Coma (cometary)
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The comet Ikeya-Zhang exhibiting a bright, condensed coma (March 2002)

In astronomy, a coma (from the Latin word for "hair") is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate.

The coma is generally made of ice and dust. Larger dust particles are left along the comet's orbital path while smaller particles are pushed away from the Sun into the comet's tail by light pressure.

This gives a comet a "fuzzy" appearance when viewed in telescopes and distinguishes it from stars. Stardust was a NASA mission to recover samples of a comet's coma.

In some cases, such as the Great Comet of 1882, a comet develops a notable anti-tail, which appears to point towards the Sun.