Fr.: étoile semblable au soleil
specific star formation rate (sSFR)
nerx-e âbize-ye diseš-e setâregân
Fr.: taux de formation d'étoiles spécifique
Star formation rate per unit → mass. More specifically, the → star formation rate in a galaxy divided by the → stellar mass of the galaxy. Observations of galaxies over a wide range of → redshifts suggest that the slope of the SFR-M* relation is about unity, which implies that their sSFR does not depend strongly on stellar mass. Specific star formation rates increase out to z ~ 2 and are constant, or perhaps slowly increasing, from z = 2 out to z = 6, though with a large scatter, sSFR ~ 2-10 Gyr-1 (Lehnert et al., 2015, A&A 577, A112, and references therein).
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.
star formation efficiency (SFE)
kârâyi-ye diseš-e setâré
Fr.: efficacité de formation d'étoiles
star formation history
târix-e diseš-e setâré
Fr.: histoire de formation d'étoiles
The → star formation rate as a function of time.
star formation quenching
osereš-e diseš-e setâregân
Fr.: assèchement de formation d'étoiles
The premature termination of star formation process in some galaxies. The ultimate quenching of star formation is caused by stripping of the gas reservoir which will finally turn into stars. A wide variety of mechanisms have been proposed to provide quenching. For example, → major mergers can transform spiral galaxies into ellipticals, and may also quench future star formation by ejecting the → interstellar medium from the galaxy via starburst, → active galactic nucleus, or shock-driven winds. In rich clusters, where merging is less efficient because of the large relative velocities of galaxies, rapid encounters or fly-bys may cause the formation of a bar and growth of a spheroidal component instead of larger scale star formation. Also, cold gas can be stripped out of the galaxy both by tidal forces and ram pressure in the intracluster medium. Similarly, the hot halo that provides future fuel for cooling and star formation may be efficiently stripped in dense environments, thus quenching further star formation (see, e.g., Kimm et al., 2009, MNRAS 394, 1131, arXiv:0810.2794).
star formation rate
nerx-e diseš-e setâré
Fr.: taux de formation d'étoiles
The rate at which a molecular cloud or a galaxy is currently converting gas into stars. It is given by the ratio of the number of stars to the star formation time-scale.
star formation region
nâhiye-ye diseš-e setâré
Fr.: région de formation d'étoiles
star formation time scale
marpel-e zamâni-ye diseš-e setâre
Fr.: échelle de temps de formation d'étoiles
The time necessary for a star to form. It depends inversely on the stellar mass.
Fr.: étoile S2
A → main sequence→ B-type star that orbits the → supermassive black hole candidate → Sgr A* in the → Galactic center. The star S2, which is bright enough for making detailed measurements, has a highly elliptical, 16-year-period orbit around Sgr A*. Near → pericenter at 120 → astronomical units, ~ 1400 → Schwarzschild radii, the star has an orbital speed of ~ 7650 km s-1, such that the first-order effects of → special relativity and → general relativity have become detectable with current capabilities (Auber et al., 2018, A&A 615, L15).
Fr.: système stellaire
Same as → stellar system.
Fr.: traînées stellaires