= Solution
On the <main sequence>, central <hydrogen burning> converts four protons into helium, positrons, <neutrinos> and released <energy>. The net reaction is $4p\to{}^4\mathrm{He}+2e^++2\nu_e$, with positron annihilation contributing to the total roughly $26.7\ \mathrm{MeV}$ release before subtracting escaping <neutrino> <energy>. In the low-<mass> <main-sequence stars> of an old <globular cluster>, the <proton–proton chain> generally dominates. The <CNO cycle> makes the same net conversion and becomes important in hotter burning regions.
After central hydrogen exhaustion, a <subgiant> contracts its helium core and establishes a <hydrogen-burning shell>. On the <red giant branch>, that shell supplies most of the <luminosity>, commonly through the temperature-sensitive <CNO cycle>, while the inert helium core grows and becomes supported mainly by <electron degeneracy pressure>. The envelope expands and its surface cools. The helium core is not yet powered by helium <nuclear fusion>.
Helium ignition in a sufficiently low-<mass> degenerate core produces the <helium flash>, removing the strong <electron degeneracy pressure> dominance. A <horizontal-branch star> subsequently burns helium in its core through the <Triple-alpha process>, $3\,{}^4\mathrm{He}\to{}^{12}\mathrm C+\gamma$, and through ${}^{12}\mathrm C(\alpha,\gamma){}^{16}\mathrm O$. A surrounding <hydrogen-burning shell> can continue simultaneously.
After central helium exhaustion, the <Asymptotic giant branch> has an inert carbon-oxygen core with a <helium-burning shell> and an outer <hydrogen-burning shell>. Alternation between quiescent burning and helium-shell thermal pulses drives mixing and substantial mass loss. Subsequent <post-asymptotic giant branch evolution> exposes a hot core that may ionize its expelled envelope as a <planetary nebula>. Finally a <white dwarf> cools mainly by loss of stored thermal <energy>, rather than sustained central <nuclear fusion>.
Back to article page
