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Globular clusters trace the formation history of the spheroidal components ofboth our Galaxy and others, which represent the bulk of star formation over thehistory of the universe. They also exhibit a range of metallicities, withmetal-poor clusters dominating the stellar halo of the Galaxy, and highermetallicity clusters found within the inner Galaxy, associated with the stellarbulge, or the thick disk. Age differences between these clusters can indicatethe sequence in which the components of the Galaxy formed, and in particularwhich clusters were formed outside the Galaxy and later swallowed along withtheir original host galaxies, and which were formed in situ. Here we present anage determination of the metal-rich globular cluster 47 Tucanae by fitting theproperties of the cluster white dwarf population, which implies an absolute ageof 9.9 (0.7) Gyr at 95% confidence. This is about 2.0 Gyr younger than inferredfor the metal-poor cluster NGC 6397 from the same models, and providesquantitative evidence that metal-rich clusters like 47 Tucanae formed laterthan the metal-poor halo clusters like NGC 6397.

An Age Difference of 2 Gyr between a Metal-Rich and a Metal-Poor Globular Cluster