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Today we celebrate the birthday of American astronomer Henrietta Swan Leavitt (born July 4, 1868, Lancaster, MA, died Dec 12, 1921, Cambridge, MA), who discovered the relationship between the luminosity and the period of Cepheid variables, pulsating stars that vary in brightness in precise periods ranging from a few days to several months.
The period–luminosity relationship, now known as "Leavitt's law" led to methods to calculate vast celestial distances.
https://en.wikipedia.org/wiki/Henrietta_Swan_Leavitt
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