An Etymological Dictionary of Astronomy and Astrophysics
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1) The glowing envelope of gas and dust that surrounds a comet's nucleus.
2) An elongated, → comet-shaped → image of a → point source of → light caused by → aberration in the → optical system. In → telescopes it is often because → off-axis rays of light striking different parts of the → objective do not focus in the same → image plane.
→ Coma Berenices; → Coma cluster; → hydrogen coma.

L. coma "hair," from Gk. kome "hair;" → hair.

Berenice's Hair. A → constellation made up of many faint stars and located near the north Galactic pole between → Canes Venatici to the north, → Virgo to the south, → Leo to the west, and → Boötes to the east. Abbreviation: Com; genitive: Comae Berenices.

→ coma; L. Berenices genitive of Berenice, a queen of Egypt, wife of Ptolemy III, who sacrificed her hair to Aphrodite, begging her husband's victory in the war with the Assyrians, who had killed his sister. While the story is an old one, the constellation is relatively new, being introduced by Tycho Brahe (1546-1601).

The nearest rich cluster of galaxies which contains more than a thousand known galaxies, is about 20 million light-years in diameter, and lies about 280 million light-years away in the → constellation  → Coma Berenices. Also known as Abell 1656.

→ coma; → cluster.

The cometary cloud of hydrogen, detectable in ultraviolet light, that is immensely bigger than even the huge visible coma it surrounds. It is produced by the dissociation of water into hydrogen and oxygen and by other processes set into motion by solar radiation and and the solar wind.

→ hydrogen, → coma.