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I've been doing some research on RR Lyrae stars and haven't been really able to find an answer to this question.

RR Lyrae are well known for their periodic magnitude, and also are usually found in certain color ranges ("RR Lyrae Color Box"). My question is: does the color of RR Lyrae Stars (G-R in particular) vary over it's period?

I would also love if anyone has a link to a topical journal article!

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2 Answers 2

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The short answer is that yes the color would have to change somewhat. All the stars in the instability strip (RR Lyrae, Cepheids, etc) are variable because they pulsate. Over the course of the pulsation, they vary in brightness, size and surface temperature.

It is the temperature change that is important here. As the temperature goes up the luminosity increases for a given amount of surface area on the star. This plus the changing size account for variations in brightness. However, as the temperature changes, the peak wavelength changes as well (as described by Wein's Law). This means that the star gets redder and bluer over the course of the pulsation cycle.

Now as to the amount of change and how measurable it would be, that I'm not sure of. At a fundamental level there has to be a change but whether it is noticable with broadband (or even narrowband) photometry, is a different question. I know people who might know (as this is what they study) and I'll ask them and see if I can find a reference as well.

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Although I'm a regular variable star observer, I don't observe RR Lyrae variables because they are not suitable targets for visual observers like myself. However, the long period Mira-type variable that I observe the most also change colour with brightness. At their dimmest, they look like glowing red coals, but they fade to white as they get brighter. In fact, I can usually spot the variable star in the field because of its striking red colour. Some good examples are Mu Cephei (Herschel's Garnet Star) and R Leporis (Hind's Crimson Star).

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