An Etymological Dictionary of Astronomy and Astrophysics
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Surrounding or occurring around a star.

Circumstellar, from → circum- + → stellar.

Any concentration of material in the form of a disk orbiting around a star. → accretion disk; → protoplanetary disk.

→ circumstellar; → disk.

Interstellar → dust grains localized around various types of stars, such as → asymptotic giant branch stars. Circumstellar dust occurs in the form of a spherical shell or a disk and is at the origin of an → infrared excess for the central star. See also → circumstellar matter.

→ circumstellar; → dust.

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.

→ circumstellar; → envelope.

The circumstances or physical conditions related to the immediate surroundings of a star.

→ circumstellar; → environment.

A zone around a star within which a planet can have temperatures that permit liquid water, depending on the luminosity of the star and the distance of the planet from it.

→ circumstellar; → habitable zone.

Maser emission from molecules in the circumstellar envelopes of → red giants, and also from regions around → protostars.

→ circumstellar; → maser.

Dust, gas and plasma around stars, generally present in the form of stellar winds or nebulae ejected by the stars.

→ circumstellar; → matter.

A stream of matter into the interstellar medium from a central star.

→ circumstellar; → outflow.

A shell of dust, molecules, and neutral gas around an evolved star resulting from an intensive mass loss phase, such as the asymptotic giant branch phase for low- and intermediate mass stars and LBVs or supernovae for massive stars.

→ circumstellar; → shell.