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 clusters1-8 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 10(7) years(9,10). Here we report, on the basis of a combination of high-resolution imaging observations and theoretical modelling, that the stars beyond the main sequence in the two-billion-year-old cluster NGC 1651, characterized by a mass of about 1.7 x 10(5) solar masses(3), can be explained only by a single-age stellar population, even though the cluster has a clearly extended main-sequence turn-off region. The most plausible explanation for 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 intermediate-age 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.Multidisciplinary SciencesSCI(E)PubMed3ARTICLEjoshuali@pku.edu.cn7531367-+51