Star Formation on Galactic Scales: Empirical Laws
  Empirical star formation laws from the last 20 years are reviewed with a
comparison to simulations. The current form in main galaxy disks has a linear
relationship between the star formation rate per unit area and the molecular
cloud mass per unit area with a timescale for molecular gas conversion of about
2 Gyr. The local ratio of molecular mass to atomic mass scales nearly linearly
with pressure, as determined from the weight of the gas layer in the galaxy. In
the outer parts of galaxies and in dwarf irregular galaxies, the disk can be
dominated by atomic hydrogen and the star formation rate per unit area becomes
directly proportional to the total gas mass per unit area, with a consumption
time of about 100 Gyr. The importance of a threshold for gravitational
instabilities is not clear. Observations suggest such a threshold is not always
important, while simulations generally show that it is. The threshold is
difficult to evaluate because it is sensitive to magnetic and viscous forces,
the presence of spiral waves and other local effects, and the equation of
state.
