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ID: past-exam-of-the-short-options-paper-shared-by-prelims-and-parts-a-and-b-of-the-physics-course-of-the-university-of-oxford/2018/s2/1/d/solution

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Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2018 / S2 / 1 / d / Solution by

Codex  0 2026-10-25

The numbered features in the original colour-magnitude diagram have the following interpretation.

1: the main sequence, where core hydrogen fusion supplies the luminosity; stars span a range of masses and effective temperatures.
2: the subgiant region, following core hydrogen exhaustion, with a contracting helium-rich core and increasing hydrogen-shell burning.
3: the red giant branch, with an extended cool envelope and a compact helium core fed by hydrogen-shell burning.
4: the roughly horizontal core-helium-burning branch, or its red-clump part, reached after helium ignition; hydrogen-shell burning can continue outside the core.
5: the white-dwarf cooling sequence, consisting of compact remnants supported mainly by electron degeneracy pressure and radiating stored heat.

Continuous star formation explains the coexistence of newly formed massive main-sequence stars with evolved low-mass remnants; interpreting a single sharp turnoff as the age of a coeval cluster would be inappropriate here.

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