This is the story of a woman named Henrietta. She was a computer.
Back before the advent of the technology on which you are reading this, before the original IBM PC, before the Commodore VIC-20, before the Altair, before Kernighan and Ritchie developed UNIX and the C programming language, before the ENIAC, before mathematician and physicist John von Neumann laid down the mathematical basics of modern computing architectures, the word “computer” was a job title. The word was applied to people whose job it was to tally up numbers.
One such computer was Henrietta Swan Leavitt. While employed as such by the Harvard College Observatory, she was tasked by astronomer Edward Pickering with the job of examining photographic plates to measure and catalog the brightness of variable stars. While tabulating the brightness of such stars in the Magellanic Clouds, she noticed something interesting. The brighter these variable stars appeared to be, the longer the period during which their brightness varied appeared to be. In 1908, she published her observations.1 This was followed by a period of intense scrutiny of Cepheid variables in the Small Magellanic Cloud, which confirmed that Cepheid variables with greater brightness indeed had longer periodicity. Furthermore, since these stars were all confined to the Small Magellanic cloud (whose distance could be estimated with reasonable accuracy through existing methods), they were all of a roughly equal distance from the Earth. The consequence of this observation was the realization that the periodicity of these variable stars was not just related to their apparent brightness, but also to their intrinsic brightness. She published the results of these further studies in 1912.2
In the following year, Ejnar Hertzsprung measured the distances to several Cepheid variables within the Milky Way, thus refining the usefulness of these stars as a method for estimating distances over large scales. This paved the way for Edwin Hubble‘s work in the twenties. Using measurements of Cepheid variables in various “spiral nebulae,” Hubble was able to estimate their distances and prove that they lay far beyond our own Milky Way galaxy, that they were in fact galaxies themselves. By the end of that decade, Hubble had combined these distance measurements with redshift data to demonstrate that the universe was expanding.3
In 1925, Swedish mathematician Gösta Mittag-Leffler was preparing to nominate her for the 1926 Nobel Prize in Physics for her pivotal contribution, only to be informed that she had succombed to cancer four years earlier. By finding the first “standard candle” for measuring astronomical distances, Henrietta Leavitt had made it possible to scale higher up the cosmic distance ladder, opening up the path for modern cosmology as we know it.
1. Leavitt, Henrietta S. “1777 Variables in the Magellanic Clouds“. Annals of Harvard College Observatory. LX(IV) (1908) 87-110
2. Miss Leavitt in Pickering, Edward C. “Periods of 25 Variable Stars in the Small Magellanic Cloud” Harvard College Observatory Circular 173 (1912) 1-3.
3. E. Hubble, “A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae”. Proceedings of the National Academy of Sciences, 15:168-173 (1929).
For More Information:
Fernie, J.D. (December 1969). “The Period-Luminosity Relation: A Historical Review”. Publications of the Astronomical Society of the Pacific 81 (483): 707.
Edwin Hubble, The Realm of the Nebulae, Dover Publications (1958) (archive.org)
Interstellar Medium and the Milky Way Period-Luminosity Relation for Variable Stars