Who is Henrietta Swan Leavitt(1868-1921)?
She was an American astronomer who attended Oberlin College for two years, later transferred to the Society for the Collegiate Instruction of Women, from which she graduated in 1892. She became a volunteer assistant in the Harvard Observatory, and was employed in the observatory's great project of determining the brightnesses of all measurable stars.
In this work she was associated with two other female astronomers: Williamina Fleming and Annie Jump Cannon.

Leavitt soon managed to advance to the position of head of the photographic stellar photometry department. In 1907 a new phase of the work began with the plan to precise photographically standardised values for stellar magnitudes.

Leavitt began with a sequence of 46 stars in the vicinity of the north celestial pole, determining their magnitudes and then those of a much larger sample in the same region.

Leavitt's most important achievement was her discovery in 1912 that in a certain class of variable stars, the Cepheid variables, the period of the cycle of change in brightness is regular and is determined by the actual luminosity of the star.
Her work allowed American astronomers such as Hubble and Shapley to determine the distances of many Cepheid stars and as a consequence the distance of the star clusters and galaxies in which they were observed.