STAR FORMATION EFFICIENCY AND GRAVITATIONAL STABILITY OF GASEOUS DISKS IN GALAXIES In summary, gravitational instability is the essential factor of promoting SF in spiral galaxies. In the unstable or marginally stable disks SFE depends very little on the gas density, that is SFR is proportional to the mass of the gas. Becoming overstable, gas layer slows down its SFE, especially for the regions where the volume gas density is low, however the formation of stars still continues at the rate which is much lower now than in the past. The value of SFE for these overstable regions is determined by the local density of gaseous (or stellar) disk.