  Observations suggest that star formation occurs in only one or two crossing
times for a range of scales spanning a factor of 1000. These observations
include (1) measurements of embedded cluster ages in comparison with the cloud
core dynamical times, (2) measurements of the age difference versus separation
for clusters in the Large Magellanic Clouds in comparison with the crossing
time versus size correlation for molecular clouds, (3) hierarchical structure
of embedded young clusters, and (4) the high fraction of dense clouds that
contain star formation. Such a short overall time scale for star formation
implies that sources of turbulent energy or internal feedback are not required
to explain or extend cloud lifetimes, and that star and protostar interactions
cannot be important for the stellar initial mass function. Stars appear in a
cloud as if they freeze out of the gas, preserving the turbulent-driven gas
structure in their birth locations. The galaxy-wide star formation rate avoids
the Zuckerman-Evans catastrophe, which has long been a concern for molecular
clouds that evolve this quickly, because the multifractal structure of
interstellar gas ensures that only a small fraction of the mass is able to form
stars. Star formation on large scales operates slower than on small scales, but
in most cases the whole process is over in only a few dynamical times.
