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

The quality of a material that allows light to pass through, but only diffusely, so that objects on the other side cannot be clearly distinguished. → transparent.

From L. translucent-, p.p. of translucere "to shine through," from → trans- + lucere "to shine," related to lux "light," lucidus "clear," luna, "moon;" Fr. lumière "light;" cf. Pers. ruz "day," rowšan "bright, clear," rowzan "window, aperture;" foruq "light," afruxtan "to light, kindle;" Mid.Pers. rôšn "light; bright, luminous," rôc "day;" O.Pers. raucah-rocânak "window;" O.Pers. raocah- "light, luminous; daylight;" Av. raocana- "bright, shining, radiant;" akin to Skt. rocaná- "bright, shining," roka- "brightness, light;" Gk. leukos "white, clear;" O.E. leoht, leht, from W.Gmc. *leukhtam (cf. O.Fris. liacht, M.Du. lucht, Ger. Licht), from PIE *leuk- "light, brightness."

Tarâtâb, from tarâ-, → trans- + tâb present stem of tâbidan "to shine," variants tâftan "to shine," tafsidan "to become hot;" Mid.Pers. tâftan "to heat, burn, shine;" taftan "to become hot;" Parthian t'b "to shine;" Av. tāp-, taf- "to warm up, heat," tafsat "became hot," tāpaiieiti "to create warmth;" cf. Skt. tap- "to heat, be/become hot; to spoil, injure, damage; to suffer," tapati "burns;" L. tepere "to be warm," tepidus "warm;" PIE base *tep- "to be warm."

A type of → interstellar medium cloud where → carbon (C), in → ionized atomic form and protected from → interstellar radiation, transforms into neutral atomic or molecular form. The chemistry in this regime is qualitatively different than in the → diffuse molecular clouds, both because of the decreasing electron fraction and because of the abundance of the highly reactive C atoms. The translucent cloud regime is the least well understood of all the cloud types. This is partly because of a relative lack of observational data, but also because theoretical models do not all agree on the chemical behavior in this transition region. In some models, there is a zone where the abundance of C exceeds that of C⁺ and CO; in others the peak abundance of C falls below that of C+ and CO. To cope with this uncertainty, Snow & McCall (2006) propose a working definition of translucent cloud material as gas with C⁺ fraction < 0.5 and CO fraction < 0.9. This definition reflects the fact that C+ is no longer the dominant form of carbon as it converts to neutral or molecular form, but also excludes the → dense molecular clouds, where carbon is almost exclusively CO (Snow & McCall, 2006, ARA&A 44, 367).

→ translucent; → cloud.