Long before your laptop computer and the computers that took us to the Moon, there was another type of computer. In the early 20^th century, women who made calculations and reduced astronomical data were known as “computers.” The hours were long and the pay was minimal. Computers were paid 25 cents an hour (about $6.30 in today’s dollars) and worked seven hours a day, six days a week. Their calculations, however, laid important groundwork for future astronomers and led to some of the most important astronomical discoveries.

One of the most famous “computers” of the day was Annie Jump Cannon, who worked at the Harvard College Observatory. Hired in 1890, Cannon’s job was to devise a classification system for all of the known stars in the sky based on their spectra. Cannon simplified two earlier schemes into one that astronomers still use today. Her classification had just seven letters arranged by decreasing temperature: OBAFGKM (to remember this order, astronomy students use the mnemonic “Oh, be a fine guy/girl kiss me”). Over the course of her life, Cannon classified the spectra of over 350,000 stars—legend has it that she could look at any stellar spectra and classify it in just three seconds.

Working alongside Cannon was Henrietta Swan Leavitt. Constantly battling illness and hearing loss, Leavitt was only able to be sporadically employed at the Harvard College Observatory. These ailments, however, did not stop her from making one of the most important astronomical discoveries of the 20^th century. Leavitt studied variable stars, which vary in brightness over the course of days, weeks, or even months. Leavitt discovered a number of these stars in the small Magellanic cloud, two irregular dwarf galaxies visible from the southern hemisphere, and published her results in the Annals of the Astronomical Observatory of Harvard College in 1908. Most importantly, Henrietta singled out 16 of those variables, noting that “the brighter variables have the longer periods.”

This insightful observation, now known as the “Period-Luminosity Relationship,” allowed astronomers to determine vast distances in the Universe that previously were unable to be measured. Notably, by applying this relationship to Cepheid Variables found in the Andromeda nebula, Edward Hubble discovered that it was so far away, it had to lie outside of the Milky Way—making it the first galaxy to be discovered. Leavitt’s discovery was so important that in 1924, Gösta Mittag-Leffler of the Swedish Academy of Sciences tried to nominate her for the Nobel Prize. Unfortunately, Henrietta died of cancer three years before this, and the Nobel Prize is not awarded posthumously.

Though Cannon and Leavitt’s work was fundamental to astronomical research in the early 20^th century, they were still limited by their role as “computers.” Computers reduced data, but did not create the data nor interpret their results. On the opposite coast, however, a woman named Phoebe Waterman was breaking down those barriers. Though Waterman had spent time as a “computer” at the Mt. Wilson Observatory in California, in 1911 she applied, and was accepted to, Berkeley’s graduate school for astronomy.

By this time, Annie Jump Cannon’s spectral classification system was becoming the world-wide standard. For her Ph.D. thesis, Waterman tested whether Cannon’s system still applied for the spectra of very hot stars when examined in different ways. She was directed to use the Lick 91-cenimeter (36-inch) refractor and its premier spectrograph in California, and even modify it, to take these new spectra and verify the universality of Cannon's system. Waterman thus became the first woman to use a very large telescope by herself to conduct research for her thesis. The Museum’s Exploring the Universe gallery displays the spectrograph that Waterman used to take these observations.

Back at Harvard, another woman, Cecilia Payne, was also working to become a full-fledged astronomer. Educated at Cambridge, Payne realized that research opportunities for women in England were scarce, so she became a graduate student at Harvard in 1923. While a graduate student, Payne studied the spectra of low-temperature giant stars to determine what the atmospheres of stars are made of. In the 1920s, astronomers generally believed that the stars were made of the same elements as the Earth’s crust: iron, oxygen, silicon, etc. Contrary to this conventional wisdom, Payne found in her 1925 thesis, Stellar Atmospheres, that hydrogen and helium were much more abundant in stellar atmospheres than in the Earth’s crust.

Payne sent her thesis to Henry Norris Russell of Princeton, a leading expert on stars, who rejected her conclusions. Admitting to an abundance of hydrogen would have required a full rethinking of the theory of how stars work—a conclusion much too radical for a Ph.D. student to make. Payne changed her conclusion to highlight the similarities between the ratios of the elements in the spectra and in the Earth’s crust rather than focus on the abundance of hydrogen and helium.

During the next four years, however, evidence in favor of Payne’s findings began piling up. Finally, in 1929, Russell admitted that her findings were correct, writing his own paper that convinced astronomers everywhere of this monumental change.

Payne’s original thesis, described by Astronomer Otto Struve as the “the most brilliant Ph.D. thesis ever written in astronomy,” was just the beginning of her long and illustrious astronomical career. Payne was the first person to receive a graduate degree in astronomy from Radcliffe College (now a part of Harvard), and continued to do research at the Observatory throughout the rest of her life. Though she produced fundamental work in the fields of spectroscopy and variable stars, for much of her career she did not hold a full teaching position and was paid much less than her male contemporaries. Finally, in 1956, Payne became the first woman from within Harvard’s Faculty of Arts and Sciences to advance to full professor and later became the first woman to be a department head at Harvard.

Each of these women pioneered their own astronomical research in the face of the social and professional constraints that women faced during the early 20^th century. From “computers” to full faculty members, these four women expanded both our view of the universe and the role of women in it. Here at the Museum, we are working to highlight these women’s stories, and other women like them. What stories are you interested in hearing about these four women? Or are there others you’re interested in learning about? Let us know in the comments.