= Solution
For a young <open cluster>, the <Hertzsprung-Russell diagram> contains a populated <main sequence> reaching hot, luminous, high-mass stars, with a high-mass <main-sequence turnoff> determined by its age. Cool low-mass members may still lie on tracks of <pre-main-sequence stars> above the lower main sequence. Depending on how young the cluster is, a few evolved blue or red supergiants may already be present; an extremely young cluster need not have any. It lacks the dominant old low-mass red-giant and horizontal-branch populations of an old globular cluster.
\Image[/past-exam-of-the-short-options-paper-shared-by-prelims-and-parts-a-and-b-of-the-physics-course-of-the-university-of-oxford/2012-s26-cluster-hr-diagrams.png]
{title=Schematic temperature-luminosity diagrams of young and old star clusters}
{description=Temperature decreases to the right. The young cluster reaches a hot luminous turnoff and contains possible pre-main-sequence stars; the old cluster has a cool low-mass turnoff, red giant and horizontal branches, and a white-dwarf sequence. Curves are illustrative population sketches, not fitted isochrones.}
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