Order of shells formed in a massive star

Question

In addition to the question with the picture here, why do shells don't form the same order as the alpha process?

image credit: Gemini Observatory/NSF/C.Aspin

Answer 1

There is a reasonable section on this in the wikipedia pages on Neon-burning and oxygen-burning if you can extract the relevant information.

Oxygen-16 is a really stable, "doubly-magic" nucleus, in that it contains complete shells of both protons and neutrons. The addition of an alpha particle through fusion with a He nucleus doesn't occur quite so easily for Oxygen-16 as it does for adding an alpha particle to Neon-20.

In fact "oxygen-burning" occurs most readily by combining an oxygen nucleus with another oxygen nucleus, partly I guess because the He nuclei get vacuumed up by reactions with carbon and neon first. As a result, the Coulomb barrier is much higher for oxygen-burning and it needs high temperatures than neon-burning. Thus the oxygen-burning and neon-burning shells appear "out of order" compared with how you might think they would be.