Fr.: région active
Fr.: région bleue
The portion of the → visible spectrum lying between 455 and 492 nm.
broad-line region (BLR)
Fr.: région à raies larges
The inner region of a → quasar or an → active galactic nucleus exhibiting broad → spectral lines which indicate ionized matter moving with speeds in excess of 10,000 km sec-1, probably due to the presence of an → accretion disk surrounding a → supermassive black hole. Also called Type I AGN. See also → obscuring torus.
compact H II region
nâhiye-ye H II-ye hampak
Fr.: région H II compacte
A Galactic H II region with an electron density ≥ 103 cm-3 and of a linear dimension ≤ 1 pc.
corotating interaction region (CIR)
nâhiye-ye andaržireš-e hamcarxandé
Fr.: région d'interaction en corotation
A spiral-shaped density enhancement formed around a star when fast stellar winds collide with slower material. This large-scale wind structure can extend from the stellar surface to possibly several tens of stellar radii. The CIRs can be produced by intensity irregularities at the stellar surface, such as dark and bright spots, magnetic loops and fields, or non-radial pulsations. The surface intensity variations alter the radiative wind acceleration locally, which creates streams of faster and slower wind material. CIRs are responsible for the → discrete absorption components seen in some ultraviolet → resonance lines of → hot stars (S. R. Cranmer & S. P. Owocki, 1996, ApJ 462, 469).
density-bounded H II region
nâhiye-ye H II-ye cagâli karânmand
Fr.: bornée par la densité
An → H II region which lacks enough matter to absorb all → Lyman continuum photons of the → exciting star(s). In such an H II region a part of the ionizing photons escape into the → interstellar medium. See also → ionization-bounded H II region.
Fr.: région de diffusion
A narrow boundary layer above the solar → photosphere, between two magnetic field lines, where the plasma becomes demagnetized or unfrozen. The presence of a localized magnetic region is necessary for → magnetic reconnection.
giant H II region
nahiye-ye H II-ye qulpeykar
Fr.: région H II géante
An → H II region emitting at least 1050 → Lyman continuum photons per second, or about 10 times → Orion nebula. Such an H II region should be powered by at least one O3V star or by at least a dozen → O-type and tens → B-type stars. Our nearest giant H II region is → NGC 3603. Some other Galactic giant H II regions are: → Lagoon Nebula, M17, W31, W51A, and NGC 3576.
H I region
nâhiye-ye H I
Fr.: région H I
A region of neutral (atomic) hydrogen in interstellar space. At least 95 percent of interstellar hydrogen is H I. It emits radio waves that are 21 cm long.
H II region
nâhiye-ye H II
Fr.: région H II
A type of → emission nebulae composed of very hot gas (about 104 K), mainly ionized hydrogen, created by the ultraviolet radiation of → massive stars. H II regions originate when O or early-type stars, born in → giant molecular clouds, start heating up the cold gas, causing it to become → ionized and "glow". The effective temperatures of the → exciting stars are in the range 3 x 104 to 5 x 104 K, and throughout the nebula hydrogen is ionized. Helium is → singly ionized, and other elements are mostly singly or → doubly ionized. Typical densities in the H II region are of the order 10 to 102 cm-3, ranging as high as 104 cm-3. Internal motions occur in the gas with velocities of order 10 km s-1. The spectra of H II regions are mainly composed of strong → H I→ recombination lines and → forbidden lines such as [O III], [O II], [N II]. See also → ionization-bounded H II region; → density-bounded H II region; → compact H II region; → ultracompact H II region.
H II region luminosity
tâbandegi-ye nâhiye-ye H II
Fr.: luminosité de région H II
The total number of → Lyman continuum photons emitted by an → H II region. It is usually derived using → radio continuum observations which are less affected by → interstellar extinction. The measured value is often a lower limit because of photon leakage from the H II region and absorption. See also → density-bounded H II region.
