# How long will the Universe's hydrogen reserves last for?

I recently became really interested in learning about physics and cosmology, but I still know very little. Hopefully someone with more knowledge will be able to shed some light on my questions.

Here are my presumptions (please correct me if I'm mistaken):

• $t_{0}$: Big Bang.
• The Universe expands (and thus cools) sufficiently to allow the formation of atoms.
• The early Universe mostly consists of Hydrogen and is not uniform in its geometry.
• Gravity pushes lumps of matter together until their density (temperature) is so high that nuclear fusion kicks off. The first giant stars are born.
• Some heavy elements are created in these stars and when they explode, sufficient temperatures are reached to form heavier elements still.
• All of the successive stars and planets are the product of these original stars.

My questions:

• If the amount of Hydrogen is finite in the Universe and each successive generation of stars use up most of their Hydrogen, what is the theoretical maximum number of generations of stars that our Universe can support? A ballpark figure in terms of years is fine too. ;)

• Are there some special physical processes that occur in the Universe on a large enough scale that are able to break up heavy elements (i.e., Helium) into lighter elements (i.e., Hydrogen)? Kind of like a cosmic recycling operation.

• If we discount the possibility of a Big Crunch, is the Universe pretty much headed into a time when there will be no more stars and only a bunch of black holes, planets, and other debris?

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You might want to make the title more specific. For example, "How many generations of stars will exist before the universe's nuclear fuel is depleted?" – Mark Eichenlaub Dec 29 '10 at 23:11
These questions are pretty independent so you might consider asking them separately. Although the answers to 2. and 3. are AFAIK "no" and "yes" respectively (barring Big Crunch, Brane-worlds and other speculative scenarios) and the answer for 1. will be easy multiplication if you could find the number of stars, average life-time of a star and few other numbers :-) – Marek Dec 29 '10 at 23:29
See arxiv.org/abs/astro-ph/9701131 and the very accessible papers by Dyson referenced therein (which are what I was looking for as I have read those...). It's not clear to be that the preprint discusses the fate of intelligent life, which the 1979 Dyson paper Time Without End does. Also that reference is slightly dated now. – dmckee Dec 29 '10 at 23:32
Here's a link to Dyson's paper in the Review of Modern Physics which you can probably download at your local university. Or the text is available in various other places on-line. Ah... a link which doesn't require a subscription to an expensive jaurnal. – dmckee Dec 29 '10 at 23:40
@Marek: Thanks. – plslick Dec 29 '10 at 23:48

Tis, a good question. Two related questions arise from it. The first one is, will the hydrogen all be used up in finite time? The second related one is, will star formation completely stop in finite time? They sound related, but the first result doesn't necessarily imply the same result for the second, or vice versa. I.E. a low but nonzero gas density might possibly not allow further star formation, and maybe we could have no hydrogen, but have other types of gas (or even solid objects) still collect into stellar mass objects.

I don't know for sure the answers. The rate of star formation (and hydrogen consumption) could decline slowly enough as to never formally reach zero. Or not.

We do know that a lot of gas gets blown out of galaxies by massive stars, supernova, and black hole activity, and becomes intergalactic gas -usually staying within the galaxy cluster. On a long time scale this should eventually fall back into the cluster's galaxies. So I would think the star formation rate would have a very long tail.

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Any ideas as to how much longer stars will be able to be formed? I'm trying to understand the scale of our ~14 billion years in comparison to the life expectancy of the Universe. Are we zygotes, infants, children, teenagers, ..., elderly? – plslick Dec 30 '10 at 4:13
@plslick: In an open universe we would be very young indeed. – dmckee Dec 30 '10 at 4:48
plslick: I've heard the claim that 90% of the stars that will ever be have already been formed. So it one sense we are near retirement age (like me). In another sense, I think, since I don't really study these things, that the long tail of slow rate star formation ought to be several times longer than the current age of the universe. – Omega Centauri Dec 30 '10 at 20:41

The vast majority of hydrogen in the universe is in hot gas in galaxy clusters, or cold, extremely diffuse atomic hydrogen in the intergalactic medium (Lyman alpha absorbers). Neither population is likely to ever form stars, so I think the safe answer is "forever."

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Isn't this intergalactic gas being prevented from falling into the clusters galaxies because it is millions of degrees -this gas is seen as diffuse X-ray glow. In any case to maintain that high temp requires an energy source. As stars die out, this gas should cool, and rain down into the local gravity wells, the galaxies mainly. So much of this gas should be available for late stage star formation in the by then very ancient galaxies. I don't know what fraction will do this, some must be lost to the voids between clusters, but it should keep star formation going for a long time. – Omega Centauri Dec 31 '10 at 1:41

This is really a comment but is too long.

A simpler, but related question is "How much of the primordial hydrogen has been used up so far?" or "Can we measure a difference between the percentage of primordial hydrogen (~75% H-1) or the percentage of primordial Helium (~25% He-4) and the percentages today. Short answer - no.

Well, how about a gradient of helium-4 from the galactic center to the outer arms. I think the answer is yes but it's hard for me to tell. Well, can we see a metallicity gradient (all elements heavier than He)? Yes, we can. See http://nedwww.ipac.caltech.edu/level5/Sept04/Henry/Henry2_1.html, fig 1.

Conclusion, you have asked an experimental question in astronomy, and I would like to see astronomical evidence in any answer given.

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