<p>Pre- supernova models often characterise the compactness of the core using a "compactness parameter" defined as
<span class="math-container">$$ \upsilon = \frac{(M/M_{\odot})}{R(M)/1000\ {\rm km}},$$</span>
where <span class="math-container">$M$</span> is usually chosen to be <span class="math-container">$2.5M_{\odot}$</span> and <span class="math-container">$R(M)$</span> is the radius within which <span class="math-container">$M$</span> is contained.</p>

<p>Pre supernova models by <a href="https://iopscience.iop.org/article/10.3847/1538-4365/227/2/22#apjsaa4c33bib125" rel="nofollow noreferrer">Farmer et al. (2016)</a> show that the central <span class="math-container">$2.5M_{\odot}$</span> of a massive star included the Carbon-burning core in a <span class="math-container">$15M_{\odot}$</span> (initial mass) star, but only contained the oxygen-burning core in a more massive star (models are presented up to 30<span class="math-container">$M_{\odot}$</span>. i.e. Your answer is going to be mass and composition dependent (these models are for a solar metallicity initial composition).</p>

<p>In the <span class="math-container">$15M_{\odot}$</span> model (with mass-loss), <span class="math-container">$\upsilon \sim 0.08$</span> at core collapse, which means the carbon burning core would have been contained within 31,000 km. The <span class="math-container">$30M_{\odot}$</span> model is more compact with <span class="math-container">$\upsilon= 0.58$</span>, indicating that the oxygen-burning core would have been contained within 4,300 km.</p>

<p>At collapse the iron core might have a mass of 1.4-1.8<span class="math-container">$M_{\odot}$</span>, will be supported by electron degeneracy pressure and should have a size a bit smaller than a typical carbon white dwarf (radius of a few thousand km).</p>

<p>You can compare these sizes with the size of the entire red supergiant star, which might have a radius of a few au (e.g. <a href="https://en.wikipedia.org/wiki/Betelgeuse#Diameter" rel="nofollow noreferrer">Betelgeuse</a>).</p>

<p>The shell-burning regions would be found out to slightly larger radii than this I think, but these numbers are a reasonable estimate. Looking in detail at the models I don't think that the final stages of pre- supernova evolution look anything like the strictly stratified onion-shell picture that is seen all over the internet.</p>

CC BY-SA 4.0
2020-02-23T10:46:20.870Z
I always suspected that those onion-layer diagrams were too neat. ;) FWIW, [Wikipedia](https://en.wikipedia.org/wiki/Oxygen-burning_process#Convectively_bounded_flames_and_off-center_oxygen_ignition) mentions that in stars of mass 9 - 10.3 $M_\odot$ oxygen (& neon) can commence burning off-centre, rather than in the core. I suppose that can make the structure even more messy.
PM 2Ring
CC BY-SA 4.0
2020-02-22T16:40:58.010Z
2020-02-22T19:52:57.543Z
2020-02-22T19:52:57.543Z
ProfRob
<p>Rob jeffies gave the results for what the core looks like. But for completeness that hydrogen <strike>shell</strike> envelope is big, very big. Its of order 1000 times the radius of the Sun. Or in other words if it replaced the Sun it would extend out to about Jupiter.</p>

CC BY-SA 4.0
2020-02-22T19:53:50.837Z
This is the size of the entire star, not the H-burning shell. But you are right that to draw a scale diagram you would need to know both.
ProfRob
CC BY-SA 4.0
2020-02-22T19:34:58.783Z
2020-02-22T20:05:49.803Z
2020-02-22T20:05:49.803Z
Rob
<p>Answer seems to be : In Red Super Giats the star can have 300 - 1000 the diameter of the sun and its core where nucleosynthesis occurs is only the diameter of the earth. 
This would explain why after 13.8 Billion years still 98% of the universe is made of hydrogen and helium</p>

<p>See image on page 42 of this slide show <a href="https://slideplayer.com/slide/13959320/" rel="nofollow noreferrer">https://slideplayer.com/slide/13959320/</a></p>

