Cepheid variables and Great Student Questions

Miss, why do they pulsate?

*thinks* I know I knew this once, but I need some time to refresh my memory. I’ll answer this question next lesson.

I love a good student question. Firstly, it shows they are listening (ha! Yey). Secondly, sometimes I genuinely need to do a bit of research to be able to answer it. Great! A question that requires me to learn (or relearn) something. So why does a cepheid variable star pulsate?

First step: search my own memory for the answer. Outcome: in a vague fog of a physics with astrophysics degree I can remember knowing a lot of complicated things. Usually, I only need a small thing to force huge chunks of memory into the clear. Not this time. I have vague notion that it’s to do with a stars insides. Very helpful.

Second step: Google it. Really, that’s what everyone does next. Even reading Wikipedia. Shhh. Outcome: a skim of the Wikipedia page tells me I’m right, it’s quite complicated. There’s no easy, one sentence explanation.

Third step: look at my books at home. I’ve got physics books covering most major topics. They usually help. Outcome: I’m nearly there. This book, clearly from my uni days, has the most info.


I’ve read up on the Hertzsprung Russell diagram and on the strip of instability that cepheids reside on. The description of using cepheids as distance markers has just clarified what I’ve already been teaching. The description of how it varies and linking it to some other bits of physics we don’t need the depth of at gcse is interesting. Getting down to the reason for the variation in size of the star, well, it points to another study and essentially says ‘it’s complicated’. Two other books on cosmology just explain how cepheids are helpful as distance markers and don’t even try to explain why they pulsate. A pocket astronomy (you never know!) just tells me how to locate some. the Feynman lectures don’t mention them at all.

Fourth step: email an astronomer friend for an explanation. Outcome: not had a reply yet 🙂 Other steps: ask Twitter (of course!). Try and just remember myself and fish those buried memories out of my brain. Outcome: I need to buy more books. For now, I’m going back the Peacock book to piece together a short response.

The physical basis for their variability is relatively simple. All stars gave a natural oscillation period, deriving from the crossing tine for sound waves… The fundamental period should be roughly equal to the pressure-free collapse time. This does not of course explain why some stars become unstable to radial oscillations with this period whereas others (such as the Sun) do not. What is needed is in fact a stellar structure where the opacity is sensitive to small changes in density. This arises in Cepheids because of the existence of a transition zone between singly and doubly ionized helium, which is sensitive to the small changes in temperature associated with small radial perturbations. Why this should arise in evolved stars but not main-sequence stars require detailed modelling… However, this doesn’t prevent us using the fact of variability if and where it occurs.

I’d love to get more questions like this from my thinking, curious students. I might need to make a series of these blog posts as I had another of these types of questions on the same day ‘how does a sound wave reflect off the open end of a pipe?’ Good question…

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One Response to Cepheid variables and Great Student Questions

  1. audsley says:

    My dad sent me this: When the star is at its hottest the Helium atom looses its two electrons which causes the double ionised Helium to become excited and the star to expands. The star becomes dimmer because the double ionised Helium becomes more opaque – less bright. After the star expands it cools and picks up electrons from the star’s magna and then contracts. The cycle then starts over again. Hope this helps.

    He has not previously revealed astrophysics depths!


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