  Star complexes are the largest globular regions of star formation in
galaxies. If there is a spiral density wave, nuclear ring, tidal arm, or other
well-defined stellar structure, then gravitational instabilities in the gaseous
component produce giant cloud complexes with a spacing of about three times the
width. These gas complexes form star complexes, giving the familiar beads on a
string of star formation along spiral arms, or nuclear hotspots in the case of
a ring. Turbulence compression, supernovae, and self-gravitational contraction
inside the giant clouds produce a nearly scale-free structure, including giant
molecular clouds that form OB associations and molecular cloud cores that form
clusters. Without stellar density waves or similar structures, random
gravitational instabilities form flocculent spirals and these fragment into
star complexes, OB associations and star clusters in the same way. The largest
coherent star-forming regions are the flocculent arms themselves. At the core
of the hierarchy are the very dense clumps in which individual and binary stars
form. The overall star formation rate in a galaxy appears to be regulated by
gravitational collapse on large scales, giving the Kennicutt/Schmidt law
scaling with density, but the efficiency factor in front of this scaling law
depends on the fraction of the gas that is in a dense form. Turbulence
compression probably contributes to this fraction. The CO version of the
Schmidt law, recently derived by Heyer et al. (2004), follows from the
turbulent hierarchy as well, as do the local efficiencies of star formation in
OB associations and clusters. The efficiency of star formation increases with
cloud density, and this is why most stars form in clusters that are initially
self-bound.
