The potential of white dwarf cosmochronology

In the light of recent significant progress on both the observational and t heoretical fronts, we review the status of white dwarf stars as cosmochrono meters. These objects represent the end products of stellar evolution for t he vast majority of stars and, as such, can be used to constrain the ages o f various populations of evolved stars in the Galaxy. For example, the olde st white dwarfs in the solar neighborhood (the remnants of the very first g eneration of intermediate-mass stars in the Galactic disk) are still visibl e and can be used, in conjunction with cooling theory, to estimate the age of the disk. More recent observations suggest the tantalizing possibility t hat a population of very old white dwarfs inhabits the Galactic halo. Such a population may contribute significantly to baryonic "dark" matter in the Milky Way and may be used to obtain an independent estimate of the age of t he halo. In addition, white dwarf cosmochronology is likely to play a very significant role in the coming era of giant 8-10 m telescopes when faint wh ite dwarf populations should be routinely discovered and studied in open an d globular clusters.