Broad-band colour evolution of star clusters

Abstract

We briefly review the main features in the broad-band colour evolution of star clusters, over the complete age interval from 10^7 to 10^(10) yr. The emphasis is in the problem of age-dating distant young clusters (<2 Gyr) from their integrated colours. It is shown that U-B and B-V are less sensitive to metallicity than colours involving red pass-bands, like V-I, at least up to ages of some few Gyr. Since U-B and B-V are determined by well-understood and well-populated evolutionary stages, they are also less affected by theoretical uncertainties and by the ubiquitous effect of stochastic colour dispersion. The latter effects become important for the V-K colour. Thus, we argue that U-B and B-V are, presently, the more suitable broad-band colours for age-dating distant clusters. For other potentially useful colours like V-R and V-I, empirical tests of their evolution are still missing.