 Solution

ID: past-exam-of-the-short-options-paper-of-the-physics-prelims-of-the-university-of-oxford/2022/s02/1/c/solution

A main-sequence star is supported by hydrostatic equilibrium and powered predominantly by hydrogen nuclear fusion in its core. More massive main-sequence stars are generally hotter and more luminous and consume their fuel faster.
On the red giant branch, core hydrogen is exhausted. Hydrogen-shell nuclear fusion surrounds an inert helium core; the envelope expands and cools while the luminosity rises. In low-mass stars the contracting helium core becomes supported by electron degeneracy pressure before helium ignition.
A horizontal-branch star has core helium nuclear fusion and usually hydrogen-shell nuclear fusion. Its luminosity varies comparatively little across the branch, while envelope mass produces a wide range of effective temperatures. It follows helium ignition in a low-mass red giant.
A white dwarf is a compact exposed remnant, typically with a carbon-oxygen interior after ordinary low- or intermediate-mass stellar evolution. Electron degeneracy pressure supports it; sustained core nuclear fusion has ended. It can be hot yet faint because its radius is small, and it moves down its cooling sequence as it loses stored thermal energy.

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