CC BY-SA 4.0
2020-02-25T12:28:54.453Z
But what is the radius of the iron ash core? And the silicon fusing core? etc
usernumber
CC BY-SA 4.0
2020-02-25T11:47:00.963Z
2020-02-25T11:47:00.963Z
Caspar Noyons
<p>I'm sure we've all seen the diagrams of various layers of element fusion from hydrogen to silicon in a star that's just about to go supernova.</p>

<p><a href="https://i.sstatic.net/sVXxq.png" rel="noreferrer"><img src="https://i.sstatic.net/sVXxq.png" alt="onion shell diagram of fusing layers"></a>
(Picture from <a href="https://courses.lumenlearning.com/" rel="noreferrer">courses.lumenlearning.com</a>)</p>

<p>I suspect these pictures grossly exaggerate the relative radii at which these fusing layers occur for the sake of readability. <strong>What would a more accurate to-scale picture of these fusing layers look like?</strong></p>

<p>Now, I assume the answer will vary significantly based on the mass of the star, and I suspect in certain mass regimes some layers will not be fusing at all. I can think of a few other variables that may affect the answer too.</p>

<p>So in <a href="https://en.wikipedia.org/wiki/Cover_your_ass" rel="noreferrer">CYA</a> fashion, I'll leave it up to the answerer to identify specific illustrative or interesting cases, as I'm not after a specific answer but rather a general feel for how large some of the layers are in comparison to the others.</p>

CC BY-SA 4.0
2019-09-18T10:39:39.717Z
While there could be well defined borders where fusion processes stop and/or transition, I would like to point out that in terms of composition/P/T.... continuity is a condition all over the star radius, for core active stars. To say that the layers are there for processes but not for a sort of "visible appearance". Just for sake of discussion.
Alchimista
CC BY-SA 4.0
2019-10-19T13:07:19.147Z
Sorry, I don't know where to find the core shell sizes, but at the end of https://en.wikipedia.org/wiki/Type_II_supernova#Formation there's a table of the burning processes in a 25 solar mass star core that gives the mean density of each shell, and the duration of each process. The table links to the articles detailing each burning process.
PM 2Ring
CC BY-SA 4.0
2020-02-22T14:22:43.657Z
Im looking for this as well, have you found more information about this topic?
In this picture fusion looks like to take place in a very small core but no precise numbers are given https://supernova.eso.org/exhibition/images/0406_3_DUM/ In this article the proportion star/core is given for red supergiant https://sites.ualberta.ca/~pogosyan/teaching/ASTRO_122/lect18/lecture18.html
Caspar Noyons
<star><supernova><core>
What do the fusing 'onion layers' of a pre-supernova star look like to scale?
Ingolifs
# What do the fusing 'onion layers' of a pre-supernova star look like to scale?

(Asked by: username_0 on 2019-09-18)

I'm sure we've all seen the diagrams of various layers of element fusion from hydrogen to silicon in a star that's just about to go supernova.

[![onion shell diagram of fusing layers](https://i.sstatic.net/sVXxq.png)](https://i.sstatic.net/sVXxq.png)
(Picture from [courses.lumenlearning.com](https://courses.lumenlearning.com/))

I suspect these pictures grossly exaggerate the relative radii at which these fusing layers occur for the sake of readability. **What would a more accurate to-scale picture of these fusing layers look like?**

Now, I assume the answer will vary significantly based on the mass of the star, and I suspect in certain mass regimes some layers will not be fusing at all. I can think of a few other variables that may affect the answer too.

So in [CYA](https://en.wikipedia.org/wiki/Cover_your_ass) fashion, I'll leave it up to the answerer to identify specific illustrative or interesting cases, as I'm not after a specific answer but rather a general feel for how large some of the layers are in comparison to the others.

