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The asymptotic giant branch (AGB) phase is the final stage of nuclear burningfor low-mass stars. Although Milky Way globular clusters are now known toharbour (at least) two generations of stars they still provide relativelyhomogeneous samples of stars that are used to constrain stellar evolutiontheory. It is predicted by stellar models that the majority of cluster starswith masses around the current turn-off mass (that is, the mass of the starsthat are currently leaving the main sequence phase) will evolve through the AGBphase. Here we report that all of the second-generation stars in the globularcluster NGC 6752 -- 70 per cent of the cluster population -- fail to reach theAGB phase. Through spectroscopic abundance measurements, we found that everyAGB star in our sample has a low sodium abundance, indicating that they areexclusively first-generation stars. This implies that many clusters cannotreliably be used for star counts to test stellar evolution timescales if theAGB population is included. We have no clear explanation for this observation.

Sodium content as a predictor of the advanced evolution of globular cluster stars