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Stargazing Why is Oxygen so abundant?

  1. May 16, 2018 #1
    Basically H is most abundant obviously....
    He is next, also obvious.....
    Why is oxygen the next most abundant element?
    and by the way.... I am a big boy I can handle answers of the sort: "Because that is just how it is...." but any more scientific insight is more than welcome!

    Thanks,
    Morlaf
     
    Last edited by a moderator: May 20, 2018 at 11:45 AM
  2. jcsd
  3. May 16, 2018 #2

    Bandersnatch

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    Oxygen is just what stars produce the most of, after helium.
    There's a large number of different fusion reactions that progressively burn heavier and heavier elements throughout the lifetime of a star. Different reactions, with different products, have different sensitivity to temperature and density, and are subject to various resonances making some pathways occur faster than others.
    This gives rise to higher abundance of some elements over others.

    I don't know how deep you want to go into this. Maybe have a look at this article::
    https://pdfs.semanticscholar.org/a3c7/d2929ed0e42b9ec9caab51e3986c38853dcd.pdf
    Starting on page 3, it describes in quite maths-less terms the whole fusion process vis-a-vis oxygen production.
    There's also some graphs showing how the abundances change, if you can read them.
     
  4. May 16, 2018 #3

    what a terrific article..... thank you so much!!!
    now I can go enlighten my 6-year-old!
    clever dads knows it all!
     
  5. May 16, 2018 #4

    mfb

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    Towards the end of their life stars fuse helium to carbon and then carbon plus helium to oxygen. Only heavier stars can continue fusion reactions - oxygen and helium to neon, oxygen and oxygen to silicon or phosphorus and a couple of other reactions. In this case the atoms are spread over a large number of elements, and they are produced by heavy stars only, so they are not very common.

    He+He->Be doesn't work as Beryllium-8 decays back to two helium nuclei extremely fast.
     
  6. May 16, 2018 #5
    this is what I am trying to get to. Why is Be so unstable? is it merely because it just is..... a bit like gravity just is?
     
  7. May 16, 2018 #6

    Bandersnatch

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    According to the article linked above, the oxygen abundance in our galaxy is mainly due to those same massive stars.
     
  8. May 16, 2018 #7
    That´s actually easy.
    2 is a magical number.
    4 is not itself a magical number. And not far enough from 2. And easily 2+2.
     
  9. May 16, 2018 #8
    so as 4 is not so "magical" we go to the next one which is 4+4=8 which seems to be at least semi-magical?
     
  10. May 16, 2018 #9
    Simple though not complete and not quite correct explanation.
    The next one is 4+2=6 which is not yet magical but further from 2.
    But the next is 6+2=8, which is magical.
    And 8+2=10 is not quite magical again.
    Though the actual explanation is more complex - for some reason, Ne-20 is wrong spin and parity resonance.

    Simple rule is that 2 is the most magical and easiest number for simple quantum mechanical number. A pair of opposite spin electrons, neutrons and protons. Very hard to take anything away, and very hard to add anything. He is a noble gas in chemistry, and He-4 nucleus is very stable to nuclear reactions.

    In case of electrons, the next magic number/noble gas is 10, so neon. The strong force field has different subshells compared to electric field, so the next magic number for nucleons is 8 rather than 10.
    Try adding an extra electron to He, and you will find that Li is chemically very active and easily gives up the surplus, misfit electron to become a He-like cation. Add more, and the extra electrons make a bit better fit - boron or carbon are not chemically as inert as neon, but neither are they as active and eager to give up excess electrons as lithium is.

    Try adding an extra neutron or a proton to an alpha particle, and it will not fit at all - there is no bound nucleus of 5 nucleon, the extra nucleon will just bounce off. Try adding a bit more, and the extra nucleons will make a slightly better fit to each other - beginning with 6, there are bound nuclei with every mass number, but 8 has no stable ones.

    So, try fusing He and Be will be completely skipped. Too unstable. C is more stable, but not magic, which is why most He fusion will pass through C as well and stop at O... but not go forward to Ne.
     
