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Period-Luminosity Relation



The period-luminosity relation for Cepheid-type variable stars—the curve showing how their brightness varies over time - as established by Harlow Shapley in 1918. An astronomer henceforth could observe the period, or time from one maximum brightness to the next maximum, for any other Cepheid variable star, and then read off the graph the star's absolute magnitude. Comparison of this estimated absolute ("real") magnitude with the observed apparent magnitude yields the distance, since brightness diminishes with the square of the distance.

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