= Solution
A <Hertzsprung-Russell diagram> plots <stellar luminosity> upward and <effective temperature> increasing toward the left. The <main sequence> runs from hot luminous stars at upper left to cool faint stars at lower right, with the Sun near $T_{\rm eff}\simeq5800\,\mathrm K$, $L=L_\odot$.
In the <evolution of a solar-mass star>, core hydrogen fusion initially supplies the luminosity. As core hydrogen is exhausted, hydrogen-shell fusion around a contracting helium core carries the star through the <subgiant> and <red giant> phases toward higher luminosity and lower surface temperature. Helium ignition is followed by core helium fusion with a hydrogen-burning shell, placing a solar-mass star in the <red clump>. The later <Asymptotic giant branch> has hydrogen- and helium-burning shells around a carbon-oxygen core. Envelope ejection exposes a hot central star of a <planetary nebula>; residual shell burning ends and the resulting <white dwarf> radiates stored thermal energy as it cools.
\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-s2-solar-evolution-hr.png]
{title=Schematic solar-mass evolution on an H-R diagram, with the dominant energy sources labelled}
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