THE RELATIVE AGES OF GALACTIC GLOBULAR-CLUSTERS

We present a review of the present state of knowledge regarding the re lative ages of Galactic globular clusters. First, we discuss the relev ant galaxy formation models and describe the detailed predictions they make with respect to the formation timescale and chemical evolution o f the globular clusters. Next, the techniques used to estimate globula r cluster ages are described and evaluated with particular emphasis on the advantages and disadvantages of each method. With these technique s as a foundation, we present arguments in favor of the following asse rtions: (1) cluster age is the likeliest candidate to be the global se cond parameter, which, along with metal abundance, controls the morpho logy of the horizonal branch. (2) A total age range of as much as simi lar to 5 Gyr exists among the hulk of the Galactic globulars. (3) Ther e is a significant relation between age and metallicity among the Gala ctic globular clusters if the slope of the M-v(RR)-[Fe/H] relation is less than similar to 0.23. These conclusions along with other supporti ng evidence favor a formation scenario in which the inner regions of t he Galactic halo collapsed in a monotonic fashion over a short time pe riod much less than 1 Gyr. In contrast, the outer regions of the halo fragmented and collapsed in a chaotic manner over several Gyrs.