In astronomy, a coma (from the Greek κόμη, "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. This gives a comet a "fuzzy" appearance when viewed in telescopes and distinguishes it from stars.
The coma is generally made of ice and dust. Water dominates up to 90% of the volatiles that outflow from the nucleus when the comet is within 3-4 AU of the Sun. The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization. The solar wind plays a minor role in the destruction of water compared to photochemistry. 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.
About a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun. The Great Comet of 1811 also had a coma roughly the diameter of the Sun. Even though the coma can become quite large, its size can actually decrease about the time it crosses the orbit of Mars around 1.5 AU from the Sun. At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, enlarging the tail.
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 visible antitail or dust tail, which points in a different direction and when the viewing angle and parallax are just right may appear to point in the opposite direction from the normal ion tail.
See also