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I'm confused: Big Bang nucleosynthesis is adamant about the 1 neutron to 7 proton ratio which yields 75% hydrogen to 25% helium (with a nominal amount of partially-reacted deuterium and heavier lithium). But everything I read about the interstellar medium gives a helium figure closer to 10%, so what happened to the missing 60% of the helium in the Universe?

Of the gas in the ISM, 89% of atoms are hydrogen and 9% are helium, with 2% of atoms being elements heavier than hydrogen or helium, which are called "metals" in astronomical parlance.

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I think this may be a simple misunderstanding.

There is a 3:1 mass ratio of Hydrogen to Helium.

This ammounts to only 8% of the atoms being Helium atoms.

See wikipedia on the Big Bang Nucleosynthesis. The last sentence of the first section addresses your confusion exactly.

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Both figures are correct because they refer to slightly different things. The baryonic matter in the universe is about 25% helium by mass, but only about 10% helium by number.

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The current thinking is that higher-mass stars have managed to "eat" all the Helium since their temperatures are hotter and they are capable of burning it (along with Hydrogen) in nucleosynthesis. So any Helium produced by lower-mass stars that escapes into the ISM eventually gets captured by heavier stars and is destroyed.

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While high-mass stars do fuse helium to form heavy elements, helium is not preferentially captured from the interstellar medium. The chemical composition of the ISM is quite close to its chemical composition an hour after the big bang. – rob May 27 '14 at 5:37

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