Context: Globular clusters are tightly-bound, ancient stellar systems containing hundreds of thousands to a million stars. On the order of 10 billion years old, globular clusters formed at an age in the early universe in which metals (elements heavier than helium) were not as abundant as they are today.
Aims & Methods: Using the Modules for Experiments in Stellar Astrophysics (MESA), an open-source 1D stellar evolution code, a population of metal-poor stars were evolved from the pre-main sequence up to 12 billion years. From the resulting Hertzsprung-Russell diagram, an isochrone of this simulated globular cluster was constructed.
Results & Conclusions: As expected, higher mass stars, with masses greater than 0.8 solar masses, were found to be much further along their evolutionary track than low mass stars. Stars greater than 0.8 solar masses were found to be in the red giant branch stage, while the star with the highest mass was found to be approaching the white dwarf stage. Comparing the expected main sequence lifetime of the highest mass star (~ 17 Gyr) with the simulated lifetime (~ 11 Gyr) led to the idea that metallicity plays a fundamental role in the lifetime of stars.