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Question on stellar absorption spectra

  1. Sep 10, 2005 #1
    Why don't stellar spectra show absorption lines for carbon, oxygen or nitrogen if they are such abundant elements. Is there a 'spectral type' that does?
    Thanks
    Dave
     
  2. jcsd
  3. Sep 10, 2005 #2

    Labguy

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    It is because H is most predominant in any star's photosphere and some other elements can, and some can't, make thier way to the photosphere. It is only from the photosphere that the emissions we can see are produced, and any absorption lines. It is the "Binding Energy" of each elements propensity to "hang on" to electrons that allows, or doesn't allow, that element to be in enough quantity at the photosphere to permit absorption lines (that we can measure).

    At:
    http://www.astronomynotes.com/starprop/s12.htm#msprop there is a decent explanation about half-way down the page:
     
  4. Sep 10, 2005 #3

    Janus

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    Class O stars show lines of doubly and triply ionized oxygen and nitrogen.
    Class N stars show carbon bands
     
  5. Sep 10, 2005 #4
    So basically what you are saying is that C, N, O are not present in the photosphere in sufficient quantities under the right temperature conditions in relation to their respective binding energies to produce significant absorption spectra? However there are a couple of spectral types where you might see them? O stars are the hottest stars, so I am surprised that O and N can be seen. Any references would be terrific.

    I greatly appreciate your responses!
    Dave
     
  6. Sep 11, 2005 #5

    SpaceTiger

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    To add to what has already been said...

    One could just as well ask why hydrogen lines are weaker than calcium lines when hydrogen is a million times more abundant. Unfortunately, there is no simple answer for any given ion or transition, but in general, the strength of an absorption line is determined by:

    1) The abundance of the atom or molecule in question.
    2) The frequency of the ionization state from which the transition can be produced.
    3) The frequency of the energy state of the ionization state from which the transition can be produced (for example, the n=2 state of hydrogen vs. n=1).
    4) The oscillator strength of the transition vs. other transitions from the same energy state (e.g. n=2->1 vs. n=3->1).

    The functions that govern these things are by no means simple, so one shouldn't take the abundances as a sole indicator of line strengths. Items 2 and 3 are determined primarily by the density and temperature in the photosphere and item 4 is determined by atomic physics.
     
  7. Sep 11, 2005 #6

    Astronuc

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    According to http://csep10.phys.utk.edu/astr162/lect/energy/cno-pp.html
    and there is obviously a temperature dependence on the CNO cycle - as the start temperature increases, so does the rate of CNO fusion - as well as age, which would determine composition, besides the initial composition.

    Now, is there a database that describes the proportion of PP / CNO energy generation for say, the 200 brightest stars?

    http://www.palmbeachastro.org/stars.htm (200 brightest stars)

    http://www.seds.org/Maps/Stars_en/ (brightest stars up through M=2.5)

    Presumably, compositions have been determined from optical spectroscopy.
     
  8. Sep 11, 2005 #7

    SpaceTiger

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    At the moment, the sun can't dredge up the products of nuclear burning from its core because it's only convective about a third of the way down, so the initial conditions are pretty much the whole story for abundances in the photosphere.
     
  9. Sep 11, 2005 #8

    Astronuc

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    So the comment about the sun being 98-99% PP and 1-2% CNO is not necessarily valid? In which case, the core may have (or does have) a greater proportion of fusion occuring via CNO?

    I am not up on stellar structure as I would like to be, but how does the convective layer of the sun (G2, or G2V) compare with the convective layer of other stars of its type/size and with other largers stars?
     
  10. Sep 11, 2005 #9

    SpaceTiger

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    No, I'm sure that number is fine, I'm saying that the chemical composition of the core doesn't mix with the photosphere, where these lines are produced.


    Low-mass stars, like M dwarfs, are convective all the way from surface to core, while very high mass stars are convective only in the core. The convection zones of stars of the same spectral type would all be about the same, though there would be some small variations with metallicity.
     
  11. Sep 12, 2005 #10

    Labguy

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    The "brightness" of the stars you link to are all given as "Apparent Magnitude" and would have nothing at all to do with their intrinsic brightness; Absolute Magnitude. It is often the case where a dim star is so near that it appears bright, and vice-versa.
    http://www.astronomynotes.com/starprop/s4.htm

    This paragraph is true, but does not mention "medium-small" stars like our sun and a bit of the main sequence on either side of the sun. These sun-like stars have a radiative core and convective outer layers. So, we have:
    - Low mass stars: Convective cores and mantels.
    - Mid mass stars: Radiative cores and convective mantels.
    - High mass stars: Convective cores, radiative inner mantels, convective outer mantels.
    - All types, of course, have radiative photospheres.

