When discussing the history of astronomy there are a number of famous names that instantly spring to mind: Hubble, Herschel, Galileo and so on. All are of course deserving of the praise they are given for their contributions to astronomy, but there are many other names that go unrecognised, often being underappreciated for their own contributions. Perhaps there is none more so, at least with regards her impact on astronomy, than Henrietta Swan Leavitt. Where others in her time debated over our true place in the universe, her calculations paved the way to some of the most important discoveries of the 20th century, which answered that very question.
Leavitt was born on 4 July 1868 in Lancaster, Massachusetts. She was the daughter of a congregational minister and in her youth attended Oberlin College and the Society for Collegiate Instruction of Women (later Radcliffe College), the latter of which she graduated from with a bachelor’s degree in 1892. In the last year of college she had taken a course in astronomy and took to the subject with great enthusiasm. It was around this time that she sadly suffered a serious illness that left her severely deaf, but her love for astronomy remained.
The late 19th and early 20th centuries, however, were not an enjoyable time for female astronomers. Women were not allowed to operate telescopes and often instead found themselves performing somewhat mundane tasks for supposedly more-important men. So it came to pass that in 1893 Leavitt began working at the Harvard College Observatory under American astronomer Edward Pickering. She was part of a group known as the Harvard Computers, or Pickering’s Harem, a group of skilled women employed by Pickering for a pittance of about 30 cents an hour to sift through mountains of astronomical data.
Each woman was given a different set of data to catalogue and analyse. Leavitt was assigned to study variable stars, those whose luminosity varied over time but whose exact workings were poorly understood. In her studies she found thousands of variable stars in the Magellanic Clouds and she noticed a pattern, namely that the brighter a variable star was, the longer its period of variability. She published her results in 1908 in the Annals of the Astronomical Observatory of Harvard College.
The type of stars she focused on were so-called Cepheid variables, a class of very luminous variable stars. She was the first to notice this strong correlation between their luminosity and their period of pulsation. Using the assumption that these stars were all at a similar distance from Earth in the Magellanic Cloud, she calculated the period-luminosity relationship of the stars.
This discovery had huge connotations for astronomy as a whole. The period-luminosity relationship enabled Cepheids to be used as standard candles, or in other words distance markers. They were incredibly useful for working out the distances to objects that were too far to be calculated by other methods. In the 1920s astronomer Edwin Hubble detected Leavitt’s Cepheid variables in Andromeda and was able to conclude that it was another galaxy rather than a part of the Milky Way, placing us in just one of hundreds of billions of galaxies. Similarly, astronomer Harlow Shapley used Cepheid variables to deduce that our Sun was not at the centre of the galaxy, but rather in the outer regions. Leavitt’s discovery turned astronomy on its head and for the first time enabled astronomers to begin working out just where we fit into the universe.
Leavitt, however, went largely unrecognised for her work and in 1921 she died after losing a battle with cancer. In 1924 a Swedish mathematician, Gösta Mittag-Leffler, attempted to nominate Leavitt for a Nobel Prize, only to discover she had died three years earlier. Nonetheless the influence of her work remains plain to see and it is thanks to her dogged determination that we now have a much greater understanding of the universe than ever before. Perhaps now, when discussing important names that have significantly contributed to astronomy, Henrietta Leavitt should be one of the first on the list.
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