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.

Absorption features in the spectrum of stars identified in the ultraviolet, visible, and infrared regions. They have an interstellar origin, but despite extensive efforts, their carrier(s) have not yet been clearly identified. See also → Aromatic Infrared Bands; → polycyclic aromatic hydrocarbons.

→ diffuse; → interstellar; → band.

The chemical element or composition that is supposedly at the origin of a → diffuse interstellar band (DIB).

→ diffuse; → interstellar; → band; → carrier.

An → interstellar cloud in which hydrogen is completely dissociated and which is less dense and dusty than → molecular clouds. In diffuse interstellar clouds photoabsorption of the background → ultraviolet (UV) radiation field is an important dissociating and ionizing process. Typical densities and temperatures of diffuse clouds are 10² to 10³ cm^-3 and 20 to 100 K respectively. Because of modest extinctions (≤ 1 mag), → photodissociation processes are important in diffuse clouds preventing the formation of larger molecules.

→ diffuse; → interstellar; → cloud.

The interstellar matter outside condensed molecular clouds. Diffuse interstellar medium consists of a hot intercloud medium, a warm intercloud medium, and a cold neutral medium with hydrogen atom densities n_H ~ 0.003, ~ 0.25, and ~ 40 cm^-3, and mean gas → kinetic temperatures T_k ~ 5 x 10⁵, ~ 10⁴, and 80 K, respectively.

→ diffuse; → interstellar; → medium.

Referring to the medium or objects lying in the space between stars in our Galaxy or outer galaxies.

→ inter- + → stellar.

The absorption of light from stars and other objects by intervening gas and dust in interstellar space.

→ interstellar; → absorption.

A huge structure of ionized gas blown into the interstellar medium by the winds of supernovae and massive stars. → Local Bubble.

→ interstellar; → bubble.

An aggregation of gas and dust in the → interstellar medium containing large quantities of atoms, molecules, and dust. There are several types of interstellar clouds, such as → diffuse interstellar clouds, → dark clouds, → molecular clouds.

→ diffuse; → cloud.

An aggregation of → dust grains in the space between stars. Interstellar dust absorbs, scatters, and polarizes the light from distant stars, causing the → interstellar extinction. Large dark regions in the plane of the Milky Way and other galaxies are caused by → intervening clouds of dust. The conclusive proof for the presence of a general and selective interstellar absorption is due to R. J. Trumpler (1930). See also → reddening; → very small grain; → big grain; → PAH.

→ interstellar; → dust.

→ dust grain.

→ interstellar; → dust; → grain.