Why Are Pulsating Red Giant Stars Important?
Pulsating red giants (apart from the Mira stars) have been much less studied than the Cepheids for instance, mostly because of their irregularity; it takes years or decades of observations to fully understand these stars. In the 1970’s, the late Olin Eggen spent several years surveying, studying, and classifying these stars. In a series of papers (e.g. Eggen 1973), he defined small-, medium- and large-amplitude red variables (SARVs, MARVs and LARVs), and even a group of ultra-small-amplitude variables which he called “Sigma Librae Stars”. The “official” classification scheme of the General Catalogue of Variable Stars divides pulsating red giants into semi-regular (SR) variables with either “persistent periodicity” (SRa) or “poorly expressed periodicity” (SRb); there are also slow irregular (L) variables. Pulsating red supergiants are classified by analogy as SRc or Lc. Now we know that there is a complete spectrum of behavior among the red giants, from almost periodic to quite irregula
