In Search of the Early Universe. I. the Oldest Stars

Abstract

The epoch of galaxyformation can be dated in a numberof ways. This lectureexplores the question of how we can date the oldest identifiable stars. It is shown that reliable ages can only be found for stars that belong to star clusters and that it is still not possible to age-date them to an accuracy of better than approximately 2 Gyr. Indirectevidence also comes from the abundances of the elements in old field stars, but these data are not able to produce precise ages, though they do indicate indirectly the great age of certain stars. Only a very few galaxies have so far been age-dated reliably. These include the galaxies that are near enough to obtain accurate main sequence turn-off magnitudes and colors using, especially, the Hubble Space Telescope. The oldest galaxies so studied have ages of 14.5Gyr, but with uncertainties of at least 2 Gyr, which include both observational and theoretical uncertainties.