Halo Star Evolution

In this review, I will discuss a few problems which point to the need for improved stellar evolution models of halo stars. Current stellar evolution models do not match the observed $^7$Li abundance patterns, suggesting that the input physics and/or the assumptions used in constructing the models are in need of revision. It appears that all halo stars have suffered some $^7$Li depletion, implying that the primordial $^7$Li abundance is higher than that presently observed in hot halo stars. Observations of abundances of various elements in globular cluster giant branch stars have suggested for some time now that some form of deep mixing, which is not present in theoretical models, occurs in halo stars. The driving mechanism for this mixing, and its incorporation into stellar models remain one of the key problems in stellar modeling. Current theoretical isochrones are able to provide a good match to observed colour-magnitude diagrams. However, there is some evidence that the theoretical luminosity functions are in disagreement with observations. This is an area which requires further study, as it suggests that the relative main sequence/giant branch lifetimes predicted by the models are incorrect. A discussion of some of the uncertainties involved in determining the ages of globular clusters is presented. The absolute ages of globular clusters provide a lower bound to the age of the universe, and so are of great interest to cosmologists. Unfortunately, present uncertainties in stellar models lead to a rather large range in the inferred ages of globular clusters of 11 -- 18 Gyr.