice in the comet vaporizes and forms a huge sphere
of expanding flourescent gas. This gas can be blown back by the solar
wind, creating one or two long tails pointing away from the Sun.
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