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One of the great Sagan quotes is that we are made of star stuff - meaning the atoms in our bodies were formed from stellar nucleosynthesis.

However, what about the hydrogen in a glass of water? Would it be correct to say that all of the hydrogen in that glass was made during the epoch of recombination in the early universe? (I am specifically referring to the hydrogen atoms, not the H2 molecules, or oxygen for that matter)

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Hydrogen wasn't really made at recombination, but in the hadron epoch, starting at one millisecond after Big Bang, and lasting about 1 second. Sure, it wasn't neutral hydrogen, but it was hydrogen nonetheless.

As you mention, neutral hydrogen was made at the epoch of recombination. But since the water doesn't contain free, neutral hydrogen atoms, this epoch isn't of much significance to the glass of water.

Water molecules was of course first formed after oxygen was created which, together with all other elements than hydrogen, helium, and lithium, was made in stars. The first stars came around a few hundred million years after the Big Bang, started enriching the Universe with heavier elements, and rather soon after, water molecules were present (Bialy et al. (2015))

Since you emphasize the word "all", if you want to be pedantic, not every single hydrogen nucleus was made during the hadron epoch, since you also have random fissions of helium nuclei in stars. They're just completely outnumbered by the number of fusions of hydrogen to helium.

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  • $\begingroup$ Thanks for your answer - I hadn't considered the fission process in stars as a source of hydrogen. However, why do you say "this epoch (recombination) isn't of much significance to the glass of water"? Since no hydrogen molecules could exist if it weren't for neutral hydrogen atoms existing in the first place, this epoch must be very significant $\endgroup$ – Amphibio Mar 2 '16 at 19:01
  • $\begingroup$ That's true, but the same can be said for the oxygen atoms, which weren't even present at recombination, and were ionized much, much later when they were created. And probably, the hydrogen that oxygen associates with, is hydrogen that had already been "processed" by stars, i.e. ionized and later recombined. So in that respect, the epoch of recombination doesn't matter. But of course, if that epoch had never been, we wouldn't have stars in the first place, since the gas must be neutral before you can even form a star. $\endgroup$ – pela Mar 2 '16 at 19:08

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