2.3. The Red Giant Branch (RGB)

The RGB is a very bright evolved phase of stellar evolution, where the star is burning H in a shell around its He core. For a given metallicity the RGB red and blue limits are given by the young and old limits (respectively) of the stars populating it (for ages 1 Gyr). As a stellar population ages the RGB moves to the red, and for constant metallicity, the blue edge is determined by the age of the oldest stars. However increasing the metallicity of a stellar population will also produce exactly the same effect as aging, and also makes the RGB redder. This is the age-metallicity degeneracy problem. The result is that if there is metallicity evolution within a galaxy, it can be impossible to uniquely disentangle effects due to age and metallicity on the basis of the optical colours of the RGB alone, especially in low metallicity systems such as dI galaxies (cf. Da Costa 1998)