  11. May 16, 2018 #10
    Yes, exactly: most of the oxygen you're breathing was made by stars that exploded.

    See the feature article on oxygen-deficient galaxies in the April 2018 Sky & Telescope and note the second paragraph: "A galaxy without oxygen is like a forest without fallen leaves. Massive stars create lots of oxygen during their bright but brief lives, then hurl the element into space when they explode."
     
  12. May 16, 2018 #11

    phinds

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    Actually half or more of the oxygen you are breathing is made via photosynthesis by phyloplankton in the sea. The rest is produced by photosynthesis by plants on land. Oxygen is highly reactive and were there no regeneration of it our atmosphere would become depleted of it.
     
  13. May 16, 2018 #12

    Bandersnatch

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    Surely, you mean all of it.
    Anyway, the comment you were quoting was specifically about from which stars does it come. Apparently, and somewhat counter-intuitively, it's the massive stars, in which fusion progresses past oxygen, that are the main source of the most abundant isotopes.

    That's rearrangement, not production.
     
  14. May 16, 2018 #13

    phinds

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    Yeah, I understand that, but nonetheless, it's production of a type and were it not done there would be little to no free oxygen on Earth.
     
  15. May 16, 2018 #14
    How much, if any, is made by stars that shed a planetary nebula, but leave a white dwarf that does not explode?
     
  16. May 16, 2018 #15

    Bandersnatch

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    Thanks. My bad. I completely overlooked the 'that exploded' part, and thought it was a stellar vs primordial issue.
     
  17. May 18, 2018 at 4:32 AM #16
    thanks guys....... some really in-depth info and some more palatable explanations. lovely mixture!
     
  18. May 19, 2018 at 9:52 AM #17

    JMz

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    Likely very little: If oxygen gets into the planetary nebula, then it wasn't in the core. So the core contained something even heavier than oxygen. So the star is quite massive. So it almost certainly exploded instead. But I don't have the numbers.

    Having said that, it's true that some pre-supernova stars eject their atmospheres (Wolf-Rayet types, eta Carinae). Whether they dredge up oxygen to mix into their atmospheres beforehand, I do not know.
     
  19. May 19, 2018 at 10:18 AM #18

    JMz

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    Not that fast, because this overlooks what does happen: Be-8 is unstable by only 95 kEv, a pittance in nuclear-energy terms. It definitely shows the effect of "magic", even though it just barely doesn't qualify.

    The 2 He-4's therefore "stick together" in a resonance, in the would-be ground state, for much longer than they otherwise would, much longer than merely "bouncing off". And that gives time for a 3rd He-4 to hit them and make -- ta-da! -- C-12. This is known as the triple-alpha process, for obvious reasons, but it's not an ordinary 3-way collision. That would be virtually impossible under normal stellar-core conditions. It's extremely strongly mediated by that resonance and the extra time it affords.

    Needing 3 particles still means, however, that the density has to be much higher than the would be for a two-way, or for two consecutive two-way reactions. And that means that the reaction has a somewhat different distribution within the stars that do it, much more strongly focused into the highest-density parts of the core. That has consequences to the structure of post-main-sequence stars, to the structure and even existence of red giants, and to what their planetary nebula and supernovae can dump into the interstellar medium.

    A historical note: This was Edwin Salpeter's contribution. It was a resolution to a severe paradox of the time: There was no known way to make elements beyond He-4 in quantities that could possibly escape enough stars to fill the universe with heavier atoms in the observed quantities. (FWIW, I studied under him. An excellent teacher as well as a first-rate scientist.)
     
  20. May 20, 2018 at 5:51 AM #19
    It does not really show effects of "magic" at 8. It DOES show the effects of magic at 4.
    Compare Be-8 with B-8.
    Which is immensely longer lived - yet releases huge amounts of energy on decay to the unstable Be-8.
     
  21. May 20, 2018 at 9:46 AM #20

    JMz

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    I don't know much about boron. I was commenting on the situation in stellar cores that have exhausted their hydrogen and must either make do with He in some way or die.
     
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