An Etymological Dictionary of Astronomy and Astrophysics
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An aggregate of small particles loosely stuck together. Same as → porous dust grain.

→ fluffy; → dust; → grain.

The mass ratio of gas to dust. It amounts to approximately 100 in the → interstellar medium, but may vary in → molecular clouds and → circumstellar disks due to dust → grain evaporation, → dust settling, → condensation of gas, etc. The gas-to-dust ratio depends on the → metallicity. It is larger in galaxies with lower metallicity.

→ gas; → dust; → ratio.

A member of the most extreme galaxies in terms of their luminosities and unusual hot → dust temperatures. The → infrared emission from HDOGs is dominated by obscured accretion onto a central → supermassive black hole (SMBH), in most cases without significant contribution from → star formation. The large contrast between the underlying → host galaxy and the hyper-luminous emission from the → active galactic nucleus (AGN) implies that either the SMBH is much more massive than expected for the stellar mass of its host, or is radiating well above its → Eddington limit. The most extreme of these remarkable systems known is → W2246-0526.

→ hot; → dust; → obscure; → galaxy.

Particles of dust in the → interplanetary medium. They are left-overs from the beginning of the solar system or from other sources such as sublimating comets. Their existence was first deduced from observations of → zodiacal light.

→ interplanetary; → dust.

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.

A cloud of dust particles that happens to lie on the → line of sight between the → observer and the → object.

→ intervening; → dust.

A fine, powder-like dust covering the Moon's surface. → regolith. It is formed when meteoroids crash on the Moon's surface, heating and pulverizing rocks, which contain silica and metals. Since there is no wind or water to smooth rough edges, the tiny grains are sharp and jagged, and cling to nearly everything. Their main chemical compositions are SiO₂ (about 45%) and Al₂O₃ (about 15%). The dust grains have an average size of 19 microns (40% smaller than hair).

→ lunar; → dust.

A violent atmospheric disturbance on Mars marked by high amounts of dust, especially during spring and summer seasons of the planet southern hemisphere. The elongated orbit of Mars has several important consequences. During southern spring and summer, Mars travels near its → perihelion, while its southern pole is tilted toward the Sun. Therefore, its surface receives much more heat . The atmosphere's temperature near surface raises and since the upper layers of the atmosphere are cold, warm air moves up and takes dust particles upward. Each several years big storms occur and cover significant portions of the planet such that dust stays in the atmosphere for several weeks or months. See also → Mars' calendar.

→ Mars; → dust; → storm.

A model concerned with the distribution in size of → interstellar grains to account for observations of → interstellar extinction from 0.11 μm to 1.0 μm. The → distribution has the form N(a)da ∝ a^-3.5da, where a is the grain radius. It extends from 5 nm to 1 μm for → graphite and over a narrower range for other materials.

MNR, the initials of authors J. S. Mathis, W. Rumpl, and K. H. Nordsiek (1977, ApJ 217, 425), who introduced the mode; → dust; → model.

Dust grains at → nanoscale sizes found in various astrophysical environments, such as → interstellar medium and → interplanetary space of our Solar System.

→ nano-; → dust.

A type of → interstellar dust grain made up of an aggregate of components with a hollow structure. Various processes operating in interstellar and → circumstellar media are believed to produce inhomogeneous and porous dust grains. Porous grains can produce more → extinction per unit mass than their combined individual dust components. They are generally cooler than compact grains (see, e.g., Iati et al. 2001, MNRAS 322, 749).

→ porous; → dust; → grain.