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 / 2 / 3

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.

An old globular cluster has lost its original massive main-sequence members. Its main-sequence turnoff lies near a low stellar mass, typically around $0.8M_{\odot }$ for an age of order $10^{10}$ years. The diagram shows a lower main sequence, a subgiant branch, a prominent red giant branch, a horizontal branch of core-helium-burning stars, an Asymptotic giant branch and a faint white dwarf cooling sequence. The preceding schematic labels these features. A few blue stragglers may lie above the old turnoff because interaction or merger rejuvenated them; they do not imply a young age for the bulk population.