red HB star
Fr.: étoile RHB
Same as → red horizontal branch star.
red horizontal branch star
setâre-ye sorx-e šâxe-ye ofoqi
Fr.: étoile rouge de la branche horizontale
A star found on the red part of the → horizontal branch. According to theoretical models, these stars result from the evolution of stars with a mass around 0.8 Msun, higher than that giving rise to → BHB stars. Upon helium burning in their cores, the remnant envelope of the red giant collapses.
Fr.: étoile rougie
A star whose light has undergone → reddening.
Fr.: étoile RHB
Same as → red horizontal branch star.
setâre-ye Ap-ye tond navandé
Fr.: étoile Ap à oscillation rapide
Same as → rapidly oscillating Ap star
setâre-ye carxân, ~ carxandé
Fr.: étoile en rotation
A star that has a non-zero → angular velocity. In a rotating star, the → centrifugal forces reduce the → effective gravity according to the latitude and also introduce deviations from sphericity. In a rotating star, the equations of stellar structure need to be modified. The usual spherical coordinates must be replaced by new coordinates characterizing the → equipotentials. See also → von Zeipel theorem.
RR Lyrae star
setâre-ye RR Cang
Fr.: étoile RR Lyrae
A member of a large class of → pulsating stars of type A2-F6 with periods less than 1 day. They are similar to → Cepheids, except that their periods are much shorter and are less luminous. RR Lyrae stars belong to → Population II and are often found in → globular clusters (hence one of their older names cluster variables) or elsewhere in the → galactic halo. They are used as distance indicators (→ standard candle) out to more than 200 kpc.
Fr.: étoile en fuite
A massive, young, and hot star that is moving quickly through space. Runaways are probably propelled through space from a binary star when its companion has exploded as a supernova, or ejected from a stellar cluster by the dynamical interactions in the system.
Setâré, → star; gorizân present participle of goriz-, gorixtan "to escape; to flee, run away;" Mid.Pers. virextan; Proto-Iranian *vi-raik, from vi- "apart, asunder" + *raik; Av. raek- "to leave, set free, let off;" Mid./Mod.Pers. reg/rig (in mordé-rig "inheritance"); Skt. ric- "to leave," rinakti "gives up, evacuates;" Gk. leipein "to leave;" L. linquere "to leave;" from PIE *linkw-, from *leikw- "to leave behind" (cf. Goth. leihvan; O.E. lænan "to lend;" O.H.G. lihan "to borrow;" O.N. lan "loan").
S Doradus star
setâre-ye S Zarrin-mâhi
Fr.: étoiles S Doradus
A type of massive, → blue supergiant, → variable star, also known as a → Hubble-Sandage variable or a → Luminous Blue Variable (LBV). S Doradus stars are the most luminous stars in the Galaxy and are easily identified in other nearby galaxies. They are named after the prototype, S Doradus, in the → Large Magellanic Cloud.
Fr.: étoile de type S
A → red giant of → spectral type S whose spectrum is dominated by → molecular bands arising from → zirconium → oxide (ZrO). S stars also have strong → cyanogen bands and contain spectral lines of → lithium and → technetium. Almost all S stars are → long-period variables.
S, letter of alphabet; → star.
setâre-ye gune-ye S
Fr.: étoile de type S
Same as → S star.
SAO Star Catalog
kâtâlog-e setâre-yi-ye SAO
Fr.: catalogue SAO
A general whole-sky catalog compiled by the Smithsonian Astrophysical Observatory which results from the combination of several earlier catalogs. The compilation gives positions and proper motions for 258,997 stars, of which 8,712 are double and 499 variable, with an average distribution of 6 stars per square degree. The star positions have an average standard deviation of 0''.2 at their original epochs (0''.5 at epoch 1963.5). The equinox is 1950.0 and the system that of the FK4.
scattering of stars
Fr.: diffusion des étoiles
The progressive increase of random motions of → disk stars with increasing stellar → ages. While some initial random motion seems likely in the disturbed conditions of disks when the oldest stars formed, the observation is generally attributed to scattering processes. Both massive gas → clumps and → spiral waves are considered as scattering agents (J. A. Sellwood & J. J. Binney, 2002, astro-ph/0203510 and references therein).
second generation star
setâre-ye âzâneš-e dovom
Fr.: étoile de deuxième génération
Fr.: étoile secondaire
sequential star formation
diseš-e peyâye-yi-e setâré
Fr.: formation séquentielle d'étoiles
The formation of second-generation stars in a → molecular cloud, as triggered by the presence of → massive stars. The observation that some nearby → OB associations contain distinct, spatially separate subgroups of → OB stars in a sequence of monotonically changing age led Blaauw (1964, ARA&A 2, 213) to suggest that star formation in fact occurs in sequential bursts during the lifetimes of the corresponding molecular clouds. The first quantitative model of this mechanism was presented by Elmegreen and Lada (1977, ApJ 214, 725), who showed that the powerful ultraviolet photons of the massive star create an → ionization front which advances in the molecular cloud and is preceded by a → shock front. The compressed neutral gas lying between the ionization and shock fronts is gravitationally unstable and collapses in time-scales of a few million years to form a new generation of massive stars. The propagation of successive births of OB groups would produce a chain of associations presenting a gradient of age. Elmegreen and Lada estimated the propagation velocity to be 5 km s-1. For a region with a length larger than 100 pc, this would imply an age difference of the order of 20 million years between the extremities. See also → stimulated star formation, → triggered star formation; → collect and collapse model.
Fr.: étoile SHB
Same as → supra-horizontal branch star.
Fr.: étoile à enveloppe
A main-sequence star, usually of spectral class B to F, whose spectrum shows bright emission lines superimposed on the normal absorption lines. The emission spectrum is explained by the presence of a circumstellar shell of gas surrounding the star at the equator. Shell stars are fast rotators.
Fr.: étoile filante
Colloquial name for → meteor.
Shooting, from shoot (v.); M.E. shoten; O.E. sceotan "to shoot" (cf. O.N. skjota, Du. schieten, Ger. schießen), from PIE base *skeud- "to shoot, to chase, to throw;" → star.
Šahâb, → meteor.
Slowly Pulsating B star (SPB)
setâre-ye âhesté tapande-ye gune-ye B
Fr.: étoile B pulsante à longue période
A member of a class of → B stars that are situated along the → main sequence with → spectral types ranging from B2 to B9 and masses from 3 to 7 → solar masses. In the → H-R diagram the SPB group lies below → beta Cephei variables, which are more massive. SPBs show light and line-profile variations that are multi-periodic with periods of the order of days. This variability is understood in terms of non-radial → stellar pulsations, and their → oscillation modes are high-order → g modes. Theoretical models attribute the pulsational nature of SPBs to the → kappa mechanism, acting in the metal → opacity bump at 2 x 105 K. Their g-mode pulsations penetrate deep into the stellar interior, making these objects very promising for → asteroseismology. Several oscillation modes are excited simultaneously, resulting in periodicities on time scales of the order of months or even years. The prototype of this group is 53 Per. First introduced as a distinct class by Waelkens (1991, A&A 246, 453).