The rate of star formation in normal disk galaxies.

Photometry of the integrated Hα emission in a large sample of field spiral and irregular galaxies has been used to obtain quantitative estimates of the total star formation rate (SFR) in the galaxies. The photoionization properties of a stellar population have been modeled for a variety of choices for the initial mass function (IMF). Excellent agreement with the observed galaxy colors and Hα emission is obtained with models using an IMF slope close to Salpeter's original value. The extinction-corrected star formation rates are large, as high as 20 M_sun; yr^-1 in giant Sc galaxies. The current rates in late-type galaxies are comparable to the past rates averaged over the age of the disk. Little evidence is found for a strong correlation between the SFR and average gas density. Comparison of the present SFRs with the remaining supply of interstellar gas yields consumption time scales of only a few times 10⁹ years in most cases.