An Etymological Dictionary of Astronomy and Astrophysics
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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.

The dimming of light traveling in the interstellar space due to the combined effects of absorption and scattering by interstellar dust particles. Interstellar extinction increases at shorter (bluer) wavelengths, resulting in → interstellar reddening.

→ interstellar; → extinction.

Gas, mostly hydrogen, in the interstellar space found in a variety of forms: molecular, atomic, ionized, plasma.

→ interstellar; → gas.

Irregularly shaped → dust grains that occur in the → interstellar medium. They are mostly composed of carbon and/or silicates and measure a fraction of micron across.

→ interstellar; → grains.

A spectral line formed in the interstellar medium, in particular an absorption line which does not participate in the periodic Doppler shift of intrinsic absorption lines in a binary star.

→ interstellar; → line.

A large-scale, weak magnetic field, with an estimated strength of about 1 to 5 microgauss, that pervades the disk of the Milky Way Galaxy and controls the alignment of interstellar dust grains.

→ interstellar; → magnetic; → field.

A maser phenomenon created by young stars and → protostars in the surrounding dense → molecular clouds of gas and dust. See also → circumstellar maser; → ammonia maser; → methanol maser; → OH maser; → water maser.

→ interstellar; → maser.

The gas and dust that exists in open space between the stars. See also → interstellar medium.

→ interstellar; → matter.

The environment containing the → interstellar matter, consisting of gas (mostly hydrogen) and dust. Even at its densest phase, the interstellar medium is emptier than the best vacuum man can create in the laboratory, but because space is so vast, the interstellar medium still adds up to a huge amount of mass.

→ interstellar; → medium.

Any molecule that occurs naturally in clouds of gas and dust in the interstellar medium. So far more than 140 species have been discovered, many of which nonexistent on Earth.

→ interstellar; → molecule.

A body other than a → star or → substellar object located in → interstellar space and not → gravitationally bound to a star. Its → hyperbolic orbit would indicate an object not bound to the Sun. The first known ISO is → 1I/'Oumuamua. ISOs are icy → planetesimals that are expected to behave like the → long-period comets of the solar system; volatile ices sublimate when the ISO approaches the Sun, developing a → coma and a → dust tail -- features that should make them bright and therefore easy to spot. The rocky ISOs, on the other hand, only reflect sunlight. As their → albedo is expected to be extremely low they become dark (after eons of bombardment by high-energy cosmic rays), they would be extremely faint and hard to detect (Hainaut et al., 2018, The Messenger 173, 13).

→ interstellar; → object.