On July 4, 1868, a day as today 150 years ago, Henrietta Swan Leavitt was born in the city of Lancaster (Massachusetts, United States). After graduating from Oberlin University and Radcliffe University, the young woman joined the Observatory of Harvard College . He was part of a group of baptized women Harem Pickering – a dubious good name in honor of Edward Pickering director of the observatory .

His boss decided that all were women to pay them a lower salary – 1945 twenty-five cents per hour – for tedious work and that in most cases were not recognized in scientific publications. Henrietta Leavitt with his companions, was counting stars photographic plates obtained by researchers from the observatories of Harvard and Arequipa (Peru)

was soon entrusted with the nickname of astronomer calculator . Leavitt not only had to write numbers and scrupulously look at the plates, but he also recorded their size – in keeping with the brightness of the star – and compared it with the data obtained in the past. The American discovered variable stars in the Magellanic Clouds two galaxies near the Milky Way. Four years later, Henrietta Leavitt publishes her discovery in the journal Annals of the Harvard Observatory where she also calculates the pulsation periods of the stars.

"It is remarkable that In this painting, the brightest stars have the longest periods "said the researcher in this work. An insightful observation that corroborates the existence of the Cepheids a type of pulsating stars whose brightness varies periodically over time. Some time later, it was Edward Pickering – and not Leavitt – who signed a circular at Harvard in which the observations of the astronomer and his companions were collected. Another sample of the discrimination experienced by women in the history of science

Henrietta Leavitt showed that Cepheids with the same pulsation period have the same luminosity . This established the relationship between period and luminosity, a key observation to identify the scale of distance that is used when calculating distances from galaxies . Shortly after, the astronomer Ejnar Hertzsprug estimated that the small cloud of Magellan was 30,000 light-years away, writing a new chapter in the way we understand the universe. The impact of his work led the mathematician Gösta Mittag-Leffler to nominate Leavitt for the Nobel Prize . Unfortunately, the astronomer had died in 1921, four years before his initiative.