Cepheid
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A Cepheid is a pulsating variable star. This type of star undergoes a rhythmic pulsation as indicated by its regular pattern of changing brightness as a function of time. The period of pulsation has been demonstrated to be directly related to a Cepheid's intrinsic brightness making observations of these stars one of the most powerful tools for determining distance known to modern day astronomy. The existence of this period-luminosity relationship was a point of contention during the 1920 Curtis-Shapley debate.