Paid by the hour to count stars on glass, she found the ruler that measures the universe.
At the Harvard College Observatory, a room of women called "computers" sat over glass photographic plates, each one a patch of night sky shot through a telescope, and logged every speck on them. Henrietta Swan Leavitt was one of them. She was going deaf. She was paid by the hour. Her job was to find variable stars, the ones that brighten and fade, by comparing plates taken on different nights. She found more of them than anyone alive.
In the Small Magellanic Cloud, a star cloud visible only from the southern hemisphere, she noticed something nobody had asked her to look for: the brighter a certain kind of pulsing star was, the slower it pulsed. That meant a star's rhythm gave away how bright it really was, and so how far off it lay. Within a dozen years her rule had shown that the Milky Way is only one galaxy among many.
A patient life
The lifeline
1868
Born 4 July in Lancaster, Massachusetts, a minister's daughter
1892
Graduates from the women's college that became Radcliffe, after one astronomy course
1895
Volunteers at Harvard Observatory; a staff post follows in 1902
1908
Publishes 1,777 variables in the Magellanic Clouds, plus a hint at a pattern
1912
States the period–luminosity relation from 25 stars
1921
Dies of cancer in Cambridge on 12 December, aged 53
By the numbers
The measure of her
2,400+
Variable stars she discovered, about half of all known in her day
1,777
Variables catalogued in a single 1908 paper on the Magellanic Clouds
25
Cepheid stars, from 1.25 to 127 days in period, behind the 1912 law
30¢
Her hourly wage as a Harvard computer, as usually reported
What she found, and what it unlocked
The work
1908
"1777 Variables in the Magellanic Clouds", in the Annals of Harvard College Observatory. Almost in passing she notes that "the brighter variables have the longer periods."
1912
Harvard Circular 173. She plots 25 Cepheids in the Small Magellanic Cloud and finds that brightness rises steadily with period. Because they all lie at about the same distance, the relation is real. It is now called the Leavitt law.
1913
The North Polar Sequence, her standard scale of stellar brightness from photographs, is adopted by the International Committee on Photographic Magnitudes.
1913–18
Others build on it. Ejnar Hertzsprung calibrates her scale and estimates the first distance to the Small Magellanic Cloud. Harlow Shapley uses Cepheids to size the Milky Way.
1924
Edwin Hubble finds Cepheids in Andromeda. Her law puts it nearly a million light-years away, far outside our galaxy, and the universe grows enormously larger.
Footnote, then foundation
Recognition
In her lifetime
Hired to catalogue other people's plates, not to set her own research
The 1912 law appeared under director Edward Pickering's name, with a note that it was "prepared by Miss Leavitt"
Increasingly deaf after an illness in the 1890s
In 1925 a Swedish mathematician wrote to propose her for a Nobel Prize. She had died more than three years earlier.
Today
The period–luminosity relation is widely called the Leavitt law
Cepheids are still a key rung on the ladder used to measure the expansion of the universe
A lunar crater and asteroid 5383 Leavitt bear her name
Her story is told in Dava Sobel's The Glass Universe and the play Silent Sky
A sky she never saw overhead
Harvard College Observatory
Her work was done in Cambridge, Massachusetts, but the Magellanic Clouds never rise above the Massachusetts horizon. The plates she studied were exposed at Harvard's southern station in Arequipa, Peru, then shipped north to the observatory's plate stacks. There she compared them a few square inches at a time, with a magnifier.
1839
Harvard College Observatory founded
1890
The southern station settles at Arequipa, Peru, about 2,400 m up
~500,000
Glass plates in Harvard's collection, the world's largest
"A straight line can readily be drawn among each of the two series of points … thus showing that there is a simple relation between the brightness of the variables and their periods."
— Henrietta Swan Leavitt · Harvard Circular 173, 1912
