Career
In 1893, Leavitt began working at the Harvard College Observatory as one of the women human "computers" hired by Edward Charles Pickering to measure and catalog the brightness of stars as they appeared in the observatory's photographic plate collection. (In the early 1900s, women were not allowed to operate telescopes). Because Leavitt had independent means, Pickering initially did not have to pay her. Later, she received $10.50 a week for her work.
Pickering assigned Leavitt to study "variable stars," whose luminosity varies over time. According to science writer Jeremy Bernstein, "variable stars had been of interest for years, but when she was studying those plates, I doubt Pickering thought she would make a significant discovery — one that would eventually change astronomy." Leavitt noted thousands of variable stars in images of the Magellanic Clouds. In 1908 she published her results in the Annals of the Astronomical Observatory of Harvard College, noting that a few of the variables showed a pattern: brighter ones appeared to have longer periods. After further study, she confirmed in 1912 that the Cepheid variables with greater intrinsic luminosity did have longer periods, and that the relationship was quite close and predictable.
Leavitt used the simplifying assumption that all of the Cepheids within each Magellanic Cloud were at approximately the same distances from the earth, so that their intrinsic brightness could be deduced from their apparent brightness (as measured from the photographic plates) and from the distance to each of the clouds. "Since the variables are probably at nearly the same distance from the Earth, their periods are apparently associated with their actual emission of light, as determined by their mass, density, and surface brightness."
Her discovery is known as the "period-luminosity relationship": The logarithm of the period is linearly related to the star's average, intrinsic luminosity (which is defined as a logarithm of the amount of power radiated by the star in the visible spectrum). In Leavitt's words, taken from her study of 1,777 variable stars recorded on Harvard's photographic plates, "a straight line can be readily drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the variable and their periods".
Read more about this topic: Henrietta Swan Leavitt
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