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6. t0 - AGES OF THE OLDEST STARS

The ages of stars can be derived quite independently from the expansion age of the Universe (obtained by integrating the Friedmann equation), and have long been used as a point of comparison and constraint on cosmology; for example, globular cluster age-dating, nucleocosmochronology, and white-dwarf cooling estimates for the Galactic disk. The reader is referred to earlier reviews on these topics by Renzini (1991), Schramm (1989). For the purposes of this review, I briefly consider only two types of age determinations: those based on Galactic globular clusters, and a new estimate of the age based on a measurement of radioactive thorium in a metal poor Galactic halo star.