How much star formation in our galaxy is simply due to the spontaneous gravitational collapse of a cold molecular cloud and how much requires the collapse to be ‘triggered’ by external forces? Massive stars have strong radiation pressure and stellar winds. Thus, within molecular clouds, it is widely accepted that star formation can be triggered by ionization or shock fronts from these massive stars. There are two major mechanisms for this: (1) the Radiation Driven Implosion (RDI) model and (2) the Collect and Collapse (C&C) model. In the RDI model, the ionization front from the massive star ablates the surface of the surrounding clouds and produces bright rims and pillars. This process drives inwards a compressional shock that induces star formation. In the C&C model, the ionization and wind shock fronts sweep up material into a high density shell that then becomes gravitationally unstable and collapses. Getman et al. (2012) estimate that the contribution of triggered star formation to the IC 1396 HII region is greater than 14%.
Triggered star formation does not require new, hot stars to occur. A similar situation can arise following a supernova explosion in a dense region. The diagram below from the Spitzer Space Telescope illustrates the process in this case.