Density of star formation in the universe

The mean star formation rate per unit volume in the universe is estimated, from the distribution of colors of spiral galaxies, to be 0.05 (H0/100)2 M/sub sun/ yr-1 Mpc-1. Implications of this star formation density the gas within spiral galaxies is being consumed on a time scale of about 0.25 of the Hubble time; stars in the disks of spiral galaxies form on about the Hubble time scale; and the amount of gas associated with galaxies was significantly greater in the recent past: for example, being twice the present value by a redshift of 0.3 and at least 3 times the present value by a redshift of 1