**Tags:** star, supernova, core

> **Comments:**
> **username_1**: While there could be well defined borders where fusion processes stop and/or transition, I would like to point out that in terms of composition/P/T.... continuity is a condition all over the star radius, for core active stars. To say that the layers are there for processes but not for a sort of "visible appearance". Just for sake of discussion.  
> **username_2**: Sorry, I don't know where to find the core shell sizes, but at the end of https://en.wikipedia.org/wiki/Type_II_supernova#Formation there's a table of the burning processes in a 25 solar mass star core that gives the mean density of each shell, and the duration of each process. The table links to the articles detailing each burning process.  
> **username_3**: Im looking for this as well, have you found more information about this topic? In this picture fusion looks like to take place in a very small core but no precise numbers are given https://supernova.eso.org/exhibition/images/0406_3_DUM/ In this article the proportion star/core is given for red supergiant https://sites.ualberta.ca/~pogosyan/teaching/ASTRO_122/lect18/lecture18.html  

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## 3 answers

### Answer by username_4 on 2020-02-22. Score: 3

Pre- supernova models often characterise the compactness of the core using a "compactness parameter" defined as
$$ \upsilon = \frac{(M/M\_{\odot})}{R(M)/1000\ {\rm km}},$$
where $M$ is usually chosen to be $2.5M\_{\odot}$ and $R(M)$ is the radius within which $M$ is contained.

Pre supernova models by [Farmer et al. (2016)](https://iopscience.iop.org/article/10.3847/1538-4365/227/2/22#apjsaa4c33bib125) show that the central $2.5M\_{\odot}$ of a massive star included the Carbon-burning core in a $15M\_{\odot}$ (initial mass) star, but only contained the oxygen-burning core in a more massive star (models are presented up to 30$M\_{\odot}$. i.e. Your answer is going to be mass and composition dependent (these models are for a solar metallicity initial composition).

In the $15M\_{\odot}$ model (with mass-loss), $\upsilon \sim 0.08$ at core collapse, which means the carbon burning core would have been contained within 31,000 km. The $30M\_{\odot}$ model is more compact with $\upsilon= 0.58$, indicating that the oxygen-burning core would have been contained within 4,300 km.

At collapse the iron core might have a mass of 1.4-1.8$M\_{\odot}$, will be supported by electron degeneracy pressure and should have a size a bit smaller than a typical carbon white dwarf (radius of a few thousand km).

You can compare these sizes with the size of the entire red supergiant star, which might have a radius of a few au (e.g. [Betelgeuse](https://en.wikipedia.org/wiki/Betelgeuse#Diameter)).

The shell-burning regions would be found out to slightly larger radii than this I think, but these numbers are a reasonable estimate. Looking in detail at the models I don't think that the final stages of pre- supernova evolution look anything like the strictly stratified onion-shell picture that is seen all over the internet.

> **Comments:**
> **username_2**: I always suspected that those onion-layer diagrams were too neat. ;) FWIW, [Wikipedia](https://en.wikipedia.org/wiki/Oxygen-burning_process#Convectively_bounded_flames_and_off-center_oxygen_ignition) mentions that in stars of mass 9 - 10.3 $M_\odot$ oxygen (& neon) can commence burning off-centre, rather than in the core. I suppose that can make the structure even more messy.  

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### Answer by username_5 on 2020-02-22. Score: 0

Rob jeffies gave the results for what the core looks like. But for completeness that hydrogen shell envelope is big, very big. Its of order 1000 times the radius of the Sun. Or in other words if it replaced the Sun it would extend out to about Jupiter.

> **Comments:**
> **username_4**: This is the size of the entire star, not the H-burning shell. But you are right that to draw a scale diagram you would need to know both.  

---

### Answer by username_3 on 2020-02-25. Score: -1

Answer seems to be : In Red Super Giats the star can have 300 - 1000 the diameter of the sun and its core where nucleosynthesis occurs is only the diameter of the earth.
This would explain why after 13.8 Billion years still 98% of the universe is made of hydrogen and helium

See image on page 42 of this slide show <https://slideplayer.com/slide/13959320/>

> **Comments:**
> **username_6**: But what is the radius of the iron ash core? And the silicon fusing core? etc  

---

