A main sequence star will fuse some of its hydrogen, but not all. In massive stars ($>1.5M_\odot$) the core is convective but the rest of the atmosphere radiative and hence does not mix much: as it undergoes shell fusion it will produce an onion-like structure with unused hydrogen on top. Solar mass stars only do this up to helium, but again leave a mantle of unused hydrogen. Stars less than $0.35M_\odot$ are fully convective and can in principle use up all hydrogen. However, I suspect this is not complete except for very low-mass M dwarves that have trillions of years to mature.

Looking at planetary nebulae, I have seen statements that they are about 90 percent hydrogen, 10 percent helium. This seems to fit with papers I have found (example, example) although in some cases helium may reach 29% (example). Given that for a G star about half of the mass is ejected, that would suggest a fraction $0.9\times 0.5 = 0.45$ of unused hydrogen. But surely the fractions will be different for other masses.

So, to sum up, what is known about the fraction of hydrogen that is never fused over a stellar lifetime?


The number we would use for the amount of hydrogen a sun-like star would be expected to fuse during its main sequence phase is about 0.1 Solar masses, or 10%, to an order of magnitude.

  • 2
    $\begingroup$ But the question is "over its lifespan". $\endgroup$
    – ProfRob
    Aug 30 '21 at 9:20
  • 1
    $\begingroup$ Please add further details to expand on your answer, such as working code or documentation citations. $\endgroup$
    – Community Bot
    Aug 30 '21 at 13:33
  • $\begingroup$ "Lifespan" is ambiguous. It all depends on what the OP needs the value for. If "lifespan" means "over an amount of time equal to its average lifetime", then you would expect the amount fused during the main sequence, since that's the vast majority of the star's lifetime. $\endgroup$ Aug 31 '21 at 0:25

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