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 clusters1,2,3,4,5,6,7,8 are usually interpreted as evidence of internal age spreads of more than 300 million years2,4,5, 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 years9,10. 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 × 105 solar masses3, 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 clusters2,3,5,11, suggesting that an extended main-sequence turn-off region does not necessarily imply the presence of a significant internal age dispersion.
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We thank S. de Mink, Y. Huang and X. Chen for discussions and assistance. Partial financial support for this work was provided by the National Natural Science Foundation of China through grants 11073001, 11373010 and 11473037.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Radial brightness density profile of NGC 1651.
The 1σ uncertainties shown are due to Poisson noise.
Extended Data Figure 2 Background decontamination.
a, Original colour–magnitude diagram of NGC 1651. b, Field-star colour–magnitude diagram. c, Field-star-decontaminated NGC 1651 colour–magnitude diagram.
Extended Data Figure 3 Constraints on the maximum likely age dispersion.
Number distribution, N (including 1σ standard deviations), of the deviations in magnitude, ΔB, of our subgiant-branch sample, as in Fig. 2. The black dashed lines at the top indicate typical ΔB values for isochrones of different ages, as indicated.
Extended Data Figure 4 Evolutionary tracks for extremes in stellar rotation rates.
Red, non-rotating stars; blue, stellar rotation at 95% of the critical break-up rate (ω = 0.95). Both tracks apply to 1.7 solar-mass stars. , solar luminosity; Teff, effective temperature.
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Li, C., de Grijs, R. & Deng, L. The exclusion of a significant range of ages in a massive star cluster. Nature 516, 367–369 (2014). https://doi.org/10.1038/nature13969
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