On both Wikipedia (Stellar evolution) and Lumen Learning (The Evolution of more Massive Stars) the Neon layer is depicted as being external to the Oxygen layer despite the fact that neon is heavier and denser than oxygen....
Why would this be so?
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$\begingroup$ "neon is heavier and denser than oxygen" Where are you getting that information? What does it mean to be "heavier"? Larger atomic weight? Is solar oxygen monoatomic? $\endgroup$– AcccumulationCommented Jul 16 at 1:07
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4$\begingroup$ @Acccumulation When the temperature is in the tens or hundreds of millions of degrees, then, yes, all the atoms will be monatomic (and probably fully ionized). $\endgroup$– Peter ErwinCommented Jul 16 at 19:28
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2 Answers
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Because oxygen burning commences at higher temperatures than neon burning.
Neon production takes place chiefly in the C/O core of massive stars at the end of helium burning. The predominant reaction is $$^{12}{\rm C}\ +\ ^{12}{\rm C} \rightarrow \ ^{20}{\rm Ne}\ + ^{4}{\rm He}\, .$$
The first step in neon burning is then the photodisintegration of neon to produce the more stable $^{16}$O nucleus and an alpha particle, followed by the capture of alpha particles onto other neon nuclei to make magnesium. This takes place at lower temperatures than oxygen burning. $$^{20}{\rm Ne}\ + \gamma \rightarrow\ ^{16}{\rm O} + \ ^{4}{\rm He}\ .$$
The net result is that neon is destroyed in favour of oxygen in a shell that is interior to a shell where neon can survive unburned.
i.e. The oxygen layer is where it is hot enough to burn carbon (and all lighter elements) and to photodisintegrate neon (and thus produce more oxygen and magnesium), but too cool to burn oxygen. The dominant element would be oxygen. At slightly lower temperatures (and hence further out in the star), it is still hot enough to burn carbon (hence producing neon), but too cool to photodisintegrate the neon. The dominant element would be neon. At even lower temperatures it is too cool to burn carbon, but still hot enough to fuse hydrogen and helium. The dominant element would be carbon.
The atomic weight of an element is of little importance in deciding the stratification. All of these layers are comparatively well-mixed by convection and one would have to think about radiative as well as gravitational accelerations. The elemental stratification is mostly governed by temperature - though it is probably not as well-ordered as suggsted by pop-sci diagrams.
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Although Neon is heavier than Oxygen, the reaction Ne+He-> Mg occurs at a lower temperature than the reaction O+He->Ne
This is because the oxygen nucleus is especially stable (with 8 protons and 8 neutrons, it is said to be "doubly magic", this particular arrangement of nucleons is highly favoured) And so to burn oxygen requires especially high temperatures.
The onion-skin structure of a massive evolved star is sorted by temperature, with the high-temperature oxygen-burning layer being internal to the lower-temperature Neon-burning layer
Despite the importance of the alpha-process, it isn't the only type of fusion occurring. Neon can be produced (for example) directly from carbon by the C+C->Ne+He process. Photodisintigration also becomes significant. After Neon burning starts in the core, oxygen and magnesium will build up in the core of the star, until temperature reach the point at which oxygen burning can begin.
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$\begingroup$ That sounds like a rather teleological explanation. "After Neon burning starts in the core, oxygen and magnesium will build up in the core of the star" -- but why? Shouldn't the oxygen, being lighter, float on top of the neon until all the neon has burned away? Sure, perhaps it's too cold for oxygen to burn up there, but why would it care about that -- and care enough to ignore its own buoyancy relative to neon and build up in the core instead? $\endgroup$ Commented Jul 15 at 15:33
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1$\begingroup$ @Troposphere I'm no stellar physicist but I'd guess there are a number of factors, its not a stationary system with the layers being still and perfectly separated. There are other forces than gravity acting on the gases, there's radiation pressure and electromagnetic forces. I'd imagine there's significant convection too. $\endgroup$ Commented Jul 17 at 7:17
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$\begingroup$ @Alan, I would understand an answer that said, "the onion model is wrong; the elements don't actually form discrete layers". But neither of the two high-rep answers make that claim. So even if it's not gravity, there must be something that causes the oxygen for form a layer instead of just mixing with the neon. Hmmm.... perhaps the correct answer would be something like "because the bottom of the neon layer is where oxygen is created, and the neon-burning region shifts outwards too fast for the freshly-made oxygen to drift out in front of it"? $\endgroup$ Commented Jul 18 at 12:09
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1$\begingroup$ There are layers of burning, not layers of elements. The Neon layer is the layer in which neon is burning, there could be oxygen etc mixed in the layer too, but it won't have reached the temperature for oxygen burning, so the oxygen would be inert in that layer. $\endgroup$– James KCommented Jul 18 at 22:26
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$\begingroup$ @James: I see. Since that correction is not stated particularly clearly at commons.wikimedia.org/wiki/File:Evolved_star_fusion_shells.svg ("concentric shells of Hydrogen (H), Helium (He), Carbon (C), Neon/Magnesium (Ne), Oxygen (O) and Silicon (Si) plasma") nor in the articles that use the image, perhaps it deserves to be made more prominently in the answer? $\endgroup$ Commented Jul 20 at 15:37
