Delays of star formation in galaxies

Evidence for the extended supression of star formation in galaxies is reviewed, and possible mechanisms for these delays are examined. Observations of metallic abundances and kinetics of stars are presented as evidence for gaps of up to 10 billion years between periods of star formation in the Milky Way and other galaxies. Consideration is then given to models of star formation by the gravitational collapse and subsequent fragmentation of massive gas clouds, and by the interaction of interstellar gas clouds, and factors controlling the rates of star formation are examined. The suppression of star formation as a result of supernova explosions leading to (1) the disruption of gravitationally bound fragments by heating, (2) the decay of the two-phase structures of interstellar gas by heating, and (3) the decrease of gas density in the galactic disk is discussed. It is shown that supernova explosion rates one to three orders of magnitude above those presently observed could serve to suppress star formation in the Galaxy.