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The Question

(Submitted November 13, 1997)

I know that hydrogen is the most common element in the Universe. Can I know from you the known percentage of hydrogen in the Universe? And where it is precisely?

The Answer

When the Universe was formed in the Big Bang, the resulting elemental matter was about three quarters hydrogen, one quarter helium, and a few parts-per-billion of lithium (by weight).

(When I say 'elemental matter', I am referring to matter made of the common chemical elements we see around us. However, one of the great mysteries of astrophysics is, we don't know what most of the Universe is made of. Between 90% and 99% of the mass of the Universe seems to be completely unknown. This 'dark matter' has so far been detected only gravitationally: galaxies and clusters of galaxies seem to be heavier (or at least have more gravitational attraction) than can be explained by summing up all the stars and gas clouds we see.

Some of this matter coalesced into galaxies and stars, although much remained as gas. As time went on, stars burned some of their hydrogen to heavier elements. These stars occasionally released their material (as winds, in supernova explosions, and in other events) so that it combined with the remaining gas, and formed new stars, and so on. All material on Earth except for the hydrogen (such as that in water), helium, and lithium is this burned material. Our Sun, and most of the stars you can see in the night sky, are later-generation stars, and we can see the material burned by earlier stars in their atmospheres.

Only a few percent of the original hydrogen and helium in the Universe has been burned this way. Most of it is still around, and so the elemental matter of the Universe is still about three quarters hydrogen, which is primarily in the form of clouds of gas and stars.

David Palmer
for Ask an Astrophysicist

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