Figure 20-3: Stellar Evolution in a Globular Cluster In the old globular cluster M55, stars with masses less than about 0.8 M_⊙ are still on the main sequence, converting hydrogen into helium in their cores. Slightly more massive stars have consumed their core hydrogen and are ascending the red giant branch; even more massive stars have begun core helium fusion and are found on the horizontal branch. The most massive stars (which still have less than 4 M_⊙) have consumed all the helium in their cores and are ascending the asymptotic giant branch.

(Adapted from D. Schade, D. VandenBerg, and F. Hartwick)