A very extensive envelope of cold gaseous materials surrounding evolved cool
stars, notably → red giants,
→ red supergiants (→ Mira variables), or
→ asymptotic giant branch stars. The typical size of such
envelopes is several thousands times that of the stellar radius and
their temperature ranges from 1000 to10 K. Circumstellar envelopes
result from mass loss from the central star (10-7 to 10-4→ solar masses per year) and expand with moderate velocities
(10 to 15 km sec-1). The low temperature of the envelope is at the
origin of the formation of molecules, which
in certain conditions provide → maser emission
(H2O, OH, SiO). Similarly, dust grains form in the envelope produce an
→ infrared excess emission.
See also: → circumstellar; → envelope.
