Two stars have the same luminosity,

PHYSICS · Multi-part · Fermi University Physics · Astrophysics & Cosmology · Stellar structure, energy generation and evolution · Stellar structure, energy generation and evolution

Stars from the Inside, Ch Pulsating stars: the kappa mechanism, the Leavitt law and asteroseismology

Problem

L = 2000\,L_\odot

, placing them at the same height on the HR diagram, but different effective temperatures: star P has

T_{\rm eff} = 6300\,\mathrm{K}

and star Q has

T_{\rm eff} = 9000\,\mathrm{K}

. The classical instability strip at this luminosity runs from about

6000\,\mathrm{K}

(red edge) to about

7500\,\mathrm{K}

(blue edge). ((a)) Predict which star, if either, pulsates as a classical Cepheid. ((b)) Explain, in terms of the depth of the He II ionisation zone and the thermal timescale of the layers above it, why the other star does not.

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