The Woman Whose Calculations Revealed The Size Of The Universe

Henrietta Leavitt was born July 4, 1868, in Lancaster, Massachusetts, to the Rev. George Roswell Leavitt and his wife, Henrietta Swan Kendrick. Her father was a Congregational minister. The family was pious and strongly supported female education. Leavitt entered Oberlin College in 1885 and transferred to the Society for the Collegiate Instruction of Women in Cambridge in 1888 (later Radcliffe College), where she studied analytic geometry and calculus as well as classics.

First and foremost, Leavitt was a mathematician, not an astronomer. It wasn’t until 1891-1892, her senior year, that she took her first astronomy class, given by Harvard instructor Arthur Searle, and was so taken with it that she stayed on for another year of work in astronomy. The next year, she was awarded a certificate verifying that she had done enough work to qualify for a bachelor’s degree from Harvard, which still hadn’t begun to award women degrees. Astronomy had found her late, but it held her attention for the rest of her life.

Leavitt spent 1892-1895 touring America and Europe following the completion of her college education, but contracted a serious and lingering illness during the trip that rendered her permanently deaf. The nature of the ailment is unknown. Some later sources speculate it was scarlet fever. Leavitt did not abandon her scientific pursuits but instead returned to Cambridge to engage in astronomical work.

Leavitt worked as an unpaid volunteer at the Harvard College Observatory in 1895. The director, Edward Charles Pickering, had assembled a team of women. They were not mere clerks, however. They were trained mathematicians and scientists, called “Harvard Computers” by their peers and informally dubbed “Pickering’s Harem.” Leavitt’s fellow workers included Williamina Fleming, Annie Jump Cannon, and Antonia Maury. He hired women in part because the pay went further in his budget than if he employed men, but also to hire serious analytical minds to help analyze all of the data they were accumulating.

Leavitt became a member of the permanent staff of the Harvard College Observatory in 1902. She was paid 30 cents an hour. By the era’s rules, women were not allowed to work on telescopes at night, so she never actually looked through a telescope. She did her job during the day in Cambridge, Massachusetts, looking at glass photographic plates, taken by telescopes at Harvard’s Boyden Station in Arequipa, Peru, that had been shipped north to Cambridge. She would spend her entire career studying photographs rather than the sky itself.

To find the few stars that had changed in brightness out of the many thousands on the plate, Leavitt superimposed a glass positive taken on one night over a glass negative taken on another. Unchanging stars canceled each other out and disappeared from sight. The variable stars, those whose brightness rose and fell, became apparent with a visible halo. It was a straightforward technique, but it allowed her to manually process a vast area of the sky.

Pickering directed Leavitt to catalog variable stars in the Small and Large Magellanic Clouds, two satellites of the Milky Way visible from the southern hemisphere and photographed by Harvard’s telescopes in Peru. Over the course of several years from 1908 to 1912, Leavitt used her plate-overlay method to hunt through the clouds for variable stars. The observatory was building a standard scale for the measurement of starlight. Leavitt’s work was not theory but simple patience, a matter of cataloging thousands of stars one plate at a time.

Leavitt’s 1908 paper in the observatory’s Annals detailed 1,777 newly found variable stars in the two Magellanic Clouds. At the very end was a smaller but stranger story. Focusing on 16 pulsating stars known as Cepheid variables, she observed that those which were brighter were also slower, taking more time to cycle through phases of fading and shining. It was a faint observation, but it was the correct one.

Over these same years, Leavitt worked out a solution for her mystery. All of the stars in question were located within the same star cloud, the Small Magellanic Cloud, meaning they were at approximately the same distance from our planet. Thus, their differences in apparent magnitude were not due to differences in distance. Instead, they were simply due to differences in luminosity. The factor of distance had been eliminated from the equation, and only the true brightness remained.

On March 3, 1912, Harvard released a circular letter numbered 173 titled “Periods of 25 Variable Stars in the Small Magellanic Cloud.” Its opening line credited the work plainly: it “has been prepared by Miss Leavitt,” even though director Edward Pickering affixed his name to it, as was his habit. When she plotted each star’s pulsation period against its brightness, she got two neat, parallel lines. In other words, she had demonstrated that a Cepheid’s rhythm predicts its true brightness. Leavitt had created a yardstick, though she didn’t know the distance of a single step on it.