    Anyone care to guess why the massive stars would have convective cores? It is (should be) in all the books.
     
  12. Sep 12, 2005 #11

    SpaceTiger

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    That's because I mentioned it in the previous post. :wink:
     
  13. Sep 12, 2005 #12

    Labguy

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    I didn't see anything about
    except that the sun is convective about 1/3 of the way down.. :confused:

    The high-mass stars are the most interesting in terms of energy transfer; agreed?
     
  14. Sep 12, 2005 #13

    SpaceTiger

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    Astronuc was talking about the chemical abundances in the core of the sun and I was pointing out that it wouldn't matter because the convective layer did not go all the way to the core. Unless I'm misunderstanding something, this is the same thing that you're saying in the "Mid mass Stars", though I've never heard the term "mantle" used in reference to the sun. Is this to be the convective layer?


    Well, I don't disagree, per se, but perhaps you can expand on why you find them so fascinating. :smile:

    My research has been heavy in the ISM, so I find radiative transfer fascinating in general. I'll be happy to respond to the question you posed in the previous post, but it wouldn't really be a guess, so I didn't want to spoil the fun.
     
  15. Sep 12, 2005 #14

    Labguy

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    True, the term "mantel" is used most often to describe outer layers of planets, etc. In my post I should have used the terms core, radiative envelope and convective envelope.

    Fascinating only because the densities of the core are so high that even the most energetic EM radiation (gamma) can't pass freely until a less dense area is encountered, so they have convective cores, radiative inner envelopes and convective outer envelopes. Just a more complicated mechanism to transfer the core's fusion energy to the photosphere. This isn't analogous to radiative transfer in the ISM is it, ie hindered by high densities? Otherwise, the question was rhetorical, as was the previous sentence.
     
  16. Sep 12, 2005 #15

    SpaceTiger

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    There are certainly much higher densities in stars, but then radiative transfer is radiative transfer. Whether or not its analogous depends on exactly which characteristics you wish to highlight and which parts of the ISM you're talking about. I hope this isn't a rhetorical question as well... :confused:
     
  17. Sep 13, 2005 #16

    Chronos

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    It appears we are talking about Jean's mass limits here. In that case, it is not that difficult to quantify. The only difficulty is constraining the parameters, as I understand it.
     
  18. Sep 13, 2005 #17

    Chronos

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    Are we not back to the surface of last scattering thing when it comes to photon emissions from stars? I was under the impression convective layers captured and suppressed photons from escaping from stellar cores for a huge number [perhaps millions] of years.
     
  19. Sep 13, 2005 #18

    Astronuc

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    Labguy, thanks for the link. Yes, I was aware that I was referring to stars by apparent rather than intrinsic magnitude. Those are the easiest for observers on the earth to 'see', and I am sure that there are databases with a much greater quantity of stars. If anyone could point me to such a database, I would appreciate it.

    Here's what NASA has about the sun's structure - http://genesismission.jpl.nasa.gov/science/mod3_SunlightSolarHeat/SolarStructure/

    Then there is a table of the structure of the sun according to http://zebu.uoregon.edu/~js/ast121/lectures/lec22.html

    My old textbook (undergrad), from about 30 years ago, had precious little on the details. Iwould appreciate any recommendation on texts (and papers) about stellar structure.

    Now from what I have read, the core of the sun is He-rich. So much of the rest of the sun is H, with traces of C, N, and O. Clearly the sun's color and spectrum reflect the relatively cool photosphere (~6000 K) and it's predominantly H-rich composition.
     
    Last edited: Sep 13, 2005
  20. Sep 13, 2005 #19

    SpaceTiger

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    If you're not interested in the really nitty-gritty theory, then The Physics of Stars is an excellent text. It's aimed at the undergraduate level. For a more sophisticated treatment, see Stellar Structure and Evolution.
     
  21. Sep 13, 2005 #20

    SpaceTiger

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    Can you expand on the connection you're drawing to the Jean's mass? I'm afraid I'm not seeing your point.


    I've heard a range of numbers, ranging from 50,000 to 10 million years, but it's certainly much less than the time to the surface of last scattering (~10 billion years). Perhaps Labguy knows some more precise numbers from the latest models.

    If you're instead referring to the photosphere as the "surface of last scattering" for solar radiation, then you're right. However, the timescale for the diffusion of atoms from the center much longer than this. Mixing is basically impossible without convective processes.
     
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