An Etymological Dictionary of Astronomy and Astrophysics

A → sequential star formation model involving → massive stars and → H II regions. The energetic ultraviolet photons from a massive star born in a → molecular cloud drive a spherical → ionization front radially outward from the star at a velocity much higher than the → sound speed in the cold neutral gas. The supersonic expansion of the H II region through the surrounding neutral gas creates a → shock front, sweeping up an increasingly massive and dense shell of cool neutral gas. This is the collect phase of the process in which the H II region simply acts like a snowplough. If the expansion of the H II region continues for long enough, the surface density of the shell increases to the point where the shell becomes self-gravitating. The shell is then expected to collapse and fragment.
Individual fragments may then enter a non-linear collapse phase, possibly forming massive stars. This model was first proposed by Elmegreen & Lada (1977, ApJ 214, 725), who used a one-dimensional analysis. Whitworth et al. (1994, MNRAS, 268, 291) developed an analytical model for the collect and collapse process which predicts the fragmentation time, the size, number, and mass of the fragments (see also Elmegreen 1998, in ASP Conf. Ser. 148, Origins, eds. Woodward et al., p. 150 and references therein). → stimulated star formation, → triggered star formation.

See also: → collect; → collapse; → model.