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 km across, and shaped into a teardrop
by the solar wind. Together, the coma
and the nucleus form the comet's head.