Fr.: gaz ultrarelativiste
A gas composed of ultrarelativistic particles.
Fr.: céphéide à très courte période
Fr.: physique des ultra-sons
The branch of physics dealing with elastic waves of frequencies above 20 kHz to 1010 kHz propagated in solids, liquids, and gases.
Sound with a frequency lying above the audition frequency range, usually taken to be about 20 kHz. → sound wave.
axtaršenâsi-ye farâ-banafš, ~ ultar-banafš
Fr.: astronomie ultraviolette
The study of astronomical objects in the ultraviolet portions of the electromagnetic spectrum, in the waveband 3000 Å to about 10 Å. At these wavelengths, the atmosphere prevents ultraviolet radiation from reaching the Earth surface. Therefore ground-based observatories cannot observe in the ultraviolet. Only with the advent of space-based telescopes has this area of astronomy become available for research.
Fr.: catastrophe ultraviolette
A paradox encountered in the classical theory of thermal radiation (→ Rayleigh-Jeans law), whereby a → blackbody should radiate an infinite amount of energy at infinitely short wavelengths, in contradiction with what is observed. The problem was solved by Max Planck in 1900, who suggested that, rather than being continuous, the energy comes in discrete parcels called quanta. The avoidance of the ultraviolet catastrophe was one of the first great achievements of → quantum mechanics.
fozuni-ye farâbanafš, ferehbud-e ultar-banafš
Fr.: excès ultraviolet
Ultraviolet emission from an object in excess of that expected for a reference. For example, → subdwarf stars show ultraviolet excess with respect to that expected from a star with → solar metallicity at a given → effective temperature. In this case, UV excess results from smaller → line blanketing in → population II stars.
setâre-ye farâbanafš, ~ ultar-banafš
Fr.: étoile ultraviolette
A star, such as O types or hot central stars of planetary nebulae, which radiates essentially in the ultraviolet part of the electromagnetic spectrum.
1) Optics: A dark area, especially the blackest part of a shadow from which
all light is cut off. → penumbra.
From L. umbra "shade, shadow."
Sâyé "shadow," from Mid.Pers. sâyak "shadow;" Av. a-saya- "throwing no shadow;" Skt. chāya- "shadow;" Gk. skia "shade;" Rus. sijat' "to shine;" M.H.G. schinen, O.H.G. skinan, Ger. Schein "glow, shine;" PIE base *skai- "bright."
The thirteenth of Uranus's known satellites and the third largest. It has a diameter of 1170 km and orbits Uranus every 4.144 days at a mean distance of about 265,980 km. Umbriel is very dark, with an albedo of 0.18. It has a heavily cratered surface. Umbriel was discovered by William Lassell (1799-1880) in 1851.
Umbriel named for an evil spirit in Alexander Pope's (1688-1744) poem The Rape of the Lock. Moreover, Umbriel derives from L. → umbra "shadow," which fits the satellite's appearance.
Fr.: in-, non-
A prefix meaning "not."
M.E., O.E. un- (cf. O.Fris., O.H.G., Ger. un-, Goth. un-, Du. on-), cognate with Pers. nâ-, as below.
Nâ- "not," variants na "no, not," ma- "not" (prohitive); Mid.Pers. nê, ma "no, not;" O.Pers. naiy, nai "not;" Av. nôit, naē "not;" cf. Skt. ná "not;" L. ne-, in-, un-; Gk. ni; Lith. nè; O.C.S. ne "not;" E. un-, as above; PIE *ne-.
cašm-e biyâvar, ~ berehné
Fr.: œil nu
The eye unassisted by an optical instrument, except for eyeglasses. Same as → naked eye.
Cašm, → eye; biyâvar "without help," from bi- "without" + yâvar, variant yâr "helper; companion" (Mid.Pers. hayyâr "helper," hayyârêh "help, aid, assistance," Proto-Iranian *adyāva-bara-, cf. Av. aidū- "helpful, useful"); berehné, → naked.
Fr.: non biaisé
Not biased or prejudiced.
→ un- + →
Bi- "without" + varak, → bias.
Fr.: estimateur non biaisé
A sample statistics when the mean of the sampling distribution of that statistic can be shown to be equal to the parameter being estimated.
nemunân-e a bi-varak
Fr.: échantillon non biaisé
A sample in which every individual element in the population has an equal chance of being selected.
Fr.: modèle non blanketé
Fr.: charge non liée
Same as → free charge.
Fr.: amas non lié
A cluster of objects, such as stars or galaxies, in which the members are not tied together gravitationally. In such a cluster the members scatter through space following different directions. → bound cluster.