  Star formation is triggered in essentially three ways: (1) the pressures from
existing stars collect and squeeze nearby dense gas into gravitationally
unstable configurations, (2) random compression from supersonic turbulence
makes new clouds and clumps, some of which are gravitationally unstable, and
(3) gravitational instabilities in large parts of a galaxy disk make giant new
clouds and spiral arms that fragment by the other two processes into a
hierarchy of smaller star-forming pieces. Examples of each process are given.
Most dense clusters in the solar neighborhood were triggered by external
stellar pressures. Most clusters and young stars on larger scales are organized
into hierarchical patterns with an age-size correlation, suggestive of
turbulence. Beads-on-a-string of star formation in spiral arms and resonance
rings indicate gravitational instabilities. The turbulence model explains the
mass spectrum of clusters, the correlation between the fraction of star
formation in the form of clusters and the star formation rate, found by Larsen
& Richtler, and the correlation between the size of the largest cluster and the
number of clusters in a galaxy.
