In astronomy, the nebulous envelope around the nucleus of a comet is called its coma (from the Latin word for "hair"). It is formed when the comet passes close to the sun on its highly elliptical orbit; as the comet warms, parts of it melt and/or vaporize and trail behind the comet. The coma is generally made of ice and dust.

This gives a comet a "fuzzy" appearance when viewed in telescopes and distinguishes it from stars.