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similarity transformation tarâdiseš-e hamânandi Fr.: transformation de similarité 1) A transformation that preserves angles and changes all distances in the same ratio. → similarity; → transformation. |
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). |
star formation diseš-e setâré 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 The degree to which stars form in a system, such as a → molecular cloud or a → galaxy. It is given by the ratio of the total mass of stars to the initial gas mass: εSFE = Mstars / (Mstars + Mgas). → star formation; → efficiency. |
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; → rate. |
star formation region nâhiye-ye diseš-e setâré Fr.: région de formation d'étoiles A region in the → interstellar medium where processes of → star formation are going on or have occurred in the past. |
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. → star formation; → time scale. |
star-forming galaxy kahkešân-e disnade-ye setâré Fr.: galaxie de formation d'étoiles A galaxy that is located on the → galaxy main sequence in the plane relating → star formation rates to total stellar masses. |
star-forming region nâhiye-ye diseš-e setâré Fr.: région de formation d'étoiles A region in which → star formation is going on. |
stimulated star formation diseš-e gavâlide-ye setâré Fr.: formation stimulée d'étoiles A process in which a star is not formed spontaneously but is provoked by the action of external forces, such as pressure and shock on a molecular cloud by close-by → massive stars, → supernova explosions, etc. See also → sequential star formation. Stimulated, p.p. of → stimulate; → star formation. |
stochastic self-propagating star formation diseš-e setâregân bâ xod-tuceš-e kâturgin Fr.: formation d'étoiles par auto-propagation stochastique A mechanism that could be responsible for global → spiral structure in galaxies either by itself or in conjunction with spiral → density waves. In this mechanism, star formation is caused by → supernova-induced → shocks which compress the → interstellar medium. The → massive stars thus formed may, when they explode, induce further → star formation. If conditions are right, the process becomes self-propagating, resulting in agglomerations of young stars and hot gas which are stretched into spiral shaped features by → differential rotation. Merging of small agglomerations into larger ones may then produce large-scale spiral structure over the entire galaxy. The SSPSF model, first suggested by Mueller & Arnett (1976) was developed by Gerola & Seiden (1978). While the → density wave theory postulates that spiral structure is due to a global property of the galaxy, the SSPSF model examines the alternative viewpoint, namely that spiral structure may be induced by more local processes. The two mechanisms are not necessarily mutually exclusive, but they involve very different approaches to the modeling of galaxy evolution. The SSPSF gives a better fit than the density wave theory to the patchy spiral arms found in many spiral galaxies. However, it cannot explain → galactic bars. → stochastic; → self; → propagate; → star; → formation. |
Stratospheric Observatory for Infrared Astronomy (SOFIA) Nepâhešgâh-e Cine-sepehri barây axtaršenâsi-ye forusorx Fr.: Observatoire stratosphérique pour l'astronomie infrarouge A partnership of NASA and the German Aerospace Center, consisting of an extensively modified Boeing 747SP aircraft carrying a reflecting telescope with an effective diameter of 2.5 m. NASA Ames Research Center manages SOFIA's science and mission operations in cooperation with the Universities Space Research Association and the German SOFIA Institute. SOFIA is the largest airborne observatory in the world, with a planned 20-year lifetime. → stratospheric; → observatory; → infrared; → astronomy. |
strong force niru-ye sotorg Fr.: interaction forte The force responsible for holding quarks and gluons together to form protons, neutrons and other particles. It is the strongest of the four fundamental forces. Same as → strong interaction. |
structure formation diseš-e sâxtâr Fr.: formation des structures The study of the processes that gave rise to the apparition of matter concentrations,
such as → superclusters of galaxies,
→ galaxy clusters, and galaxies, in a homogeneous
→ expanding Universe.
Cosmic structures are believed to result from → density fluctuations
that existed in the → early Universe
before radiation and matter decoupled (→ decoupling era
or → recombination era). Initial
→ quantum fluctuations in the → inflaton field
were expanded by → inflation. Inflation amplified
them up to scales that correspond to those of galaxy clusters and beyond.
Generally, a model of structure formation includes three main ingredients: 1) background
cosmology, 2) model for fluctuation generation, and 3) types of
→ dark matter. |
tensile force niruy-e taneši Fr.: force de traction The force tending to stretch (or produce tension in) an object |
terraform ziman-disidan Fr.: terraformer To alter the environment of a planet or moon in a → terraforming process in order to make it habitable for life forms. Probably taken from noun terraform, from L. terra "earth," → terrestrial; → form. |
terraforming zamin-diseš Fr.: terraformation The hypothetical process of altering the environment (atmosphere, temperature, surface topography, or ecology) of another planet or moon to improve the chances of survival of an indigenous biology or to allow habitation by terrestrial life forms. See also → ecopoiesis. Verbal noun of → terraform. The term first appeared in a science fiction novel, Seetee Shock (1949) by Jack Williamson, an American science fiction writer; but the actual concept pre-dates this work. |
tidal force niru-ye kešandi (#) Fr.: force de marée The → gravitational force exerted on an extended body as a result of the difference in the strength of gravity between near and far parts of the body. The ocean tides on Earth result from the varying gravitational force of the Moon exerted on the Earth's oceans closest and farthest from the Moon. Tidal force, which is the → gradient of the gravitational force, varies as 1/r3. More specifically, Ftidal = dF/dr = (2GMm)/r3, where M is mass of the → primary body, m is mass of the → secondary body, r is distance between objects, and G the → gravitational constant. The total tidal force experienced across a body is equal to the tidal force (force per unit distance) multiplied by the diameter of that body: Ftt = Ftidal x 2R (provided that radius R is much smaller than r). It is obvious that the tidal force experienced by Earth at Moon's → perigee is larger than that at the → apogee. If the tidal force is stronger than a body's cohesiveness, the body will be disrupted. The minimum distance that a secondary comes to a primary before it is shattered by tidal force is called its → Roche limit. Tidal forces create → tidal heating. |
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