This graph schematically plots two populations of stars Hubble has seen in a neighboring star cluster. The graph, a simplified version of the original data, plots stellar brightness (vertical axis) against stellar color-temperature. The normal main sequence stars in the cluster are plotted on the right curve. The cooler a star is, the redder it appears, and it diminishes in brightness. Hubble has identified the very coolest and faintest normal stars in the cluster.
The curve on the left plots the white dwarf population of burned-out sunlike stars. It follows a normal "cooling slope" until the white dwarf begins to look bluer and hook toward the left. The dwarfs aren't getting hotter. Chemical changes in their atmospheres make them look cooler. This predicted "white dwarf hook" has never been seen before.