  One of the outstanding puzzles about star formation is why it proceeds so
slowly. Giant molecular clouds convert only a few percent of their gas into
stars per free-fall time, and recent observations show that this low star
formation rate is essentially constant over a range of scales from individual
cluster-forming molecular clumps in the Milky Way to entire starburst galaxies.
This striking result is perhaps the most basic fact that any theory of star
formation must explain. I argue that a model in which star formation occurs in
virialized structures at a rate regulated by supersonic turbulence can explain
this observation. The turbulence in turn is driven by star formation feedback,
which injects energy to offset radiation from isothermal shocks and keeps
star-forming structures from wandering too far from virial balance. This model
is able to reproduce observational results covering a wide range of scales,
from the formation times of young clusters to the extragalactic IR-HCN
correlation, and makes additional quantitative predictions that will be
testable in the next few years.
