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Using deep Hubble Space Telescope imaging, color-magnitude diagrams areconstructed for the globular clusters 47 Tuc and NGC 6397. As expected, becauseof its lower metal abundance, the main sequence of NGC 6397 lies well to theblue of that of 47 Tuc. A comparison of the white dwarf cooling sequences ofthe two clusters, however, demonstrates that these sequences areindistinguishable over most of their loci - a consequence of the settling outof heavy elements in the dense white dwarf atmosphere and the near equality oftheir masses. Lower quality data on M4 continues this trend to a third clusterwhose metallicity is intermediate between these two. While the path of thewhite dwarfs in the color-magnitude diagram is nearly identical in 47 Tuc andNGC 6397, the numbers of white dwarfs along the path are not. This results fromthe relatively rapid relaxation in NGC 6397 compared to 47 Tuc and provides acautionary note that simply counting objects in star clusters in randomlocations as a method of testing stellar evolutionary theory is likelydangerous unless dynamical considerations are included.

Comparing the White Dwarf Cooling Sequences in 47 Tuc and NGC 6397