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We study the high-quality CMDs of three star clusters, NGC 1831, NGC 1868 andNGC 2249 in detail, via the two most likely causes (stellar rotation and agespread) for CMDs with extended main-sequence turn-offs. The results showevident failure of stellar rotation (including resolved and unresolved binarystars) to interpret the CMDs of three star clusters, and the unexpected successof age spread. In particular, the special structures of turn-off and red clumpparts cannot be generated by stellar rotation, but age spread perfectlyreproduces the observed features. This suggests that these three clusterscontain multiple populations of stars, rather than a single population ofrotating stars and binaries. The thick subgiant branch of NGC 1831 gives thestrongest support to this. The results demonstrate that stellar rotation cannotsave the widely accepted view (simple population) of star clusters, and anextended star formation history is needed for explaining the observed CMDs. Inaddition, this work shows that a narrow subgiant branch does not correspond toa simple population. The judgement of simple population in NGC 1651 by aprevious work is not necessarily reliable. We anticipate our assay to be astarting point for more precise study of Hertzsprung-Russell diagrams of starclusters. This involves many factors, such as binaries, rotating stars, andstar formation history (SFH).

Failure of rotation and notable success of age spread to reproduce the CMDs of star clusters NGC 1831, NGC 1868 and NGC 2249