This Month in Physics History

While it was said that Pickering chose the women "to work, not to think," several proved to be highly capable astronomers. They included Annie Jump Cannon, who devised a classification system for stars that is still in use today. And then there was Henrietta Swan Leavitt, who determined how to measure distances to far-off celestial objects.

Born in 1868 to a Congressional church minister, the young Henrietta hailed from a proud Puritan heritage. She attended Oberlin College and what would later become Radcliffe College. It was a rigorous education for a woman of that era, including instruction in ancient Greek, philosophy, analytical geometry, and calculus, as well as science and the fine arts. Leavitt fell in love with astronomy during her senior year while taking a class in the subject.

Her discovery was significant because the brightness of such stars proved remarkably consistent regardless of their location in the universe. They became a useful "standard candle" in astronomy, enabling scientists to easily compute the distances to galaxies too far away for the prior method of stellar parallax observations to be useful. Within a year astronomers had used her results to determine the distance to several Cepheid variables in the Milky Way. Ultimately, her discovery convinced Harvard astronomer (and future Observatory director) Harlow Shapley that the sun was not the center of our galaxy. It also convinced Edwin Hubble that the Milky Way is not the center of the universe.

Most significantly, Hubble drew on her work with Cepheid variables to measure the distance between Earth and the Andromeda galaxy: at 2.5 million light years away, it is the nearest galaxy beyond our own. And of course, by measuring the redshift of stars, Hubble determined that our universe was not static, as astronomers previously believed, but expanding. Astronomy would never be the same.

Sadly, Leavitt died from cancer on December 12, 1921, shortly after being named head of the stellar photometry division. In his obituary, her colleague, Solon Bailey, praised not just her science, but also her personal character: She "had the happy faculty of appreciating all that was worthy and lovable in others, and was possessed of a nature so full of sunshine that, to her, all life became beautiful and full of meaning."

As director of the Observatory, Pickering published Leavitt’s work under his own name, although she was credited with "preparing" it. But at least one scientist deemed Leavitt’s contributions worthy of a Nobel Prize. In 1924, the Swedish Academy of Science’s Gӧsta Mittag-Leffler sought to nominate her for the prize and wrote to Shapley in 1926 requesting more information on her work. The ambitious Shapley informed Mittag-Leffler that Leavitt had died, rendering her ineligible, and suggested he might be a more worthy recipient for his interpretation of her findings. (He never won the Nobel Prize either.) But while Leavitt may not have won the Nobel Prize, there is an asteroid and a crater on the Moon named in her honor, as well as a play about her life and science, Silent Sky, by critically acclaimed playwright Lauren Gunderson.

Further Reading:

Johnson, G. 2005. Miss Leavitt’s Stars: The Untold Story of the Woman Who Discovered How to Measure the Universe. New York: W.W. Norton.

Sobel, D. 2016. The Glass Universe: How the Ladies of the Harvard Observatory Took the Measure of the Stars. New York: Penguin.