Fr.: gros grain
A type of → interstellar dust grains with a size ranging from 150 to 1000 Å. Big grains consist of graphite and silicates. They are in → thermal equilibrium with the radiation field and their emission can be described by a modified → blackbody radiation following from → Kirchhoff's law.
kânun-e Cassegrain (#)
Fr.: foyer Cassegrain
The main focus in → Cassegrain telescope.
→ Cassegrain telescope; → focus.
durbin Cassegrain, teleskop-e ~ (#)
Fr.: Télecope Cassegrain
A reflecting telescope whose primary mirror has a hole bored through the center to allow the reflected light from the convex secondary mirror be focused beyond the back end of the tube.
Cassegrain, named after the French priest and school teacher Laurent Cassegrain (1629-1693), who invented this system in 1672; → telescope.
dâne-ye qobâr (#)
Fr.: grains de poussière
A small, solid flake of → graphite and/or → silicates coated with water ice, found in the → interstellar medium or the → interplanetary medium. Dust grains are irregularly shaped with sizes from microns to Angstroms (→ very small grain; → big grain; → PAH). It is believed that the dust is mainly formed in the cool outer layers of → red giants and dispersed in the interstellar medium. In dense environments, such as → molecular clouds and around → protostars, dust grains grow due to collisions (→ grain growth). Therefore, dust grains in → protostellar disks should be larger than interstellar grains. Dust grains absorb and scatter optical and ultraviolet light and re-radiate in infrared (→ dust emission). Dust acts as a catalyst in the interstellar medium, because molecules form via reactions on the surface of dust grains.
fluffy dust grain
dâne-ye qobâr-e korkvâr
Fr.: grain de poussière duveteux
An aggregate of small particles loosely stuck together. Same as → porous dust grain.
1) A small, hard seed of plants, especially the seed of cereals.
M.E. grain, grein, from O.Fr. grein, from L. granum "seed;" akin to corn.
Dâné "grain, seed;" Mid.Pers. dân, dânag "seed, corn;" Av. dānô- in dānô.karš- "carrying grains; an ant;" cf. Skt. dhânâ- "corn, grain;" Tokharian B tāno "grain;" Lith. duona "corn, bread."
Fr.: coagulation des grains
Sticking together of micron- to centimetre-sized grains occurring in the interstellar and protoplanetary environments to form larger grain agglomerates.
→ grain; → coagulation.
Fr.: évaporation des grains
Conversion of dust grains into smaller grains due to high environmental temperatures.
→ grain; → evaporation.
Fr.: formation des grains
The process by which dust grains are assembled or produced.
ruyeš-e dâné (#)
Fr.: croissance des grains
The increase of dust grains to micron sizes in the interstellar environments due to various physical processes, for example mutual collisions and accumulation of ice mantles.
rupuš-e dâné (#)
Fr.: manteau de grain
A layer of icy molecules covering interstellar dust grains.
→ grain; mantle, from O.E. mentel "loose, sleeveless cloak," from L. mantellum "cloak," perhaps from a Celtic source.
Rupuš "overgarment, cloak," from ru "surface, face; aspect; appearance" (Mid.Pers. rôy, rôdh "face;" Av. raoδa- "growth," in plural form "appearance," from raod- "to grow, sprout, shoot;" cf. Skt. róha- "rising, height") + puš "covering, mantle," from pušidan "to cover; to put on" (Mid.Pers. pôšidan, pôš- "to cover; to wear;" cf. Mid.Pers. pôst; Mod.Pers. pust "skin, hide;" O.Pers. pavastā- "thin clay envelope used to protect unbaked clay tablets;" Skt. pavásta- "cover," Proto-Indo-Iranian *pauastā- "cloth").
Fr.: érosion des grains par pulvérisation
The ejection of atoms from interstellar dust grains due to impact by gas ions, which leads to grain destruction.
→ grain; sputtering, from sputter "to spit with explosive sounds," cognate with Du. sputteren.
Osparâni, verbal noun of osparândan, from os- "out of, outside," → ex- + parândan "to eject," transitive of paridan "to fly" (from Mid./Mod.Pers. par(r) "feather, wing," Av. parəna- "feather, wing;" cp. Skt. parna "feather," E. fern; PIE *porno- "feather").
interstellar dust grain
dâne-ye qobâr-e andaraxtari
Fr.: grain de poussière interstellaire
→ dust grain.
→ interstellar; → dust; → grain.
Fr.: grain interstellaire
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.
porous dust grain
dâne-ye qobâr-e porlik
Fr.: grain de poussière poreux
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).
Fr.: grain pré-solaire
A → refractory → nanoparticle embedded in → meteorites and → interplanetary dust particles whose → isotopic ratios suggest formation earlier than the Solar System.
teleskop-e Schmidt-Cassegrain, durbin-e ~ (#)
Fr.: télescope Schmidt-Cassegrain
A mixture of the → Cassegrain telescope with a very short → focal length and of a Schmidt design (due to the presence of the → corrective plate), used mainly in → amateur astronomy. The main advantage of this telescope is its compact design. However, Schmidt-Cassegrain telescopes produce fainter images with less contrast than other telescope designs with similar → aperture sizes. This is due to the comparatively large → secondary mirror required to reflect the light back the → eyepiece.
very small grain (VSG)
dâne-ye besyâr kucak
Fr.: très petit grain
A special type of carbonaceous → interstellar dust grains with a size ranging from 10 to 150 Å and consisting of tens to hundreds of atoms. In contrast to → big grains, VSGs are not in → thermal equilibrium. They can be heated to very high temperatures (~ 1000 K) by the absorption of a single photon. It is thought that VSGs are clusters of → PAH.