Identifying Cepheid Variables from Spectrum

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Discussion Overview

The discussion revolves around the identification of Cepheid variables through their spectral characteristics, particularly focusing on the interpretation of observed Doppler shifts. Participants explore whether these shifts can be definitively attributed to pulsation rather than other potential causes, such as orbital motion.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Galaxy raises a question about the attribution of Doppler shifts in Cepheid variables to pulsation instead of other causes like orbital motion.
  • Some participants suggest that the absence of a spectrum from a companion star, the measured speed of the expanding atmosphere, and the time dependence of wavelengths during pulsation could help distinguish between pulsation and orbital motion.
  • Galaxy acknowledges the importance of these considerations but questions the ability to identify a Cepheid solely from its spectrum.
  • Another participant argues that the uniform behavior of stars with the same luminosity indicates that variability is an intrinsic property of the star, suggesting pulsation over orbital motion.
  • Models of stellar dynamics, particularly regarding temperature, density, and pressure effects on helium, are mentioned as supporting the pulsation hypothesis, with alternatives being less consistent with observed data.
  • Galaxy expresses difficulty in determining the nature of the moving envelope from the spectrum shift alone, despite acknowledging the points raised by others.

Areas of Agreement / Disagreement

Participants express differing views on the ability to identify Cepheid variables based solely on spectral data. While some suggest that intrinsic properties and stellar dynamics support the pulsation model, others remain uncertain about the clarity of the spectral evidence.

Contextual Notes

The discussion highlights limitations in the ability to definitively attribute spectral shifts to pulsation without additional information, and the dependence on various assumptions regarding stellar behavior and dynamics.

Galaxy111
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Hi all,

Recently I came across 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?

Greetings
Galaxy :smile:
 
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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.
 
Thanks for an answer mgb phys. Yes, in fact all of the three have well to be considered.

Cheers
Galaxy
 
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.

Galaxy
 
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.
 
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.

Galaxy
 

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