"Hypothetical H-R Diagram of an Older Cluster. In this plot titled ""M 2001 Age: 4240 million years,"" the vertical axis is labeled ""Luminosity (LSun)"" and goes from 0.1 at the bottom to 100,000 at the top. The horizontal axis is labeled ""Surface Temperature (K)"" and goes from 40,000 on the left to 3000 on the right. The zero-age main sequence is drawn as a red diagonal line starting just above 100,000 LSun at the top of the graph down to about 4000 K at the bottom. Over-plotted on the graph are black dots representing the individual stars in the cluster. Several of the stars are plotted above and to the right of the main sequence and represent the stars that have begun to enter the giant phase of their evolution. Below about 6500 K and 5 LSun the remaining stars lie on the main sequence."