Chris Peterson wrote:
fusion isn't what powers a star in any case; the energy source is gravity
That I will politely take issue with. While the release of gravitational potential energy can act as an energy source, what powers the stars very much is nuclear fusion, with E=mc
2 showing up as the difference in mass between the atoms that were fused and the fusion product.
Once the core is inert (carbon and oxygen), you still have a shell of helium (you also have hydrogen, but that isn't the important reaction) outside that. And the tricky thing with helium fusion is that it's exquisitely sensitive to temperature, so just a few degrees of temperature rise (as from convective processes in the outer envelope) can cause a massive burst of energy output, which blows away some of the outer layers.
I can quibble about how inert the core is as helium shell burning sets in, but the literature that I am able to find and recall does back up your main point: That this is an unstable configuration that lends itself to massive energy outbursts. Also, I can see that nuclear fusion may be brought to an end by the loss of the outer layers and the pressure that they once exerted on the core.
And that process will continue ... throwing out shells of material in successive waves of activity. And there you have a planetary nebula.
You can actually see shells of material in that picture, but then are the polar outflows that give the Dumbbell Nebula its name. I take that as some evidence that the central star is still quite active. This is not like the Ring Nebula, which I suspect is quite complete and fading as its now-inert central star cools down. This planetary nebula looks to me like a work in progress.