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).
Fr.: étoile semblable au soleil
A hypothetical, very rapidly → rotating star formed in the → metal-deficient conditions of the primordial → interstellar medium. The → first stars were probably spinstars, because the lack of metals leads to faster rotation velocities. Indeed → metal-poor stars are more compact than → metal-rich ones. Stars formed from a gas whose → metallicity is below 1/2000 of the → solar metallicity could attain rotation velocities of 500-800 km s-1 (see also → Population III star). Rotation triggers → mixing processes inside the star, leading to the production of important quantities of 14N, 13C, and 22Ne (Maeder & Meynet 2012, and references therein). The production of primary 22Ne has an important impact on the → s-process → nucleosynthesis in spinstars compared to non-rotating stars. This increases by orders of magnitude the s-process → yields of → heavy elements. Spinstars would therefore have strongly influenced the properties and appearance of the first galaxies that formed in the → Universe (See G. Meynet et al. 2009, arXiv:0709.2275; C. Chiappini, 2013, Astron. Nachr. /AN 334, No. 6, 595 and references therein).
Fr.: étoiles standard
Stars for which accurate color indices and/or magnitudes exist, defining a standard system.
A huge mass of hot gas whose radiation is provided by its internal → thermonuclear reactions. A star represents a → hydrodynamic equilibrium between two opposing forces, the inward → gravitational force, which is attempting to make the mass collapse and the pressure caused by the generation of nuclear energy. Below a certain mass (0.08 → solar masses), the central pressures and temperatures are insufficient to trigger the → hydrogen fusion (→ brown dwarf). Stars have a variety of masses and sizes. → Massive stars are less common than → low-mass stars (→ initial mass function). → Star formation results from → gravitational collapse of → molecular clouds (→ fragmentation; → pre-stellar core; → protostar; → accretion). After leaving the → main sequence, they pass through several evolutionary stages (e.g., → red giant, → supergiant, → white dwarf, → supernova, → neutron star) depending on their initial masses. See also: → internal structure of stars; → spectral classification; → luminosity class; → variable star; → multiple star. The term star is sometimes loosely applied to objects that do not comply with the above specifications, but are evolutionary products of stars, such as neutron stars and white dwarfs. For ancient civilizations a star was anything appearing in the night sky, apart from perhaps the Moon.
M.E. sterre, O.E. steorra; cf. O.S. sterro, O.N. stjarna, O.Fris. stera, Du. ster, O.H.G. sterro, Ger. Stern, Goth. stairno; cognate with Gk. aster, astron, L. stella (Fr. étoile, Sp. esterella, It. stella), Bret. sterenn, Pers. setâré, as below.
Setâré, variants star, estâr, estâré, and probably axtar, → astro-, (Lori, Laki) âsâra, (Laki) hasâra, (Tabari) essâra, (Baluci) istâr, (Ossetic) st'aly, (i)sthalu, (Tâti) usdurâ; Mid.Pers. stârag, stâr; Av. star-; cf. Skt. stár-, tāra-, tārakā- "star;" akin to Gk. and L., as above; PIE base *ster- "star."
Fr.: catalogue stellaire
A listing of stars usually ordered by right ascension with observational data elements such as coordinates, magnitude, distance, proper motion, and so on.
Fr.: carte du ciel
A chart or map showing the relative apparent positions of the stars as viewed from the Earth.
→ star; chart, from M.Fr. charte "card, map," from L. charta "leaf of paper, tablet," from Gk. khartes "layer of papyrus."
xuše-ye setâre-yi (#)
Fr.: amas stellaire
1) A group of stars held together by the mutual
→ gravitational attraction of its members,
which are physically related through common origin. They are of two types:
→ open clusters and
→ globular clusters.
šomâreš-e setâré, ~ setâregân
Fr.: comptage d'étoiles
The number of stars that appear in a given region of sky, usually counted on a photographic plate or CCD image.
Fr.: dérive stellaire
The relative motion of two groups of stars in the Galaxy moving in opposite directions.
Fr.: formation d'étoiles
The process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy, star formation includes the study of the interstellar medium and molecular clouds as precursors to the star formation process as well as the study of young stellar objects.