The Relative Ages of Galactic Globular Clusters

We present a review of the present state of knowledge regarding the relativeages of Galactic globular clusters. First, we discuss the relevant galaxyformation models and describe the detailed predictions they make with respectto the formation timescale and chemical evolution of the globular clusters.Next, the techniques used to estimate globular cluster ages are described andevaluated with particular emphasis on the advantages and disadvantages of eachmethod. With these techniques as a foundation, we present arguments in favor ofthe following assertions: 1) The age of a globular cluster is the likeliestcandidate to be the global second parameter, which along with metal abundance,controls the morphology of the horizonal branch. 2) A total age range of asmuch as $\sim$5 Gyr exists among the bulk of the Galactic globulars. 3) Thereis a significant relation between age and metallicity among the Galacticglobular clusters if the slope of the \mvrr-\feh relation is less than$\sim$0.23. These conclusions along with other supporting evidence favor aformation scenario in which the inner regions of the Galactic halo collapsed ina monotonic fashion over a short time period much less than 1 Gyr. In contrast,the outer regions of the halo fragmented and collapsed in a chaotic manner overseveral Gyrs.Comment: Invited review to appear in PASP. Uses aastex. 45 pages with 9 figure