Horizontal Branch Stellar Evolution

I review aspects of the evolution of horizontal branch (HB) stars. I start by discussing current topics in the study of HB stellar evolution, including a brief review of the main determinants of the structure of low-mass core helium burning stars and of HB morphology. I describe the first' and second’ parameter effects on HB morphology, and attempt to summarize the current state of affairs and how future investigations might improve our findings in this area. I also briefly discuss the topic that has generated the most theoretical attention over the last 5 years, that of the hot end of HB sequences. I then discuss the current state of HB theory, reviewing the effect of new physics on the models, and the special considerations (partial mixing) that arise in core helium burning models. Finally, I discuss two other outstanding issues. After noting that the mass loss mechanism is not at all understood, I review detailed work by several different groups concerning the age, abundance spread, and rotational properties of RGB and HB stars in the `second parameter’ clusters M3 and M13. I conclude by discussing the HB luminosity-metallicity relation. The discrepancy between the slope of this relation from different lines of argument appears largely to have been resolved. However, the observational data and theoretical sequences (if Y ~ 0.23) are still in conflict as far as the HB luminosity zero-point is concerned.