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Type 1a supernovae are known for having very consistent energy yields, and they are caused when a carbon-oxygen white dwarf reaches the Chandrasekhar limit of about 1.4 solar masses. Since type 1a supernovae are caused by carbon detonation, it makes sense that they would have the more or less the same amount of carbon to explode with more or less the same yield.

ProfRob
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zucculent
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    An article: in Physics today Suggests that this is not well understood, with different results from modelling the fusion process in red giants and the analysis of astroseismograpy of white dwarfs. – James K Aug 09 '21 at 17:30
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    So this means the jury's still out? – zucculent Aug 09 '21 at 17:52
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    As Wikipedia says, we have an ok theory of the general process, but we aren't clear on some of the details. Computer modelling is helpful, but it gets tricky doing accurate models of high energy, high speed processes. – PM 2Ring Aug 09 '21 at 18:01
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    The main feature of a 1a SN is that it's initiated when the mass crosses the critical value, so the total amount of material involved is fairly constant. But it'd be nice to know what the effects on yield the total amount of carbon and the oxygen ratio have. – PM 2Ring Aug 09 '21 at 18:07
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    The accepted answer to this question says that fusion just starts once the thing is denes enough. Is that one possible explanation? – zucculent Aug 09 '21 at 18:13
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    Once the carbon fusion starts it proceeds very quickly, and all the carbon is consumed in a couple of seconds. Note that processes in a white dwarf are dependent on pressure, but the pressure is virtually insensitive to temperature, as explained by ProfRob here: https://physics.stackexchange.com/a/255234/123208 – PM 2Ring Aug 09 '21 at 18:53
  • @PM2Ring I understand the reasons behind carbon detonation. I'm asking why almost the exact same amount of carbon detonates every time when white dwarfs have varying masses and compositions. – zucculent Aug 21 '21 at 23:02
  • Ok, but that's slightly different to the title of your question. You should state that clearly in the body of your question (which currently doesn't contain an actual question). I suspect that nobody here has an answer that adequately covers this topic, or they'd have posted it by now. But you never know your luck... – PM 2Ring Aug 22 '21 at 04:53

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According to Nature, the typical mass fractions of helium, carbon, oxygen, and neon in white dwarfs are 33%, 50%, 15%, and 2%, respectively. This means that there is more carbon than oxygen in a white dwarf. Here's the article used: https://www.nature.com/articles/nature06318#:~:text=As%20a%20result%2C%20these%20objects%20re%2Denter%20the,Ne%20are%2033%%2C%2050%%2C%2015%%20and%202%).

Astrovis
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    Perhaps you could outline how these proportions were found and confirm that they apply to the whole star and not just the atmosphere (the link is pay walled). The object discussed is also not a typical white dwarf. – ProfRob Mar 09 '24 at 19:12