= Solution
Hydrogen-shell burning deposits helium onto a geometrically thin helium layer. Once helium ignites, the strong temperature dependence of the <Triple-alpha process> and the shell's initially weak expansion response produce the <Härm–Schwarzschild instability>. The resulting <AGB thermal pulse> drives a short-lived intershell convection zone, expands the layers above it, and temporarily extinguishes the hydrogen-burning shell before the cycle restarts.
Toward the upper end of the mass range, neutrino cooling keeps the centre cooler than an off-centre shell, so <carbon burning> can ignite off-centre under partial <electron degeneracy pressure>. Repeated flashes and an inward-moving carbon flame consume most carbon and leave a degenerate <oxygen-neon-magnesium core>, surrounded by helium- and hydrogen-burning shells.
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