The Star Formation Relation in Nearby Galaxies
  I review observational studies of the large-scale star formation process in
nearby galaxies. A wealth of new multi-wavelength data provide an unprecedented
view on the interplay of the interstellar medium and (young) stellar
populations on a few hundred parsec scale in 100+ galaxies of all types. These
observations enable us to relate detailed studies of star formation in the
Milky Way to the zoo of galaxies in the distant universe. Within the disks of
spiral galaxies, recent star formation strongly scales with the local amount of
molecular gas (as traced by CO) with a molecular gas depletion time of ~2 Gyr.
This is consistent with the picture that stars form in giant molecular clouds
that have about universal properties. Galaxy centers and starbursting galaxies
deviate from this normal trend as they show enhanced star formation per unit
gas mass suggesting systematic changes in the molecular gas properties and
especially the dense gas fraction. In the outer disks of spirals and in dwarf
galaxies, the decreasing availability of atomic gas inevitably limits the
amount of star formation, though with large local variations. The critical step
for the gas-stars circle seems therefore the formation of a molecular gas phase
that shows complex dependencies on various environmental properties and are
nowadays investigated by intensive simulational work.
