Henrietta Leavitt

‍ “The woman who discovered how to measure the Universe”

Henrietta Leavitt  (1868 – 1921) was born July 4, 1868 in Lancaster Mass.  Henrietta was the eldest of seven children, and suffered ill-health for most of her life.  

In 1885, she enrolled at Ohio’s Oberlin College, and after returning to Mass., entered the Society for Collegiate Instruction of Women (later Radcliffe college), and graduated in 1892. In her senior year, she took a course in astronomy.  Around this time she suffered a serious illness that left her severely deaf. 

In 1895,  because her family was financially relatively prosperous, and because of her interest in astronomy, she volunteered as a research assistant at Harvard observatory.  

Leavitt was assigned the task of examining photographic plates to measure and catalog the brightness of the stars.  She took particular note of stars whose brightness varied, and the period of the variation.  Leavitt found that she could accurately and consistently relate the period of a given star’s brightness cycle to its absolute magnitude, which made it possible to calculate their distance from Earth.   In 1902, she was hired on as permanent staff at $.30/hour.

In 1908 Leavitt published the paper  “1777 Variables in the Magellanic Clouds” and had discovered 969 variables in the small cloud and 808 in the large cloud.  She noted that the brighter a variable star was, the longer its period of variability.  She was the first to notice the strong correlation between their luminosity and their period of pulsation.  Assuming that these stars in the Magellanic Cloud, were at similar distances from Earth, she calculated the period-luminosity relationship of the stars.

In 1912, Edward Pickering published her findings in his own name, only referring to Leavitt as the one who “prepared” the paper.

This discovery had huge connotations for astronomy.  The period-luminosity relationshoip enabled Cepheids to be used as “standard candles” – distance markers.  This was useful for working out the distances to objects.  In the 1920’s, Edwin Hubble detected Leavitts’s Cepheid variables in Andromeda and was able to conclude that it was another galaxy.    Later, Harlow Shapley used Cepheid variables to deduce that our Sun was not at the center of the galaxy, but rather in the outer regions.

During her career, Leavitt discovered more than 2,400 variable  stars, about half of the known total in her day.

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