An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A → molecular cloud that is physically or apparently related to a star formation region.

→ associate; → molecular; → cloud.

1) A visible mass of water droplets and/or ice particles in the atmosphere above the Earth's surface.
2) → interstellar cloud.

Cloud, from O.E. clud "mass of rock," from P.Gmc. *kludas.

Abr, from Mid.Pers. awr, abr (Laki owr, Baluchi haur, Kordi Soriani hewr), Av. awra- "rain cloud, rain," cf. Skt. abhra-"thunder cloud," Gk. afros "scum, foam," L. imber "rain;" also Sk. ambha- "water," Gk. ombros "rain," PIE *mbhros "rain cloud, rain," from *mbh-.

An early type of → bubble chamber used for detecting particles of ionizing radiation. It was invented in 1900 by Charles Thomson Rees Wilson (1869-1959), a Scottish physicist, who along with Arthur Compton (1892-1962 ) received the Nobel Prize for physics in 1927.

→ cloud; → chamber.

The fraction of the sky covered by clouds. It is expressed in tenths, so that 0.0 indicates a clear sky and 1.0 (or 10/10) indicates a completely covered sky.

→ cloud; → cover.

Process by which a → collapsing → giant molecular cloud breaks into dense → clumps, eventually bringing about → pre-stellar cores.

→ cloud; → fragmentation.

A large, faintly colored arc formed usually by sunlight falling on a cloud. Also called white rainbow, fogbow, and mistbow. Cloudbow appears white because the water droplets in the cloud or fog are very small compared with those of ordinary rainbows.

→ cloud; → bow.

Any sudden and heavy fall of → rain, always of the → shower type.

→ cloud; → burst.

Ragbâr, from rag + bâr. The second component bâr, variant bârân "rain," from bâridan "to rain." The origin of the first component is not clear. Rag in Persian means "blood vein, vessel," but this sense seems irrelevant here. In Gilaki the bare râk (without bâr) means cloudburst. Râk/rag may be related (via an extinct Iranian parent) to the Skt. stem ri- "to flow, to drop, to become liquid."

Same as → cloud cover.

From cloudy, from cloudy, from → cloud + → -ness.

Light from nearby stars scattered by → dust grains in low-density outer regions of → molecular clouds. It is seen not only in the → near infrared bands JHK, but also continuously from the visible to 5 μm. Cloudshine could be considered as an intermediate between → scattering in the visible and the → coreshine effect (Foster & Goodman, 2006, ApJ 636, L105). See also

→ cloud; → shine.

A population of relatively small (typically < 2°) → high-velocity clouds, which are spatially and kinematically isolated from the gas distribution in their environment. They are thought to be located in the → intergalactic medium of the → Local Group.

→ compact; → high-velocity cloud.

Meteorology: A cloud that owes its vertical development, and possibly its origin, to convection.

→ convective; → cloud.

A relatively dense cloud of → interstellar gas, mainly molecular, whose dust particles obscure the light of stars behind it. A famous example is the → Horsehead Nebula silhouetted against the reddish glow of the → H II region IC 434. Individual dark clouds come in a range of sizes from tens of → light-years to tiny → Bok globules of only a few thousands → astronomical units.

→ dark; → cloud.

A type of → interstellar medium cloud in which → carbon (C) becomes almost completely molecular due to relatively high → extinction. The chemistry is qualitatively different from that of → diffuse molecular clouds, as the → electron abundance is very low (→ cosmic-ray ionization being the dominant source) and the reactive C is replaced by the very stable → carbon monoxide (CO). This regime is found only in → sightlines with A_V > 5-10 mag; not all such sightlines will contain dense cloud material and if dense cloud material is present it is likely to be surrounded by → translucent material. These clouds are typically → self-gravitating, and are most often observed by → infrared absorption and → millimeter wave emission methods. Their densities are typically at least 10⁴ cm^-3, and their → kinetic temperatures are typically on the order of 10-50 K in the quiescent regions. Most of the more than 140 currently known → interstellar molecules were found through observations of → microwave→ rotational transitions in such clouds, starting with the discovery of OH, followed by a host of other new detections such as CO, NH₃, H₂O, and H₂CO (Snow & McCall, 2006, ARA&A 44, 367).

→ dense; → molecular; → cloud.

A type of cloud in the → interstellar medium with low molecular content that is fully exposed to the → interstellar radiation field, and therefore nearly all its → molecules are quickly destroyed by → photodissociation. Hydrogen is mainly in → neutral atomic form (→ neutral hydrogen), and atoms with → ionization potentials less than that of hydrogen (most notably → carbon) are almost fully → ionized, providing abundant electrons. The paucity of molecules implies that very little chemistry occurs in such clouds. Many → sightlines with low → extinction seem to pass exclusively through → diffuse atomic gas. Such sightlines typically have a → column density, N_H, less than about 5 × 10²⁰ cm^-2, and are sufficiently → optically thin to be observable by means of → visible and → ultraviolet → absorption line measurements. Diffuse atomic clouds typically have a fairly low → density (~ 10-100 cm^-3), and → temperatures of 30-100 K (Snow & McCall, 2006, ARA&A 44, 367).

→ diffuse; → atomic; → gas.

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.

A type of → molecular cloud in which the → interstellar radiation field is sufficiently attenuated, so that the local fraction of → molecular hydrogen (H₂) becomes substantial (> 0.1). However, enough interstellar radiation is still present to → photoionize any atomic carbon, or to → photodissociate → carbon monoxide (CO) such that carbon is predominantly still in the form of C⁺ (> 0.5). In steady state, diffuse molecular clouds must necessarily be surrounded by diffuse atomic gas, in order to provide the → shielding of radiation. This means that most sightlines that cross a diffuse molecular cloud will also cross → diffuse atomic gas (Snow & McCall, 2006, ARA&A 44, 367).

→ diffuse; → molecular; → cloud.

A massive complex of → interstellar gas and → dust, consisting mostly of → molecular hydrogen, that typically stretches over 150 light-years and contains several hundred thousand → solar masses. Giant molecular clouds are the principal sites of star formation. → molecular cloud.

→ giant; → molecular; → cloud.

A population of neutral or partly ionized gas clouds in the → Galactic halo which are seen as high-altitude structures in the → atomic hydrogen  → 21 cm emission at high radial velocities (v_LSR > 100 km/sec). They have substantial neutral → column densities (> 10¹⁹ cm^-2) and their → metallicities range from 0.1 to about 1.0 times solar. The distances to the majority of them remain unknown. They may represent the continuing infall of matter onto the → Local Group. See also → compact high-velocity clouds.

→ high; → velocity; → cloud.

A → dark cloud characterized by a → visual extinction A_v≥ 10² mag. IRDCs are opaque even at 8 μm, and can be seen in silhouette against the bright diffuse → mid-infrared emission in the → interstellar medium.

→ infrared; → dark; → cloud.

A medium in which several molecular clouds are situated.

→ inter- + → cloud + → medium.