The exclusion of a significant range of ages in a massive star cluster

Stars spend most of their lifetimes on the main sequence in the Hertzsprung-Russell diagram. The extended main-sequence turnoff regions-containing stars leaving the main sequence after having spent all of the hydrogen in their cores-found in massive (more than a few tens of thousands of solar masses), intermediate-age (about one to three billion years old) star clusters^1-8 are usually interpreted as evidence of internal age spreads of more than 300 million years^2,4,5, although young clusters are thought to quickly lose any remaining star-forming fuel following a period of rapid gas expulsion on timescales of order 10⁷ years^9,10. Here we report, on the basis of a combination of high-resolution imaging observations and theoretical modelling, that the stars beyond themain sequence in the twobillion-year-old cluster NGC1651, characterized by a mass of about 1.7×10⁵ solar masses³, can be explained only by a single-age stellar population, even though the cluster has a clearly extended mainsequence turn-off region.Themost plausible explanationfor the existence of such extended regions invokes a population of rapidly rotating stars, although the secondary effects of the prolonged stellar lifetimes associated with such a stellar population mixture are as yet poorly understood. From preliminary analysis of previously obtained data, we find that similar morphologies are apparent in the Hertzsprung-Russell diagrams of at least five additional intermediateage star clusters^2,3,5,11, suggesting that an extended main-sequence turn-off region does not necessarily imply the presence of a significant internal age dispersion.