How much lower is the ratio of hydrogen to helium in a red giant vs a 'normal' main sequence star like our Sun?

Also, how low does the ratio get at the end of a red giant's life? (Just before it becomes a white dwarf, or goes onto the asymptotic giant branch, or whatever...)


1 Answer 1


Yes, red giants are still primarily hydrogen, even after leaving the main sequence

The problem however is, that although red giants are still primarily hydrogen, this hydrogen is "inert" i.e. this hydrogen is wayyy above the core. This means that hydrogen cannot fuse, as the conditions for that exist only at the core of the star.

The exact composition, is... well it depends on the type of star, but from what I have heard, it most probably should be somewhere between the 50-70% mark. So the ratio of hydrogen can vary from 1:2 (In larger stars) to 7:10 (In smaller stars). The reason is that stars never actually deplete all their hydrogen. This is due to the fact that above 0.5 solar masses, a star no longer convects fully, convection is now restricted to being above the core, whereas the core itself radiates fusion energy. Go above 1.5 solar masses, and it is the opposite, the core is now convecting, whereas the rest of the star is radiating that heat. This means that nearly every star, including our own Sun, tend to stratify over a short period of time.

This means that although the star still has a lot of hydrogen, the star cannot mix itself uniformly, resulting in a pile-up of heavier elements like helium and carbon at the core.

TLDR: Yeah, red giants are still primarily hydrogen even before their death, and the ratio might be somewhere between 1:2 to 7:10.

  • $\begingroup$ Whilst not disagreeing with the final conclusion, I note that red giants are burning hydrogen in a shell and at a much greater rate than on the main sequence. $\endgroup$
    – ProfRob
    Dec 25, 2022 at 12:45

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