<p>I understand the theory of the mechanism by which Cepheid variables pulsate, where helium is ionised and deionised, leading to changes in diameter.</p>
<p>However, I am unsure if the brightness changes due to the changing opacity of helium, or is it explained by the changing diameter of the star, or perhaps a combination of both?</p>
<p>We learn through Stefan's Law that luminosity is directly proportional to the surface area. Is the changing surface area causing the changing luminosity, and this change in surface area is driven by the changes in helium ionisation? Or is the change in luminosity solely brought about by helium?</p>
<p>Thanks.</p>

CC BY-SA 4.0
2021-03-09T23:03:20.707Z
Luminosity is related by $L=4\pi r^2 T^4 \sigma$. As diameter-radius increases, luminosity increases. Ionization may take part, but does not contribute much to the change in luminosity.
WarpPrime
CC BY-SA 4.0
2021-03-10T05:27:54.453Z
@fasterthanlight but answers have to address "temperature and radius of what exactly?" when a star oscillates like this the wavelength we observe visually and the wavelength of maximum radiative cooling may differ, perhaps leading to different effective photosphere radii and apparent temperatures for the two.
uhoh
<variable-star><cepheids>
Why do the pulsations of a Cepheid variable occur?
Matthew H
# Why do the pulsations of a Cepheid variable occur?

(Asked by: username_0 on 2021-03-09)

I understand the theory of the mechanism by which Cepheid variables pulsate, where helium is ionised and deionised, leading to changes in diameter.

However, I am unsure if the brightness changes due to the changing opacity of helium, or is it explained by the changing diameter of the star, or perhaps a combination of both?

We learn through Stefan's Law that luminosity is directly proportional to the surface area. Is the changing surface area causing the changing luminosity, and this change in surface area is driven by the changes in helium ionisation? Or is the change in luminosity solely brought about by helium?

Thanks.

**Tags:** variable-star, cepheids
> **Comments:**
> **username_1**: Luminosity is related by $L=4\pi r^2 T^4 \sigma$. As diameter-radius increases, luminosity increases. Ionization may take part, but does not contribute much to the change in luminosity.  
> **username_2**: @fasterthanlight but answers have to address "temperature and radius of what exactly?" when a star oscillates like this the wavelength we observe visually and the wavelength of maximum radiative cooling may differ, perhaps leading to different effective photosphere radii and apparent temperatures for the two.  

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## 0 answers

