The HR diagram of Globular Clusters: Theorist' view(s)

Abstract

I list the characteristic features of Globular Cluster (GC) HR diagrams which provide a complete test of the stellar evolution of low mass stars: morphologies describing the different evolutionary phases, number ratios and luminosity functions, which add quantitative information. Then I explore the stage of todays' understanding of the classical distance scale indicators (providing warnings against model construction which, underneath, already is based on a choice of distance), and compare them to the `new' indicators, such as the white dwarfs and the first and second kink of the low main sequence. The classical and new distance indicators are still subject to uncertainties due to tiny details of the theory, which are all of the same order of magnitude, \~0.25mag and the absolute ages of GCs can not be constrained to better than 10-16Gyr. However, most of recent theoretical and observational results (including both classical and new indicators) point more towards the lower range of ages (10-12) than to the upper range (15-16).