= 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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