[sculptor]

Why do planetary nebulae form? The usual story goes:

(1) core fusion normally heats star and prevents gravitational collapse. Fusion rate extremely temperature sensitive. Very stable. Tiny collapse causes tiny increase in temperature and pressure, causing huge increase heat production causing negative feedback fluffing up of atmosphere to stop collapse. Ok fair enough.

(2) Now we run out of hydrogen. Core fusion energy production drops. Core starts to collapse. Core collapse runs away. Power output from gravitational energy thus liberated exceeds earlier nuclear power output by huge factor. This fluffs up outer atmosphere to planetary nebula size.

Ok, why is it that in para 1, a tiny reduction in radius causes an increase in heat production by the fusion process which causes the core to expand stably, by just the right amount, so the core is self supporting, but in para 2 a tiny reduction in radius (which also leads to immense heating from gravitational collapse) does not also cause the core to thermally support itself?

What is it about the dynamics that makes (1) stable but (2) unstable?