Fr.: région de Huygens
The inner bright part of the → Orion Nebula, from which most of the radiation is emitted. It is about 5' across corresponding to 0.7 pc (for a distance of 440 pc). See O'Dell (2001, ARAA 39, 99).
Named after the Dutch astronomer Christiaan → Huygens (1629-1695), who sketched the appearance of the Orion Nebula. His drawing, the first such known sketch, was published in Systema Saturnium in 1659. First named such by O. Gingerich (1982, Ann. NY Acad. Sci. 395, 308); → region.
nâhiye-ye andarbâzu, ~ andararm
Fr.: région interbras
A low-density region separating the spiral arms of a galaxy.
Interarm, from → inter- + arm "body part," from O.E. earm "arm," from P.Gmc. *armaz (cf. M.Du., Ger. arm, O.N. armr, O.Fris. erm), from PIE base *ar- "to fit, join;" Mod.Pers. arm "arm, from the elbow to the shoulder;" Av. arma-, arəmo- "arm;" Skt. irma- "arm;" Gk. arthron "a joint," L. armus "shoulder;" → region.
Andarbâzu, from andar-, → inter-, + bâzu "arm," from Mid.Pers. bâzûk "arm;" Av. bāzu- "arm;" cf. Skt. bāhu- "arm, forearm;" Gk. pechys "forearm, arm, ell;" O.H.G. buog "shoulder;" Ger. Bug "shoulder;" Du. boeg; O.E. bôg, bôh "shoulder, bough;" E. bough " a branch of a tree;" PIE *bhaghu- "arm"); nahiyé, → region. Andararm, from andar-, → inter-, + arm, as above.
ionization-bounded H II region
nâhiye-ye H II-e yoneš-karânmand
Fr.: région H II bornée par ionisation
ionized hydrogen region
nâhiye-ye hidrožen-e yonidé (#)
Fr.: région d'hydrogène ionisé
Same as → H II region.
low-ionization nuclear emission-line region
nâhiye-ye hasteyi bâ xatt-e gosili-ye kamyoneš (#)
Fr.: Noyau de galaxie à raies d'émission de faible ionisation
Same as → LINER.
Fr.: région à raies étroites
Parkes Selected Region (PSR)
nâhiye-ye gozide-ye Parkes
Fr.: Région sélectionnée de Parkes
A catalog of 397 radio sources between declinations +20° and +27° which were compiled from a finding survey made at 635 MHz with the 64m radio telescope at the Australian National Radio Astronomy Observatory, Parkes, N.S.W. and published in 1968. Originally abbreviated PSR, this catalog, which is also called PKS, replaces and improves on four earlier lists (1964 to 1966).
photodissociation region (PDR)
nâhiye-ye šid-vâhazeš, ~ nur-vâhazeš
Fr.: région de photodissociation
A neutral region at the boundary of a → molecular cloud created by the penetration of → far ultraviolet (FUV) radiation from associated stars. The FUV radiation (6 eV ≤ hν ≤ 13.6 eV) dissociates the molecules and heats the gas and dust. A warm, atomic → H I region is thus created and the chemistry and thermal balance of the region are determined by the penetrating FUV photons. The progressive absorption of FUV photons leads to the occurrence of transitions between atomic and molecular phases, such as H I/H2 and C II/C I/CO transitions. By extension, any neutral region where the physics is controlled by FUV photons can be called a PDR, as it is the case for → diffuse interstellar clouds or the edge of → circumstellar disks. The PDR concept was first studied by A. G. G. M. Tielens and D. Hollenbach (1985, ApJ 291, 722).
A large, usually continuous segment of a surface or space; area. → H II region.
M.E., from Anglo-Fr. regioun; O.Fr. region, from L. regionem (nominative regio) "district, country, direction, boundary," from regere "to direct," cognate with Pers. râst, → right.
Nâhiyé, loan from Ar. nâHiyat.