  We present a simple phenomenological model of star formation in galaxies that
describes the process with a set of a few parameters. The star formation
efficiency and the fraction of gas and metals ejected in outflows are assumed
to be the main drivers of star formation. The Tully-Fisher and the
Faber-Jackson relations in different passbands are used as constraints in the
analysis of disk and elliptical galaxies, respectively. We find that a steep
correlation between efficiency and maximum rotation velocity is needed in disk
systems, whereas elliptical galaxies can be explained by a high star formation
efficiency, uncorrelated with the central velocity dispersion. Gas outflows are
not important in disks, whereas a correlation of outflows with galaxy mass must
be invoked in early-type galaxies to account for the observed colors.
