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 turn-off 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 are usually interpreted as evidence of cluster-internal age spreads of more than 300 million years, although young clusters are thought to quickly lose any remaining star-forming fuel following a period of rapid gas expulsion on timescales of order ${10}^7$ years. Here we report that the stars beyond the main sequence in the two billion-year-old cluster NGC 1651, characterized by a mass of $\sim 1.7\times {10}^5$ solar masses, can be explained only by a single-age stellar population, even though the cluster has clearly extended main-sequence turn-off region. The most plausible explanation for the extended main-sequence turn-offs invokes the presence of 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 intermediate-age star clusters, suggesting that an extended main-sequence turn-off does not necessarily imply the presence of a significant cinternal age dispersion.