big blue bump
quz-e âbi-ye bozorg
Fr.: grande bosse bleue
The broad continuum feature dominating the optical-ultraviolet spectra of AGNs. Most current models attribute the big blue bump to thermal emission from an optically thick accretion disk.
The hue of that portion of the visible spectrum lying between green and indigo, evoked in the human observer by radiant energy with wavelengths of approximately 420 to 490 nanometers.
From O.Fr. bleu, P.Gmc. *blæwaz, from PIE base *bhle-was "light-colored, blue, blond, yellow."
Âbi "color of water," from âb "water," Mid.Pers. âb, O.Pers./Av. âp-, Skt. âp-, PIE *âp-; → Aquarius.
blue compact dwarf galaxy
kahkešân-e kutule-ye âbi-ye hampak
Fr.: galaxie naine bleue compacte
An small → irregular galaxy undergoing → violent star formation activity. These objects appear blue by reason of containing clusters of hot, → massive stars which ionize the surrounding interstellar gas. They are chemically unevolved since their → metallicity is only 1/3 to 1/30 of the solar value. Same as → H II galaxy.
Fr.: continuum bleu
The → continuum emission of an astronomical source with wavelengths between about 492 and 455 nm.
Fr.: géante bleue
A giant star with spectral type O or B.
blue halo star
setâregân-e âbi-ye hâlé
Fr.: étoiles bleues du halo
blue HB star
Fr.: étoile BHB
Same as → blue horizontal branch star.
blue hook star
setare-ye qollab-e abi
Fr.: étoile du crochet bleu
A rare class of → horizontal branch (HB)
stars that so far have been
found in only very few Galactic → globular clusters.
These stars are such called because they
form a blue hook at the hot end of the HB in
→ far ultraviolet
(FUV) → color-magnitude diagrams.
The physical mechanism that produces blue hook populations
is still uncertain. At least two scenarios have been proposed.
blue horizontal branch star
setâre-ye âbi-ye šâxe-ye ofoqi
Fr.: étoile bleue de la branche horizontale
A member of a population of blue stars appearing on the → horizontal branch in the → Hertzsprung-Russell diagram of the Galactic → halo populations and → globular clusters. Belonging to → spectral types B3 to A0, they have evolved past the → red giant stage and are burning helium in their core.
Fr.: jet bleu
A transient optical phenomenon in the → stratosphere that emerges from the tops of → thunderstorm clouds at tremendous speeds. As their name implies, blue jets are optical ejections from the top of the electrically active core regions of thunderstorms. Following their emergence, they typically propagate upward in narrow cones at vertical speeds of roughly 100 km/s, fanning out and disappearing at heights of about 40-50 km. See also → sprite; → elve.
Fr.: fuite bleue
Leakage phenomenon in a filter, causing an unwanted response to the blue or green light.
→ blue; leak, from M.Du. leken "to drip, to leak," or from O.N. leka, cognate of O.E. leccan "to moisten," from P.Gmc. *lek- "deficiency" (cf. O.H.G. lecchen "to become dry," Ger. lechzen "to be parched with thirst").
Našt "leak," origin unknown; âbi, → blue.
Fr.: boucle bleue
An evolutionary behavior of certain stars, particularly massive stars, which return to the blue stage after becoming a red supergiant. The phenomenon appears as a blueward loop on the theoretical evolutionary tracks.
Fr.: lune bleue
The second full moon in a calendar month. For a blue moon to occur, the first of the full moons must appear at or near the beginning of the month so that the second will fall within the same month. Full moons are separated by 29 days, while most months are 30 or 31 days long; so it is possible to fit two full moons in a single month. This happens every two and a half years, on average.
The folkloric term blue Moon for the calendrical event is new, and apparently goes back to the Maine Farmers' Almanac for 1937. But its original meaning in that work was the third full Moon in a season when there were four full Moons in that season. Some have related the term to the much older English expression moon is blue, which goes back to a couplet from 1528, interpreting it as "something that occurs rarely." However in that poem the expression had a meaning of "something that was absurd." Alternatively, the term blue Moon may have been borrowed from the Chinese lunar calendar particularly in its usage among American Chinese community. In fact in that calendar when there are two full Moons in a month they use the term "blue Moon" and add a thirteenth intercalary month. → blue; → moon.
Fr.: région bleue
The portion of the → visible spectrum lying between 455 and 492 nm.
âsmân-e âbi (#)
Fr.: ciel bleu
A phenomenon which results from → Rayleigh scattering of sunlight by → atmospheric molecules. → Nitrogen and → oxygen molecules that compose about 78% and 21% of the air, respectively, are small compared to the light → wavelengths, and thus more effective at scattering shorter wavelengths of light (blue and violet). The → selective scattering by these → molecules is responsible for producing the blue skies on a clear sunny day. The sky over the horizon appears much paler in color, because the scattered blue light must pass through more air. Some of it gets scattered away again in other directions. Hence, less blue light reaches the observer's eyes.
Fr.: traînarde bleue, traînard bleu
Any of stars, often found in → globular clusters and old → open clusters, that lie on the blueward extension of the → main sequence beyond the → turnoff point. Blue stragglers have an anomalously blue color and high luminosity in comparison with other cluster members. The most probable ways in which they could form are: → mass transfer or → coalescence in → close binary systems, encounters or collisions in overcrowded cores of globular clusters.
Fr.: supergéante bleue
Fr.: aile bleue
The → line wing with wavelengths shorter than that of the emission or absorption peak.
Fr.: décalage vers le bleu
The apparent shift of the wavelength towards the shorter wavelength region of the radiation spectrum of an approaching object due to the Doppler effect.
Fr.: composante décalée vers le bleu
A constituent of a composite astronomical object which has a motion directed towards the observer, as revealed by its spectrum.