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Cepheid spectrum

  1. Sep 16, 2008 #1
    Hi all,

    Recently I came accross an interesting question:

    "...Why do astronomers attribute the observed Doppler shifts of a Cepheid variable as pulsation, rather than to some other causes, such as orbital motion? "

    OK, I have my ideas about the problem as there is a correlation between e. g. the max. blueshift (spectrum),the max surface temp. (from the Wien-displacement law I guess) and the apparent magnitude (observation) of the star.
    But still, is there a way to tell it is a cepheid from its pure spectrum without knowing more?

    Galaxy :smile:
  2. jcsd
  3. Sep 16, 2008 #2


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    Just a guess, but possibly.
    1, No spectrum from companion
    2, Speed, does the rate measured for the expanding atmosphere give a reasonable orbital radius.
    3, Wavelength time dependance, in the expanding atmosphere different wavelenghts might be emitted at different parts of the cycle as different temperatures/pressures are seen, if it was orbital they would all be moving at the same rate.
  4. Sep 16, 2008 #3
    Thanks for an answer mgb phys. Yes, in fact all of the three have well to be considered.

  5. Sep 16, 2008 #4


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  6. Sep 17, 2008 #5
    Yes - thank you Chronos. I know these calculations. Only, it doesn't really explain the tricky question. Why can we tell from the pure Doppler-shift of the Cepheid spectrum that the envelope of the star is moving and the red/blue-shift doesn't have some other cause? Interesting though. I think mgb phys has given some good starting points here.

  7. Sep 17, 2008 #6

    Vanadium 50

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    There are two IMO very good reasons to explain this as pulsation over the proposed alternatives of orbital motions:

    (1) The fact that all stars of a given luminosity behave the same way suggests that the variability is an intrinsic property of the star. If it was orbital, why are the orbits identical?

    (2) Models of stellar dynamics - particularly the effect of temperature, density and pressure on helium - give pulsation and match the observed data. Alternatives do not.
  8. Sep 19, 2008 #7
    Good point Vanadium; especially your no. 1 seems to lead to this suggestion. My initial thinking was also stellar dynamics, as it will convince as well.
    But still, from the pure pure spectrum shift it seems to be quite difficult, if anyhow possible, to tell its the moving envelope.
    Thanks again to you all.

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