> > There was no C12 anywhere in the universe until the first stars began dying. > ... outside of the stellar cores manufacturing it. Stellar cores do not pro

> > There was no C12 anywhere in the universe until the first stars began dying. > ... outside of the stellar cores manufacturing it. Stellar cores do not produce C12 unless they've already started dying. If there is hydrogen in the core, it will fuse that into helium instead of fusing helium into C12. It's not just a question of "it does happen, just super rarely" either- stars will have an inert helium core surrounded by an expanding shell of hydrogen burning for quite some time before the helium is actually able to begin burning. In the case of the Sun, it's expected to enter the red giant phase, where it burns hydrogen around its inert helium core for 1.2 billion years before it begins burning helium into C12. > So we get all the elements heavier than iron from these insane-o-novas and from ordinary supernovas, often by decay from even heavier isotopes. Do we know how much of each is from which? E.g., did all the platinum or something start from pop iii output? We have a few models of sources of elements heavier than iron. Supernova are the primary source of almost everything from oxygen up to rubidium. Once a star goes from its red dwarf stage, where it's still burning hydrogen, into its asymptotic giant branch stage, there is a thin, very hot layer around the helium core, with a dense neutron flux. Heavy elements here will absorb stray neutrons and slowly grow up to elements as heavy as lead. This accounts for many of the elements heavier than rubidium. The rest of the elements heavier than rubidium are the result of merging neutron stars. See here for more info: https://en.wikipedia.org/wiki/Stellar_nucleosynthesis#Key_reactions https://en.wikipedia.org/wiki/Stellar_nucleosynthesis#Key_re... Unrelated fun fact: for stars between about 0.5 and 2 times the mass of the Sun, nearly all of the helium burning happens all at once, in the space of a few minutes. For those brief moments, most of the energy generation in that star's galaxy is happening in that star; that star is more powerful than all the other energy sources in its galaxy combined. This is called helium flash. Surprisingly, this doesn't explode the star, all that energy is absorbed by the outer layers. Stars heavier than 2 times the mass of the Sun do not undergo helium flash, as they are heavy and massive enough to begin burning helium earlier and in a non-runaway fashion.