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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?

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    $\begingroup$ physics.SE has a good answer on this. $\endgroup$ – pela Jun 15 '18 at 8:07
  • $\begingroup$ Probably a large fraction does not get fused. Big Bang Nucleosynthesis calculations predict the early universe was about 75% hydrogen. Measurements of our galaxy show it is about 74% hydrogen. In the lifetime of the universe, almost none of the hydrogen has been fused into heavier elements within stars.. $\endgroup$ – zephyr Jun 15 '18 at 15:35
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    $\begingroup$ Pela's answer is helpful. You might want to check out The BONNSAI project, "The hydrogen profile, previous mixing, and loops in the H-R diagram during core helium burning" 2nd Source, "Chapter 9: Post-main sequence evolution through helium burning.PDF", continued ... $\endgroup$ – Rob Jun 16 '18 at 21:47
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    $\begingroup$ "Massive stars on the main sequence - Karen Köhler", "Main Sequence Stars", or search for "Astrophysical Concepts Martin Harwit Fourth Edition". -- There's a lot of variables that make for a busy chart and a lengthy explanation. Mass, rotational velocity, initial composition, etc. are all factors in the calculations - after a few hours I'll leave you with the links as a starting point and move on to an easier question ... Closing 30+ tabs. $\endgroup$ – Rob Jun 16 '18 at 22:00
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    $\begingroup$ Watching the amount of hydrogen drop... 75%... 74%... depresses me. Only 74% to go before it moves on to fusing helium... I only have a few hundred billion years left to live... $\endgroup$ – Alonda Dec 3 '18 at 0:06

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