Cepheids are pulsating variable stars, meaning that their periodic changes in brightness are associated with changes in physical size. The wavelengths at which Cepheids emit radiation also vary as the stars pulsate. So what’s happening in the X-ray when it comes to Cepheids?
Classical Cepheids and Modern Observations
X-ray images of the Cepheid δ Cephei and a non-varying nearby star. The variability of δ Cephei is evident from the two images. [NASA]Cepheid variables — specifically, Classical Cepheid variables (hereafter Cepheids) — hold a special place in astronomy. This is largely thanks to their period–luminosity relations, which were discovered by Henrietta Swan Leavitt over a hundred years ago.
In short, a Cepheid’s period is related to its brightness in a well-defined way, and we can use that knowledge to determine how far away a given Cepheid is if we measure its period. This property of Cepheids has made them critical to measuring large distances in space, which are extremely valuable in astronomy. So, it’s important that we understand Cepheid behavior very well.
Cepheid emission at shorter wavelengths (e.g., ultraviolet, X-ray) is strong when Cepheids are at their smallest. However, observations taken over the last few years have shown that Cepheids ...