Why does the luminosity of Cepheid variable stars vary, and why does the timescale of the variation tell us their intrinsic luminosity?
Some of you may wonder why some stars vary in luminosity while others do not. Here is an analogy. When you ring a bell, you need to have a bell in the first place and a hammer to make the bell vibrate and produce sound which can be received by your ear. The exact same thing is going on for the variable stars. The variable star is like a bell: it can oscillate. The structure of the star can be slightly perturbed: this is the hammer; and what we `hear’ is the changing of the luminosity. Here is a bit more technical explanation. An ordinary (non-variable) star is in hydrostatic and thermal equilibrium; when perturbed in some way so as to induce mass motions, it will gradually return to its original state. The reason for this is that a parcel of mass that has been compressed, for example, is hotter than its surroundings and thus (normally) radiates heat to its surroundings. Thus the work that went into compressing the parcel is effectively dissipated and the parcel loses some of the “bounc
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