Tuesday, August 2, 2011

Why is oxygen the third most common element in the universe?

So it appears from several sources that the three most common elements in the universe are, in order, hydrogen, helium, and oxygen.

The first two make perfect sense to me. Hydrogen especially -- it's basically just a stray proton, right? Most of them have an electron buddy, of course, but it's not hard to see how that could happen. And then helium is two protons fused together, and as far as I understand it requires the lowest amount of energy to fuse. So that makes sense. You've got a universe full of stray protons (your most common element) and when they start to clump together producing heat and pressure, you fuse pairs of them together and get your second most common element.

But whence oxygen? It's #8 in the periodic table. My simple-minded imagination would have thought the order of frequency of elements would have been roughly the same as their atomic number, allowing some idiosyncracies for what's more stable, etc. And if it weren't, I would have expected the next most common to be maybe a noble gas, or just generally something with a distinct position in the periodic table. I can't see anything special about oxygen...

I'm assuming it has to do with some idiosyncrasy of stellar evolution. Can anybody help me out here?


  1. Off the top of my head, I think it has to do with the processes by which larger atoms are built up from smaller ones. For instance helium has 2 protons and 2 neutrons (2P2N for short), beryllium is 4P4N, oxygen is 8P8N, so one can see that oxygen could be built up from helium via beryllium. Why there isn't more beryllium around is presumably because the relative probabilities of the different steps.

    1. Actually, Be8 is unstable; only Be9 is stable, and fusing it would not produce O16, the most common isotope of oxygen.

  2. Hydrogen gets fused to produce lots of helium-4 (2 protons, 2 neutrons). Fusion of helium nuclei then produces elements with 2n protons and 2n neutrons. Beryllium-8 (4 of each; made from two He nuclei) is unstable so there isn't much of that around. Carbon-12 (6+6; three He nuclei) and oxygen-16 (8+8; four He nuclei) are stable so there are plenty of those. I think the reason why there's (slightly) more oxygen than carbon is that carbon participates in more subsequent fusion reactions producing heavier elements.

  3. Very interesting. I don't know much about astrophysics, so my first step is to check out Wikipedia (hehe). For Big Bang nucleosynthesis:


    So oxygen must come from stars. But oxygen is involved in the "CNO" cycle (there's another wiki article), so it's very interesting why nitrogen is so much lower than carbon and oxygen.

    I can certainly "phone a friend," so to speak, and at least get a suggestion for where to look, without trying to dive right into the peer-reviewed literature, which most people won't have access to, unfortunately.

    1. All elements heavier than He come from stars. Elements heavier than iron come only from supernovae.

  4. ^ I don't even know my own URL. Self-promotion fail. :/

  5. Great answers. I also had stumbled on the CNO cycle via Wikipedia, after writing this, but didn't feel it was a complete answer. I feel like we are getting closer. The 1+1=2, 2+2=4, 4+4=8, and 4(Be) = unstable is making a lot of sense to me...

  6. I phoned one friend last night, and it stumped her. I'll make more calls in the near future. In the meantime, I did a search for the phrase "oxygen abundance universe" in the site ISI Web of Knowledge and found the following links, which might be interesting. At the very least, I believe you can read the abstracts of the articles that are blocked by a pay-wall.

    The origin and abundances of the chemical elements revisited

    Origin of the biologically important elements

    Big bang nucleosynthesis updated with the NACRE compilation

    The primordial abundance of Li-6 and Be-9

    The production of beryllium in the early galaxy

    Nucleosynthesis and evolution of massive metal-free stars

    Chemical composition of the early universe

    I'm likely committing a major faux-pas by not including the full citations for each article, but the links get you to the paper, where the authors, journals, and page numbers should be readily apparent (assuming no error with the links on my part, which I have established as a legitimate concern).

    You might find it useful to go to the ISI Web of Knowledge website and do a search for "oxygen nucleosynthesis abundance" or something that might get a lot of relevant hits.

  7. The CNO cycle is a red herring, it doesn't really produce Oxygen, it jut uses oxygen in a catalytic reaction.

    Once most of the hydrogen in a large enough star's core is consumed, the triple alpha process, alpha processes, and various burning processes take over and produce the heavier elements up to iron and nickel, including oxygen. (Supernova processes create the heavier elements beyond that, and certain suprnovea also create oxygen.)

    I think that since the carbon burning stage of a massive star lasts about 10E3 years and the Oxygen burning stage (combined with all remaining stages)lasts only a few months (and some of the carbon gets eventually get turned into Oxygen), that more oxygen is left unburned compared to carbon when the core collapses.

    See http://en.wikipedia.org/wiki/Stellar_nucleosynthesis

    I believe someone already link to big bang neucleosynthesis to explain why hydrogen and helium are the most abundant elements.

    As an interesting side note/ piece of trivia, most helium on Earth is neither primordial nor stellar in nature; it is the result of alpha decay of heavier radioactive elements (which are themselves stellar products).

    1. I hadn't thought about your last point before, but it certainly makes sense. Many heavy radioactive elements undergo alpha decay, and the alpha particle is simply a helium nucleus. All it then has to do is capture two electrons.

  8. Yeah, I remember reading about terrestrial helium in the book "The Cold Wars" (about superconductivity and low temperature physics) by French authors Matricon, et al.

    I linked to something like half a dozen journal articles that I found while looking through ISI Web of Knowledge, but that comment is missing (perhaps mercifully), whatever the cause may be.

    I do recommend getting access, by hook or crook, to the article "Origin of the Biologically Important Elements" by Virginia Tremble (1996). It is available via SpringerLink.com, though it was originally published in the journal ORIGINS OF LIFE AND EVOLUTION OF BIOSPHERES. (When it comes to journals, anything but of Physical Review is out of my comfort zone. And even then....)

    According to the paper, some lithium was indeed generated after the big bang, but its ratio to hydrogen is something like 10^-10, so big bang nucleosynthesis is pretty much just hydrogen and helium-4. From there, the most likely syntheses are two-body events, but the products Li-5 and Be-8 are highly unstable.

    You can check out wikipedia or the aforementioned review article, which I found pretty readable, and funny, to boot.

  9. That was a great read, thanx.
    Poor Beryllium-8 -- so unstable and it had such large potential. The shake of the dice for this universe stacked the arbitrary laws of physics against her. (ooops, was that sexist? -- re next post)