Figure 14-35 The Formation of Spiral and Elliptical Galaxies (a) If the initial star formation rate in a protogalaxy is low, it can evolve into a spiral galaxy with a disk. (b) If the initial star formation rate is rapid, no gas is left to form a disk. The result is an elliptical galaxy. (c) This graph shows how the rate of star birth (in solar masses per year) varies with age in spiral and elliptical galaxies.