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.
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.
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.
top-down structure formation
diseš-e sâxtâr az bâlâ bé pâyin
Fr.: formation des structures du haut vers le bas
A cosmological model of → structure formation in which larger structures, such as galaxy → superclusters or perhaps even the vast → filaments and → voids, form earlier and then they fragment into smaller structures such as individual galaxies. Opposite of → bottom-up structure formation.
tarâdiseš (#), tarâdis (#)
1) The act or process of transforming. The state of being transformed.
Verbal noun of → transform.
triggered star formation
diseš-e mâše-yi-ye setâré
Fr.: formation d'étoiles déclanchée
The formation of second-generation stars in a → molecular cloud, as set off by the action of → massive stars. → sequential star formation; → radiation-driven implosion.
→ trigger; → star formation.
tarâdis-e yekâyi, ~ yekâni
Fr.: transformation unitaire
A transformation whose reciprocal is equal to its Hermitian conjugate.
→ unitary; → transformation.
violent star formation
diseše surâ-ye setâregân
Fr.: formation violente d'étoiles
The concept of star formation pertaining to a variety of systems (OB associations, giant H II regions, H II galaxies, massive star clusters, etc.) that are believed to have formed large numbers of stars in a very short time.
→ violent; → star formation.