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  • Writer's pictureDale DeBakcsy

Summing the Cosmos: Henrietta Swan Leavitt and the Saga of the Cepheid Stars

Astronomy is the sifting science. Its practitioners rake the sky, star by star, collecting and cataloguing, and when they are done, they begin again, through years and decades and generations. What they leave behind are reams of papers, or stacks of photographic plates, singing to the future the shape of the sky they knew. Today, pouring over those records is the job of machines, which take the tedium largely in stride. A century ago, however, that task fell almost entirely to an army of women, called computers, the most famous of whom, somewhere amidst the bleary hours of mechanical reckoning, gave us our first glimpse at how to tally the distance to the stars.

She was Henrietta Swan Leavitt (1868–1921), and her life throbbed with a steady misery lightened here and there by glimpses of the secrets behind the pulsing night sky. She was born in 1868 to an education-loving family who supported her through college, where she took the standard round of courses and received the standard middlin-to-high grades until her attention was caught by the work being done at the nearby Harvard Observatory.

It housed the largest telescope of its time in the United States, and boasted in addition a newly established South American associate observatory that regularly sent photographic plates of the Southern sky. The mission of the observatory before the arrival of the Henry Draper project, as set by its director, Edward Pickering, was ambitious: to map every known star there is, not only their position, but also their relative brightness. While Harvard’s dual telescopes swept the sky over and over again through the decades, Pickering, as we have seen in the tale of Williamina Fleming, arranged to have an army of women trained in distinguishing star brightness, and employed at the minimum wage rate of twenty-five cents an hour.

his bullpen would nurture a new crop of astronomical talent – Annie Jump Cannon , Cecilia Payne, and the rather odd but brilliant Henrietta Leavitt. Pickering was devoted heart and soul to astronomy – he gave it his every moment, and expected the same of his workers. If their salary was low, well, so was his – telescopic financing was the least of Harvard’s priorities in the early 1900s, and yet, in spite of the terrible pay and neck-breaking hours, the bullpen and Pickering kept at their tasks for years on end, motivated by pure enthusiasm and love for the work.

The first fruit of that enthusiasm came when Leavitt, looking at plates of the Magellanic Clouds, noticed regular variations in the brightness of certain of their stars – a steady brightening and dimming, repeated at predictable intervals. The existence of such stars had been noted before – they even had a name, Cepheid Variable Stars, but through pain-staking observation and insight, Leavitt noticed something new, namely that the frequency of the stars’ pulsing was directly correlated with their brightness.

It doesn’t sound like much, but it was, in fact, the key that opened the field of three-dimensional stellar cartography, humanity’s very first tool for gauging the distances between stars. How? Well, if you have two cepheids that pulse at the same rate, you know they must be the same actual brightness. If one appears, however, to be 9 times less bright, then it must be 3 times further away, thanks to the inverse square law of light intensity.

In other words, Leavitt had found a way to calculate the relative distance of every Cepheid star in the sky. Therefore, if a Cepheid was found in a galactic cluster, by relating its brightness to a known Cepheid of similar frequency, one knew, relatively speaking, how far that galaxy must be from Earth. So, if one could find the actual distance of even one Cepheid from Earth, it would drag all of the others in the Universe with it, giving us a full distance map of the stars in the sky.

Leavitt wouldn’t live to see that day, however. Her life was spent in almost constant illness – for years at a time, she would lie at home, unable to carry on the simplest of work, knowing that Pickering and the astronomical community were anxiously waiting for her to complete her studies, anguished at her inability to rouse herself to the tasks that gave her life meaning. Growing deaf and being eaten away from the inside by stomach cancer, Leavitt just managed to complete her titanic survey of the North Polar Sequence, the most meticulously cross-referenced and standardised index of star brightness of its time, but passed away in 1921 before she could turn her attention back to the pulsing stars that had caught her and the world’s attention a decade earlier.

And that’s the real tragedy of Henrietta Leavitt – between illness and family complications, her twenty brief years in astronomy were pared down to a pittance, and of those few real working years, most of her time was given to the pet projects of Pickering, who was interested in compiling data, not theorizing. So, her work on cepheids languished, while she examined plates from a dozen different observatories to produce the important but mechanical data Pickering craved. She was a computer who could think, and if she never had the freedom to let her curiosity guide her work, at least her results were important enough to pave the way for her intellectual descendants to carve out a small measure of investigatory autonomy for themselves.

Once intense multi-national parallax studies, wed to the results of Doppler-shifted elemental spectra, allowed the ascertaining of certain Cepheid distances, the universe opened up. Soon, any galaxy we could see, we could map. We found the probable size of the universe, and therefore its probable age, and thus learned truly the humbling scope of our place in it. And at the start of it all was Henrietta Swan Leavitt, the deaf human computer who saw a correlation when she was supposed to just record data, and thereby gave us the measure of the cosmos.


We have so little actual documentation about Leavitt’s life that any book is bound to be a bit slim. I like George Johnson’s Miss Leavitt’s Stars. It puts her in the context of the dizzying rush to determine the shape and nature of the universe in the early twentieth century, and works to dispel some of the rumors that more popular accountings of her life wove into existence from nothing. I’ve read it maybe four times, and each time, it’s still a fun and enlightening romp.

If you'd like to read more about women astronomers like this one, check out my History of Women in Astronomy and Space Exploration, which you can order from Amazon, or from Pen and Sword US or UK.


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