Observations and Theory of Dynamical Triggers for Star Formation

Star formation triggering mechanisms are reviewed, including the direct compression of clouds and globules, the compression and collapse of molecular clouds at the edges of HII regions and supernovae, the expansion and collapse of giant rings and shells in galaxy disks, and the collision and collapse between two clouds. Collapse criteria are given. A comprehensive tabulation of regions where these four types of triggering have been found suggests that dynamical processes sustain and amplify a high fraction of all star formation that begins spontaneously in normal galaxy disks.