Stellar Oscillations

Discussion in 'Astronomy, Exobiology, & Cosmology' started by exchemist, Mar 16, 2017.

  1. exchemist Valued Senior Member

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    Prompted by Nebel's crank postings on this subject

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    , I had a look at the following Wiki article: https://en.wikipedia.org/wiki/Asteroseismology

    It's actually rather interesting for someone like me who has studied a bit of atomic quantum theory. Stellar oscillations are very much like electron orbitals, it seems, involving spherical harmonics - well, I suppose they, wouldn't they?

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    . One interesting thing hinted at but not described is what happens in the presence of spin. The situation in non-rotating stars is explained, and the hint is that when there is rotation the presence of a preferred axis somehow alters the oscillation pattern. I am tempted to see this as some sort of analogy with space quantisation in atoms, but I am not sure what mechanism would resolve the "degeneracy", as it were, of the spherical harmonics.

    The source of the excitation of the oscillations is also interesting, but quite a complicated story: there seem to be four different ones, depending on the type of star in question.
     
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  3. Farsight Valued Senior Member

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    I'm afraid stellar oscillations have got nothing to do with electron spin or electron orbitals.
     
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  5. origin Trump is the best argument against a democracy. Valued Senior Member

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    He didn't say that they did.
     
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  7. origin Trump is the best argument against a democracy. Valued Senior Member

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    This is an interesting article that discussed the possiblity that gravitational effects from the orbiting planets may be responsible for the oscillations. This was originally posted by nebel, but he somehow drew a completely bogus conclusion from the article.

    Langton, Adam P. Showman. Planet-induced Stellar Pulsations in HAT-P-2's Eccentric System. The Astrophysical Journal, 2017; 836 (2): L17 DOI: 10.3847/2041-8213/836/2/L17
     
  8. exchemist Valued Senior Member

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    I welcome Farsight back from his recently imposed ban for miseducation in physics.

    The parallels that struck me were first the applicability of spherical harmonics and second the apparent splitting of frequencies due to the presence of a preferred angular momentum axis in space. I realise these parallels cannot be pushed too far, but I was very struck by the familiarity of some of the terms in this paper: http://www.nature.com/articles/s41550-017-0064

    In particular I might quote this passage: " Each oscillation mode is identified by an angular degree l, which gives rise to a multiplet of (2l + 1) different components through the degeneracy lifted by the stellar rotation6. Each rotationally split component is in turn identified by an azimuthal number, m ≤ |l|. The dipolar (l = 1) mixed modes are the most suited for measuring the orientation of the spin axis in red giants13."

    We seem to have it all there, down to azimuthal quantum numbers, 2l+1 multiplets and lifting of degeneracy! In a way it is not surprising, since I guess it all comes from the maths of spherical harmonics, but it is always intellectually satisfying to see parallels, across apparently widely disparate fields.

    Cool stuff.
     
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  9. exchemist Valued Senior Member

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    Ah yes I see, these, if I read correctly, are tidal effects, exerted by a very eccentric planetary orbit on the star. That would be a fifth potential source of excitation of stellar oscillations, in addition to the more general ones listed in the Wiki article. Thanks for that.
     
  10. The God Valued Senior Member

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    Why parallel? Just because of orbital motion or something deeper?
     
  11. exchemist Valued Senior Member

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    Both are resonances, i.e. standing wave patterns, for a spherical system, that's the link.

    But the orbital motion thing I have not really thought through. In an atom, the l (azimuthal) quantum number is associated with the amount of orbital angular momentum possessed by an electron in that orbital, but that would not be relevant here. However what IS - seemingly - relevant is that the various spatial orientations available for a given l value (in QM designated by the quantum number m(l) ), apparently have different energies in the presence of rotation of the star, rather as atomic orbitals with the same l but different m(l) do in the presence of space quantisation by an external field. I'm not sure why this would be - possibly to do with centrifugal effects operating in the x-y plane but not along the z axis, or something.

    Maybe if Janus pitches up he will be able to help me.
     
    Last edited: Mar 16, 2017
  12. nebel Registered Senior Member

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    nebel is totally out of the league in this discussion, but in his simple, bode-type thinking there is a correlation between the sun's powerful dynamics and the orbital spacings of planetary orbits, if you consider them standing wave patterns as mentioned above.
     
  13. exchemist Valued Senior Member

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    Yes, I think we know that by now. But it is off-topic for this thread.

    Someone who is interested in science and not, let us say, a one-track obsessive, might learn from the thread a bit about how oscillation in stars works. That was my purpose is starting it, as it is not a subject I knew anything about, until I was prompted by you to look it up. So thanks for that.

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    As I have observed before, the cranks on this forum do actually serve a beneficial function, in that they often prompt the non-cranks to dust off, or learn, some of the relevant science.
     

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