2 Added cepheids tag
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How to find a Cepheid's pulsation period using its graph?

Basically, I've been seeing these graphs that say: Cepheid Period-Luminosity Relation and the Period axis is supposed to be "logarithmic";

How does this graph work? Since the luminosity of a star doesn't just increase, as we know, but it increases and decreases periodically, why does the magnitude keep raising? How do I find the pulsating period of such a star through a graph like this? enter image description here