Hydrogen Levels in the Universe

Our Sun is significantly enriched, having formed when the Universe was more than 9 billion years old in the plane of a spiral galaxy, one of the most enriched places in the Universe. Yet, when our Sun formed, it was still made out of — by mass — 71% hydrogen, 27% helium, and about 2% “other” stuff. If we convert that into “number of atoms” and treat the Sun as typical of the Universe, that means, over the first 9.3 billion years of the Universe, the fraction of hydrogen has gone down from 92% to 91.1%.


The Universe will go dark, but it won’t be because it ran out of hydrogen. Rather, it will be because the hydrogen that is left is unable to bind together in a large enough molecular cloud to form new stars. It’s only an estimate, but I doubt that — by number of atoms — the amount of hydrogen in the Universe will ever drop below 80%. In other words, we’re going to form plenty of helium and a large number of heavier elements, but at all moment in time, even if we ran the theoretical clock to infinity, the Universe will always be mostly hydrogen. (Which shouldn’t be too surprising; by number-of-atoms, you are mostly hydrogen!)

By mass, we may wind up with less than 50% of the Universe as hydrogen, particularly due to large galaxies and clusters of galaxies. The fact of the matter is, when the Universe is millions of times its present age, we fully expect that new stars will still be forming, but by a very different mechanism by collapsing molecular clouds millions of times the mass of the Sun.


Folksonomies: physics astronomy hydrogen

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 Ask Ethan #34: Using up the Universe’s fuel
Electronic/World Wide Web>Internet Article:  Siegel, Ethan (4/26/2014), Ask Ethan #34: Using up the Universe’s fuel, Retrieved on 2014-08-09
  • Source Material [medium.com]
  • Folksonomies: astronomy