19. Helium core flash and the tip of the red giant branch as a primary distance indicator

In his 1930 Harvard Observatory Monograph: ``Star Clusters,'' Harlow Shapley outlined various methods for the estimation of distances to Galactic globular clusters. Noteworthy among these methods is the one that assumes that the apparently brightest (giant) stars are all of the same absolute magnitude. Tested for consistency in those cases where more than one method of distance determination could be made (with RR Lyrae stars, for instance), and roughly corrected for foreground contamination (choosing the magnitude of the sixth-brightest star as a robust estimator), Shapley went on to map out the size scale of the Galaxy as defined by the old Population II globular cluster system. Although the exact details of his approach are now known to be flawed, the method as applied was of sufficient precision that a revolutionary view of the size of our Galaxy was obtained, and the Milky Way as an island universe started to take on a tangible reality of its own.

Before pursuing the historical path relating to the successful application of brightest red giant stars as extragalactic distance indicators we digress slightly in the next section to present a set of criteria that any extragalactic distance indicator might be judged by. It will rapidly become clear that many of these criteria are probably mutually exclusive in a practical sense (ultra-luminous, locally calibrated and theoretically understood), but ideals are seldom realized in full measure in the real world. Nevertheless, we can establish metrics for relative performance.