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We know that one way carbon ends up in the interstellar medium which by the way is one or the heavy elements that help form the planet.

But we also know that in the core, carbons fuse with another carbon to produce oxygen which means carbon is no longer there.

Then how does carbon end up in the interstellar medium ?

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    $\begingroup$ Does this answer your question? How small stars help with planet formation $\endgroup$
    – James K
    Apr 17, 2023 at 21:57
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    $\begingroup$ The linked question answers most of your doubts. There are a couple of additional points. Carbon doesn't fuse with carbon to produce oxygen. If the temperature is high enough carbon can fuse with helium to produce oxygen, but that only occurs in larger stars, and even in those stars, there would be a shell of carbon outside the core. So there is carbon in the core, and dredged up by convection to the outer parts of the star from where it can get into space, and see the linked answer for more details. $\endgroup$
    – James K
    Apr 17, 2023 at 22:02
  • $\begingroup$ Well, that's my question on that link as well and no, it doesn't answer this :) $\endgroup$ Apr 18, 2023 at 15:24
  • $\begingroup$ How does it not answer?? First - fix your question, is it "how does carbon get into a remnant" or "how does carbon get into the interstellar medium". You might try to fix the mistakes in the question too. Then actually read the linked question. It explains how carbon in formed and not completely used up in stars of various sizes, and then how some of that carbon can migrate to the ISM. If it doesn't answer your question, then explain very carefully what you don't understand. I think that question has an answer that is very clear. $\endgroup$
    – James K
    Apr 18, 2023 at 16:38

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First Carbon + Carbon makes Magnesium, Neon, or Sodium. It's Carbon + helium that makes Oxygen.

Secondly, it's not true that all carbon is converted into oxygen. At the end of core helium burning (where carbon is converted into oxygen), the core will have about 30% carbon and 70% oxygen. In massive stars, most of the carbon will then be converted into heavier elements as the star approaches core collapse. But some carbon can survive if it's near the outer edge of the core and thus does not reach carbon-burning temperatures. The star will also make more carbon in its helium shell, which is outside of the core, which can survive until the star explodes. Once the star undergoes a supernova the remaining carbon can be ejected into the Universe.

There is also another option, in stars ~40 times the mass of the sun the stellar winds can be strong enough to remove the surface layers of the star and expose the helium-burning core. That core is rich in carbon (as we are converting helium into carbon) and thus carbon can be ejected in the strong stellar winds of massive stars.

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  • $\begingroup$ Thanks Rob. So not all carbons participate in fusion with helium. And those which dont, start to fuse with more heavier elements at a later stage of fusion. But long story short, some percent( less than 30) end up in the outer edge shell of the core. How do they end up there and why not all carbons participate in fusions ? $\endgroup$ Apr 18, 2023 at 16:12
  • $\begingroup$ Hi Rob. One more question, where would i be able to read all this well explained ? all the details ? how shells work as well in the star and everything you mentioned ? $\endgroup$ Apr 18, 2023 at 23:32

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