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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 (Picture from 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 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.

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    $\begingroup$ 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. $\endgroup$
    – Alchimista
    Sep 18 '19 at 10:39
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    $\begingroup$ Sorry, I don't know where to find the core shell sizes, but at the end of 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. $\endgroup$
    – PM 2Ring
    Oct 19 '19 at 13:07
  • $\begingroup$ 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 supernova.eso.org/exhibition/images/0406_3_DUM In this article the proportion star/core is given for red supergiant sites.ualberta.ca/~pogosyan/teaching/ASTRO_122/lect18/… $\endgroup$
    – Caspar Noyons
    Feb 22 '20 at 14:22
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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) 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).

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.

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  • $\begingroup$ I always suspected that those onion-layer diagrams were too neat. ;) FWIW, Wikipedia 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. $\endgroup$
    – PM 2Ring
    Feb 23 '20 at 10:46
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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.

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    $\begingroup$ 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. $\endgroup$
    – ProfRob
    Feb 22 '20 at 19:53
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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/

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  • $\begingroup$ But what is the radius of the iron ash core? And the silicon fusing core? etc $\endgroup$
    – usernumber
    Feb 25 '20 at 12:28

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