Ejnar Hertzsprung, a Danish astronomer, took Leavitt’s relationship and turned it into actual numbers. By 1913, with a method known as statistical parallax that used the common motion of clusters of stars to gauge distance, he determined the true distance to 13 Cepheids in our galaxy. By plugging these values into Leavitt’s law, Hertzsprung created the first distance ladder, although his measurements were flawed because astronomers did not yet realize dust dimmed starlight. Leavitt made the ruler. Hertzsprung put the first measurements on it.

The American astronomer Harlow Shapley, who became director of the Harvard College Observatory, applied Leavitt’s law to the galaxy. In 1918, he calculated distances to globular clusters of stars found throughout the Milky Way, and concluded that the galaxy was approximately 300,000 light-years across. More recent estimates place the size at approximately 100,000 light-years after correcting for the effects of interstellar dust. In any case, the Milky Way was much larger than previously thought, and our Sun was not located at its center.

In 1920, astronomers Harlow Shapley and Heber Curtis were engaged in a public dispute over fuzzy, spiral-looking blobs of light that could be seen by telescopes. These blobs were called nebulae. Did the nebulae represent gaseous clouds that existed inside our own galaxy, or were they actually other galaxies located beyond our own? This argument was known as the Great Debate and, at the time, it was left unresolved. It would take several more years and one more use of Leavitt’s law to settle the matter.

After Edward Pickering’s death, the Harvard College Observatory’s directorship passed to Harlow Shapley, who named Leavitt Head of Stellar Photometry in March of 1921. After 26 years at Harvard, it was the first time that she had formal administrative responsibility over her own department. This came too late. Her health deteriorated rapidly in the next few months.

Henrietta Leavitt was battling aggressive stomach cancer, cared for by her mother at their house in Cambridge, Massachusetts. She died there on December 12, 1921, at the age of 53. She was buried in the Leavitt family plot at Cambridge Cemetery. Her estate, which was left to her mother, was valued at $314.91. She was not alive to witness the major discoveries that her Period-Luminosity law would soon allow.

On the night of October 5-6, 1923, American astronomer Edwin Hubble used the 100-inch Hooker Telescope at Mount Wilson Observatory to photograph the Andromeda Nebula. On the plate, he found a pulsating Cepheid variable star. He crossed out the N he had written for nova and wrote VAR in red ink instead. By applying Leavitt’s law, he calculated that Andromeda was about a million light-years distant, well outside our galaxy, the Milky Way.

That measurement answered a debate that astronomers had been having for many years. Were these spirals, of which Andromeda was one, in fact gaseous clouds in our galaxy, or were they external “island universes” with their own individual stars? The answer, as Hubble proved, was the latter. This discovery, which put an end to the Great Debate between Harlow Shapley and Heber Curtis in 1920, is considered one of the greatest discoveries in 20th-century astronomy. None of this could have been achieved without Leavitt’s Period-Luminosity law.

In 1925, the Swedish mathematician Gösta Mittag-Leffler wrote to Leavitt at Harvard, saying that he would like to nominate her for the 1926 Nobel Prize in Physics for her Period-Luminosity relationship. The observatory’s director, Harlow Shapley, wrote back to inform him that she had passed away in 1921, on December 12. Since Nobel rules did not allow posthumous nominations, the nomination never went through.

Edwin Hubble paired these Cepheid-based distances, calculated using Leavitt’s law, with research by Belgian astronomer Georges Lemaître on the shifting of light coming from galaxies, which was another indication of movement. These two data points together revealed that remote galaxies were receding from Earth at a velocity proportional to their distance. Not only was the cosmos far larger than previously believed, but it was also growing in size.

Leavitt’s name is immortalized on the far side of the Moon in a crater 66 kilometers wide, called Leavitt by the International Astronomical Union. There’s also an asteroid in the main belt of the solar system known as 5383 Leavitt. These are the only formal tributes she received. The Nobel nomination never made it in time, but her Period-Luminosity law remains one of the most influential results in the history of science.

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