Note that different students may see different quiz forms.) ... Giant stars are rare because . a) they do not form as often as main sequence stars. b) giant stars are unstable. c) the giant stage is very short compared to the main sequence stage. d) elements heavier than helium are relatively rare. In a color-magnitude diagram of a star cluster, the blue end of the main sequence is useful for defining the age of the cluster because . ... Paul J. Green pgreen@cfa.harvard.edu ...