  We present a model for the star formation process during the initial collapse
of dark matter haloes at redshifts z=0-30. We derive a simple expression for
the characteristic stellar mass scale during this initial burst of star
formation. In our picture, this characteristic scale reflects both the minimum
temperature to which the gas can cool (determined by the metallicity and the
temperature of the cosmic microwave background) and the pressure of overlying
baryons in the collapsing halo. This prescription reproduces both the large
mass scales found in simulations of Population III star formation and the near
solar values observed for star formation at low redshift.
