Who first noticed that for Cepheid variable stars, there was a direct relation between the luminosity and the period of the variation in their brightness?

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Henrietta Leavitt first noticed the direct relation between the luminosity and the period of variation for Cepheid variable stars while working at the Harvard Observatory in the early 1900s. This discovery allowed astronomers to use these stars as "standard candles" to measure distances in the universe. Henrietta Leavitt, a Harvard College Observatory astronomer, first noticed in 1912 that the Cepheid variable stars in the Small Magellanic Cloud followed a direct relation between the luminosity and the period of their variation in brightness. This observation was critical in developing a method to measure the distances to faraway galaxies.

Who First Noticed That for Cepheid Variable Stars, There Was