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Stars spend most of their lifetimes on the "main sequence" in the Hertzsprung--Russell diagram, which relates stellar surface temperatures to luminosities. The extended main-sequence turn-off regions found in massive (in excess of a few tens of thousands of solar masses), intermediate-age (1-3 billion year-old) star clusters are commonly interpreted as evidence of cluster-internal age spreads of greater than 300 million years, although young clusters are thought to quickly lose any star-forming gas on timescales of order 10 million years. KIAA faculty member Richard de Grijs and his PhD student Chengyuan Li have found that the stars beyond the main sequence in the 2 billion-year-old, massive Large Magellanic Cloud cluster NGC 1651 can be explained only by a single-age stellar population, even though it has a clearly extended main-sequence turn-off. 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. In retrospect, they find that similar morphologies are apparent in the Hertzsprung-Russell diagrams of at least five additional, intermediate-age star clusters, thus suggesting that an extended main-sequence turn-off does not necessarily imply the presence of a significant cluster-internal age dispersion. These latest results resolve nearly a decade of debate among scientists; the resulting paper is published in the 17 December 2014 issue of the journal Nature.
Figure caption: Although the extended main-sequence turn-off region could be interpreted as the result of an age spread of about 450 million years among the NGC 1651 member stars, the narrow width of the subgiant branch precludes a spread of more than 80 million year and suggests that the cluster is better represented by a truly single-generation stellar population.