The envelope of gas and dust that surrounds the solid nucleus of an active comet (see nucleus, cometary). The coma often appears as a teardrop, being largely shaped by the solar wind flowing around the comet. Near perihelion, the coma may be 100 000 km wide. A coma does not usually form until the comet is within 3–4 au of the Sun. Coma production has, however, been recorded from Chiron at over 11 au.