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I Chemical Elements produced inside the Sun

  1. Jun 23, 2018 #41
    I was wrong about solar mass stars having convective cores - they're radiative. But this doesn't negate the fact that temperatures in the cores of solar mass stars aren't high enough to produce elements beyond carbon, oxygen, and nitrogen, and the C-N-O cycle doesn't occur in solar mass stars. The s-process and r-process elements are produced exclusively in stars of 8 solar masses and greater.
     
  2. Jun 25, 2018 #42

    stefan r

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    This paper says most of the s-process elements come from stars with mass 1Θ to 3Θ.
    They are asymptotic giant branch stars. The temperatures are far above what is needed for CNO cycle. They have helium burning in explosive flashes. The s-process elements reach the surface in a series of 3rd dredge ups.

    R-coronae Borealis has a mass less than the sun. It lacks hydrogen and there is a class of objects call R-coronae borealis variables most of which also have a hydrogen shortage. The variability comes from puffs of carbon soot blowing out into a planetary nebula.

    Sakurai's object is worth reading about too. It has 0.6 solar mass. It was observed coughing up s-process elements in 1996.

    The Sun has some CNO cycling according to wikipedia:
     
  3. Jun 25, 2018 #43
    The paper states that the s-process nuclei are produced in stars of 1.5-3 solar masses not 1-3 solar masses, but your point is well taken. Also, Ratman's comments in post #40 are entirely relevant!
     
  4. Jun 27, 2018 #44
    Sun is not now hot enough to produce carbon and oxygen out of helium by triple alpha process.
    However, Sun is hot enough to interconvert preexisting carbon, nitrogen and oxygen by CNO cycle.
    Which other preexisting elements can Sun convert?
     
  5. Jul 1, 2018 #45
    It seems, given what was said here so far, that, in a realistic model of the Sun, it may have (or have had at some time) the complete set of chemical elements inside, as it has come from a supernovae explosion. It seems that all these elements, specially the heavier ones, are unlikely to survive for long time.
    I understand that, once all the heavier ones have been broken in small pieces, and due to the Sun's thermodynamic condition, the reposition of the complete set of chemical elements turns out to be impossible. Would this description be a reasonable synthesis of all that we had here?
     
    Last edited: Jul 2, 2018
  6. Jul 2, 2018 #46
    How much fission does actually happen in Sun?
     
  7. Jul 2, 2018 #47

    mfb

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    The sun will keep having some uranium (and its decay products) for hundreds of billions of years, much longer than its lifetime as star.
     
  8. Jul 2, 2018 #48
    100 Gyr is 22 half-lives for U-238.
     
  9. Jul 2, 2018 #49
    When we speak of half-life we mean the atom left alone, don´t we?
    Inside the Sun the mean durations of practically every chemical element tend to be far shorter than these estimates (half-lives), aren´t they?
     
  10. Jul 2, 2018 #50
    Every RADIOACTIVE element...the abundances of these elements are far too small for chain reactions to change their half-lives appreciably!
     
  11. Jul 3, 2018 #51

    mfb

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    It means the number of atoms will drop by a factor of 5.5 million. The number of uranium atoms in the Sun will go down from about 1042.5 to 1036.
    We expect the last atom to decay after about 140 half lives, or 630 billion years, neglecting that the Sun constantly gets new uranium atoms from the outside. Atoms are small, there are many of them.
    The environment in the Sun doesn't change radioactive decays in any relevant way.
     
  12. Jul 3, 2018 #52
    Sun produces a lot of D.
    Sun does not contain much of it, because it readily reacts. A major fate of d is
    d+p→3He+γ
    despite being an electromagnetic process.
    This keeps the abundance of d low, so processes like
    d+d→3He+n
    have a low branching fraction
    p is not the only common nucleus in Sun. But reaction
    d+α→6Li+γ
    is also electromagnetic, and faces a high Coulomb barrier.
    How about reactions like
    d+12C→13C+n?
    It is a strong process, not electromagnetic.
    And the proton does not need to get across the Coulomb barrier...
    Is this a common fate for metals in Sun?
     
  13. Jul 3, 2018 #53

    mfb

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    You mean d+12C→13C+p?
    The Coulomb barrier is still there - the nuclei have to get close together to make the neutron transfer possible.
    Didn't check the energy balance of the reaction. 13C is part of the CNO cycle. This reaction, if possible, mixes the two fusion chains.
     
  14. Jul 3, 2018 #54

    stefan r

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    These two statements look contradictory. My understanding was that uranium 235 fissions when it is hit with a neutron. Uranium 238 can fission with fast neutrons and becomes plutonium 239 (via Np239) when exposed to slow neutrons. Plutonium 239 has a much shorter half life than U238. Pu239 will alpha decay to U235 or fission when exposed to neutrons.

    "Fission reaction" is not the same as "radioactive decay". DaTario actually wrote "mean durations of chemical elements" so reactions should count too.
     
  15. Jul 4, 2018 #55
    Different posters, different beliefs as to facts.
    Sun is big so Sun produces a large number of free neutrons.
    However, these neutrons are a small fractions of all nuclei present in Sun, being so big. Induced fission is a small effect in the Sun.
    The major free neutron producing reactions for s-process are
    13C+α→16O+n
    22Ne+α→25Mg+n
    These reactions have a high Coulomb barrier, and Sun is not hot enough for them.
    The likely reaction is
    d+d→3He+n
    but this is a minor side reaction because of competing reaction
    d+p→3He+γ
    Therefore the free neutron flux in Sun is modest at present.
    How is the radial distribution of that neutron flux?
     
  16. Jul 4, 2018 #56

    mfb

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    Exactly.
    Not that the neutrons would matter - in absolute numbers there are a lot of them, but they get absorbed by other elements quickly.
     
  17. Jul 4, 2018 #57
    Mainly protium.
     
  18. Jul 4, 2018 #58
    There aren't many free neutrons in the solar core - d+d -> He-4 NOT He-3 + n, and the half-life of free neutrons is only fifteen minutes!
     
  19. Jul 5, 2018 #59

    mfb

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    d + d -> He-4 + photon is a very rare reaction as it needs the electromagnetic interaction. d + d -> He-3 + n and d + d -> T + p are much more common (about 50% each).
    Doesn't matter, neutrons in matter are nearly always caught (~microseconds, probably even less in the Sun) before they decay. You have to carefully keep them away from matter to observe their decays.
     
  20. Jul 7, 2018 #60
    New information. The R-process seems to be an exclusive result of a kilonova, the merger of two neutron stars. We could discuss the latest on the maximum mass of a neutron star, but the kilonova, will be less than about 3 solar masses total. A kilonova was recently detected by LIGO then by lots of telescopes all across the electromagnetic spectrum. https://www.vox.com/science-and-hea...igo-gravitational-waves-neutron-star-kilonova The result was almost purely R-process elements, pretty much matching not only the expected abundances, but theoretical calculations. The result is that more than 90% and probably all of the R-process elements come from kilonovas.

    I guess you could say that each of the (two) stars involved were originally high enough mass (but not too high) to go supernova and leave a neutron star. Then the neutron stars have to spiral in for the kilonova--there are some pairs out there which won't merge for tens of billions of years. So kilonovas are pretty rare, and galaxies without one in their history are missing R-process elements. Oh, and you get a short gamma-ray burst. Lots of newly settled science from that one event.

    To relate this to the original post, almost every R-process atom in the sun came from a kilonova.
     
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