  Measurements of H-alpha, HI, and CO distributions in 61 normal spiral
galaxies are combined with published far-infrared and CO observations of 36
infrared-selected starburst galaxies, in order to study the form of the global
star formation law, over the full range of gas densities and star formation
rates (SFRs) observed in galaxies. The disk-averaged SFRs and gas densities for
the combined sample are well represented by a Schmidt law with index N =
1.4+-0.15. The Schmidt law provides a surprisingly tight parametrization of the
global star formation law, extending over several orders of magnitude in SFR
and gas density. An alternative formulation of the star formation law, in which
the SFR is presumed to scale with the ratio of the gas density to the average
orbital timescale, also fits the data very well. Both descriptions provide
potentially useful "recipes" for modelling the SFR in numerical simulations of
galaxy formation and evolution.
