The age of the Galactic Globular Clusters

Abstract

The Galactic globular clusters are believed to be among the most ancient objects for which reliable ages can be determined. As the Universe can not be younger than the oldest object it contains, the oldest Galactic globular clusters provide one of the few most important constraints that one can have on cosmological models. Latest estimates indicate that the absolute age of the oldest globular clusters is 14 ± 3 Gyr. The calibration of absolute ages is still subject to observational and theoretical uncertainties at the ≈ 20 % level, and represents a major limitation on our ability to test cosmological models. However, relative ages are starting to be much better known due to the super colour-magnitude diagrams that have been obtained through the use of CCD detectors on large telescopes and the Hubble Space Telescope. The available data are consistent with the majority of Galactic globular clusters being virtually coeval but with a minority having significantly lower ages. The existence of “prehistoric” clusters with ages of around 50 Gyr, as hypothesised in the quasi-steady state cosmology, should be readily recognised.