Triggered Star Formation
  Triggered star formation in bright rims and shells is reviewed. Shells are
commonly observed in the Milky Way and other galaxies, but most diffuse shells
seen in HI or the infrared do not have obvious triggered star formation. Dense
molecular shells and pillars around HII regions often do have such triggering,
although sometimes it is difficult to see what is triggered and what stars
formed in the gas before the pressure disturbances. Pillar regions without
clear age gradients could have their stars scattered by the gravity of the
heads. Criteria and timescales for triggering are reviewed. The insensitivity
of the average star formation rate in a galaxy to anything but the molecular
mass suggests that triggering is one of many processes that lead to
gravitational collapse and star formation.
