Henrietta Swan Leavitt: The woman who measured the universe


In the early 20th Century, Henrietta Swan Leavitt’s contribution to the field of astronomy completely changed our perception and understanding of the universe. Despite women not being allowed to use telescopes, her research led to one of the biggest scientific discoveries of our time.

Born in Massachusetts on July 4th in 1868, Leavitt was the daughter of a church minister. Conscientious and bright from a very early age, she received her degree from The Society for the Collegiate Instruction for Women (now Radcliffe Institute), and it was in her fourth year of study that she took an astronomy class.

Following a recommendation from a friend, she took up a position at Harvard College Observatory under the astronomer Edward Pickering. Here she began her role as a ‘Harvard computer’.

The Harvard Computers were a group of women employed to do clerical work and data entry. This was the ‘woman’s work’ that was afforded to women after the civil war.

Leavitt initially took up a volunteer position but left due to a debilitating illness that left her profoundly deaf. She spent several years at home with her parents but returned full-time to the Observatory in 1902. On her return she was paid around 30 cents an hour, half as much as the men in the similar roles.

Over 80 women were part of the group during Pickering’s time as a director at Harvard. Despite their collective and individual talents they were known as ‘Pickering’s Harem’ within the scientific community.

Pickering’s Harem, 1913. Photo belonged to Annie Jump Cannon.

Collectively the Harvard Computers had a huge impact on science and astronomy. Pickering’s first hire was actually his maid, Williamina Fleming, whom he deployed after her became frustrated with his male assistant and announced that even a maid could do the work. Fleming discovered the Horsehead Nebula in 1888.

Credit and Copyright: Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT
Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT

The Harvard Computers also included leader Annie Jump Cannon who led the cataloging of over a million stars and Margaret Harwood who became the first female director of the Maria Mitchell Observatory.

So what was Leavitt’s role within this group of extraordinary women?

She studied ‘variable stars’, the luminosity of which changes over time. Rather than using a telescope, Leavitt assessed photographic plates. She catalogued over 2000 variable stars during her time at Harvard.

Her work with a certain class of star – Cepheid variables – showed that some of the stars had a pattern in their periods of brightness. Their visibility in the sky dips and falls over a period of time, sometimes over many weeks. Brighter Cepheids have longer periods of variability. Leavitt noted that there was ‘a remarkable relation between the brightness of these variables and the length of their period’.

This NASA/ESA Hubble Space Telescope image shows the globular cluster NGC 1854, a gathering of white and blue stars in the southern constellation of Dorado (The Dolphinfish). NGC 1854 is located about 135 000 light-years away, in the Large Magellanic Cloud (LMC), one of our closest cosmic neighbours and a satellite galaxy of the Milky Way. The LMC is a hotbed of vigorous star formation. Rich in interstellar gas and dust, the galaxy is home to approximately 60 globular clusters and 700 open clusters. These clusters are frequently the subject of astronomical research, as the Large Magellanic Cloud and its little sister, the Small Magellanic Cloud, are the only systems known to contain clusters at all stages of evolution. Hubble is often used to study these clusters as its extremely high-resolution cameras can resolve individual stars, even at the clusters’ crowded cores, revealing their mass, size and degree of evolution.
This image of the Magellanic Clouds was rather ironically taken by the Hubble telescope. ESA/Hubble & NASA.

Leavitt’s theory was confirmed while studying the Magellanic Clouds, a star system just outside the Milky Way. The Magellanic Clouds were all assumed to be approximately the same distance from earth, which meant that astronomers could use their apparently brightness and actual brightness to determine how far away their really were.

The discovery meant that scientists were able to determine how far away every single star was from Earth and from each other. The stars and Leavitt’s calculations could be used to measure any distance within space. Scientists could use this information to determine how big the universe really was. But Leavitt wasn’t one of those scientists. She wasn’t permitted to continue her research.

There are many scientific advancements in the early 20th Century whose impact wasn’t clear at the time – splitting the atom, the Schwarzschild radius – but the importance of Leavitt’s work became clear very quickly.

A year after Leavitt’s discovery, The Danish astronomer Ejnar Hertzsprung used Leavitt’s work to measure the distance of several Cepheid variable stars in the Milky Way.

Edwin Hubble used Leavitt’s research to determine that there were Cepheids outside of our own galaxy. The discovery that The Milky Way was just one of many galaxies forever changed our understanding of modern astronomy.

Today astronomers estimate more than 100 billion galaxies in the observable universe.

X-ray: NASA/CXC/SAO; Optical: Detlef Hartmann; Infrared: NASA/JPL-Caltech
X-ray: NASA/CXC/SAO; Optical: Detlef Hartmann; Infrared: NASA/JPL-Caltech

Leavitt’s work hasn’t gone entirely uncelebrated – Asteroid 5383 Leavitt is named after her and there’s also a crater called Leavitt on the moon –  but her acknowledgement was left too late. She was to be nominated for a Nobel Prize by Swedish scientist Professor Mittag-Leffler.

“Honoured Miss Leavitt,

What my friend and colleague Professor von Zeipel of Uppsala has told me about your admirable discovery of the empirical law touching the connection between magnitude and period-length for the S. Cephei-variables of the Little Magellan’s cloud, has impressed me so deeply that I feel seriously inclined to nominate you to the Nobel prize in physics for 1926, although I must confess that my knowledge of the matter is as yet rather incomplete.” – Professor Mittag-Leffler

However, Professor Mittag wrote to Leavitt in 1925, unaware that she had died of cancer in 1921. There’s no saying if she’d have ever won, but the Nobel Prize isn’t awarded posthumously.

Astronomer and head of Harvard Observatory, Harlow Shapely, used Leavitt’s work to determine that the sun was not at the centre of our galaxy and he replied to the letter:

Miss Leavitt’s work on the variable stars… has afforded us a very powerful tool in measuring great stellar distances.

To me personally it has also been of highest service, for it was my privilege to interpret the observations of Miss Leavitt, place it on a basis of absolute brightness, and extending it to the variables of the globular clusters, use it in my measures of the Milky Way. – Harlow Shapely.

Published in The Harvard College Observatory: The First Four Directorships, 1839-1919. By Bessie Zaban Jones, Lyle Gifford Boyd

Most telling is the lack of acknowledgement from Edwin Hubble himself. He never credited Leavitt’s research in his own discovery. Of course, scientists use each other’s research to further their own all the time, racing to reach discoveries. Sharing information is the very basis of the growth of our own intelligence. And there’s certainly no doubting the impact of Hubble’s Law on future scientific advancements. But he certainly didn’t do it alone.

History is often diluted into a series of factoids and quotes and memorable dates. It’s Hubble’s discovery that people remember. History books gloss over the what came before, but there’s no doubt that Hubble’s work, which was made using the largest telescope in the world, was a direct result of the clerical work undertaken by Henrietta Swan Leavitt. A scientist who was never allowed to operate a telescope because she was a woman.


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