<p>There is a reasonable section on this in the wikipedia pages on <a href="https://en.wikipedia.org/wiki/Neon-burning_process" rel="nofollow noreferrer">Neon-burning</a> and <a href="https://en.wikipedia.org/wiki/Oxygen-burning_process#Pre-oxygen_burning" rel="nofollow noreferrer">oxygen-burning</a> if you can extract the relevant information.</p>
<p>Oxygen-16 is a really stable, &quot;doubly-magic&quot; 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.</p>
<p>In fact &quot;oxygen-burning&quot; 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 &quot;out of order&quot; compared with how you might think they would be.</p>

CC BY-SA 4.0
2021-06-19T12:41:45.390Z
2021-06-20T06:30:27.233Z
2021-06-20T06:30:27.233Z
ProfRob
<p>In addition to the question with the picture <a href="https://astronomy.stackexchange.com/questions/44430/do-elements-like-magnesium-and-sulfur-form-an-layer-on-massive-stars">here</a>, why do shells don't form the same order as the <a href="https://en.wikipedia.org/wiki/Alpha_process#:%7E:text=The%20alpha%20process%2C%20also%20known,only%20helium%2C%20and%20produces%20carbon.&amp;text=Silicon%20and%20calcium%20are%20purely%20alpha%20process%20elements." rel="noreferrer">alpha process</a>?
<a href="https://i.sstatic.net/MWJ63.png" rel="noreferrer"><img src="https://i.sstatic.net/MWJ63.png" alt="enter image description here" /></a></p>
<p>image credit: Gemini Observatory/NSF/C.Aspin</p>

<stellar-evolution>
Order of shells formed in a massive star
Jack the Ranger
# Order of shells formed in a massive star

(Asked by: username_0 on 2021-06-19)

In addition to the question with the picture [here](https://astronomy.stackexchange.com/questions/44430/do-elements-like-magnesium-and-sulfur-form-an-layer-on-massive-stars), why do shells don't form the same order as the [alpha process](https://en.wikipedia.org/wiki/Alpha_process#:%7E:text=The%20alpha%20process%2C%20also%20known,only%20helium%2C%20and%20produces%20carbon.&text=Silicon%20and%20calcium%20are%20purely%20alpha%20process%20elements.)?
[![enter image description here](https://i.sstatic.net/MWJ63.png)](https://i.sstatic.net/MWJ63.png)

image credit: Gemini Observatory/NSF/C.Aspin

**Tags:** stellar-evolution


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## 1 answers

### Answer by username_1 on 2021-06-19. Score: 14 | Accepted answer

There is a reasonable section on this in the wikipedia pages on [Neon-burning](https://en.wikipedia.org/wiki/Neon-burning_process) and [oxygen-burning](https://en.wikipedia.org/wiki/Oxygen-burning_process#Pre-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.


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