 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.