• International Year of Astronomy - Henrietta Swan Leavitt
Henrietta Leavitt at her desk in the Harvard College Observatory. Date unknown. American Institute of Physics

International Year of Astronomy - Henrietta Swan Leavitt

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Henrietta Swan Leavitt (1868 - 1921)

If Henrietta Leavitt had provided the key to determine the size of the cosmos, then it was Edwin Powell Hubble who inserted it in the lock and provided the observations that allowed it to be turned” –David and Matthew Clark in Measuring the Cosmos


Henrietta Leavitt was a turn of the century American astronomer most famous for discovering the period-luminosity relation in stars of variable luminosity, thus allowing the determination of distances within and in the neighborhood of the Milky Way galaxy.

In 1895, Leavitt became a member of a group of women hired by the director of the Harvard College Observatory, Edward Pickering, to reduce data and carry out astronomical calculations. Several members of this group, collectively known as the “Harvard Computers”, became famous astronomers of their day and their pioneering work is considered fundamental in the modern field of stellar astrophysics.

In 1907, Leavitt began work on creating a standard for accurately estimating stellar magnitudes—that is, the apparent brightness of stars as seen from Earth—from photographic plates. The standards were published in the 1910s, were adopted by the International Committee on Photographic Magnitudes, and remained in general use until improved technology made possible measurements of far greater accuracy.

Leavitt’s expertise allowed her to study variable stars, which are stars whose luminosity (total energy output) varies regularly and predictably, usually with periods of a few hours to days. Cepheid variables, the brightest kind of variable stars, are unstable yellow giant stars that expand and contract over regular time intervals as they convert helium into carbon in their cores.

Based on her study of Cepheid variables in the Small Magellanic Cloud (which are all about the same distance from Earth), Leavitt discovered a relationship between the average luminosity of Cepheids and their period of variability. Cepheids with longer period have greater luminosity. This relationship is useful to determine a star’s distance. By observing a star’s variation in magnitude over days or weeks, an astronomer can then estimate the average luminosity of the star. A comparison of this (intrinsic) luminosity with the average (apparent) magnitude yields the star’s distance from the Solar System.

Famed astronomer Edwin Hubble used Leavitt’s period-luminosity relation to show that Cepheids in what was then known as the “Andromeda Nebula” yielded a much greater distance than what was previously believed. Andromeda was surely not part of our galaxy, and in fact was an entirely separate galaxy similar to the Milky Way.

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