  In a new simple model I reconcile two contradictory views on the factors that
determine the rate at which molecular clouds form stars -- internal structure
vs. external, environmental influences -- providing a unified picture for the
regulation of star formation in galaxies. In the presence of external pressure,
the pressure gradient set up within a self-gravitating isothermal cloud leads
to a non-uniform density distribution. Thus the local environment of a cloud
influences its internal structure. In the simple equilibrium model, the
fraction of gas at high density in the cloud interior is determined simply by
the cloud surface density, which is itself inherited from the pressure in the
immediate surroundings. This idea is tested using measurements of the
properties of local clouds, which are found to show remarkable agreement with
the simple equilibrium model. The model also naturally predicts the star
formation relation observed on cloud scales and, at the same time, provides a
mapping between this relation and the closer-to-linear molecular star formation
relation measured on larger scales in galaxies. The key is that pressure
regulates not only the molecular content of the ISM but also the cloud surface
density. I provide a straightforward prescription for the pressure regulation
of star formation that can be directly implemented in numerical models.
Predictions for the dense gas fraction and star formation efficiency measured
on large-scales within galaxies are also presented, establishing the basis for
a new picture of star formation regulated by galactic environment.
