Date context
1868–1921
Research in photographic plates
Leavitt began working at Harvard College Observatory in 1893, initially as a volunteer and later in a paid position. Comparing plates taken on different nights, she identified variable stars and contributed to the measurement and standardization of stellar magnitudes.
References: [1]
Choosing comparable stars
She studied Cepheid variables in the Small Magellanic Cloud. Because those stars lay at approximately the same distance from Earth, differences in their observed brightness could be compared as differences in intrinsic luminosity, reducing the complication introduced by unequal distances.
Period and luminosity
Her 1912 study of 25 variables showed that longer-period Cepheids were generally more luminous. The relationship allowed observers to infer intrinsic brightness from a measured period. Converting it into an actual distance required an independent calibration of the luminosity scale.
From a relation to a distance tool
After calibration, the relation could be combined with apparent brightness to estimate distance. Hubble later used Cepheids in investigations of other galaxies. Leavitt established the observational relationship; its calibration and subsequent galactic applications involved further work by several astronomers.