HR diagram (technically, this is a CMD) showing the locus of single age populations, taken from various galactic clusters. The right scale shows the age corresponding to a main sequence turn off at the given absolute magnitude.
In young clusters the light is dominated by the upper (blue) main sequence. In older clusters it is dominated by red giants.
Click on the above image to start a Flash animation showing the evolution of the HR diagram with age. Select the "Line Representation" to match the figure on the left.
Animation from Pearson Education Inc.