An Etymological Dictionary of Astronomy and Astrophysics
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A concept of → star formation in which → high-mass stars and → low-mass stars form in different physical conditions involving different → molecular clouds. Following the pioneering suggestion of Herbig (1962), successive investigations have generally supported the idea that star formation proceeds bimodally with respect to stellar mass. The star formation rate appears to differ both spatially and temporally for low mass and → massive stars. This is of considerable importance for galactic evolution, since the low-mass stars lock up mass and are long-lived, low luminosity survivors to the present epoch, whereas massive stars are short-lived, recycle and enrich interstellar gas, and leave dark remnants while producing a high luminosity per unit of mass (Silk, J., 1988, in Galactic and Extragalactic Star Formation, p. 503, eds. R. E. Pudritz and M. Fich).

→ bimodal; → star; → formation.

An intense → star formation activity in a region of → interstellar medium or, more globally, in a → galaxy. It is characterized by a → star formation rate which is much higher than the corresponding average. Same as → starburst.

→ burst; → star; → formation.

Formation of stars, especially → massive stars, in group as opposed to individual formation.

→ collective; → star; → formation.

A crucial period in the history of the → Universe, when the bulk of stars in massive galaxies were likely formed. Observations of young stars in distant galaxies at different times in the past have indicated that the → star formation rate peaked at the → redshift of z ~ 2, some 10 billion years ago, before declining by a factor of around ten to its present value (P. Madau & Dickinson, 2014, arXiv:1403.0007).

→ cosmic; → star; → formation; → peak.

Massive star formation outside → OB associations. Recent observational findings suggest that → massive star formation is a collective process. In other words, massive stars form in → cluster environments and the mass of the most massive star in a cluster is correlated with the mass of the cluster itself. Nevertheless, other observational results give grounds for supposing that massive stars do not necessarily form in clusters but that they can be formed as isolated stars or in very small groups. According to statistical studies nearly 95% of Galactic → O star population is located in clusters or OB associations. This means that a small percentage, about 5%, of high mass stars may form in isolation. Isolation is meant not traceable to an origin in an OB association. This definition therefore excludes → runaway massive stars, which are thought to result from either dynamical interaction in massive dense clusters, or via a kick from a → supernova explosion in a → binary system. Alternatively, isolated massive star has been defined as follows: An O-type star belonging to a cluster whose total mass is < 100 Msun and moreover is devoid of → B stars (Selier et al. 2011, A&A 529, A40 and references therein).

→ isolated; → massive star; → formation.

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.

→ sequential; → star formation.

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).

→ specific; → star; → formation; → rate.

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.

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: ε_SFE = M_stars / (M_stars + M_gas).

→ star formation; → efficiency.

The → star formation rate as a function of time.

→ star; → formation; → history.

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; → quench.

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.

A region in the → interstellar medium where processes of → star formation are going on or have occurred in the past.

→ star; → formation; → region.

The time necessary for a star to form. It depends inversely on the stellar mass.

→ star formation; → time scale.

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.

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.

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.

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.