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.