A

David

Darling

coma

comet coma, nucleus and tail

A coma is a glowing envelope of gas and dust that surrounds a comet's nucleus when the comet comes within a few astronomical units of the Sun (although Chiron has been seen to develop a coma at 11 AU). At perihelion, the coma is typically about 100,000 kilometers across, and shaped into a teardrop by the solar wind. Together, the coma and the nucleus form the comet's head.