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.: transformation de similarité
1) A transformation that preserves angles and changes all distances in the same ratio.
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).
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.
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.
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.
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
tarâdis-e yekâyi, ~ yekâni
Fr.: transformation unitaire
A transformation whose reciprocal is equal to its Hermitian conjugate.
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.