How Henrietta Swan Leavitt Helped Build a Yardstick to Measure the Universe
In the early 1920s, the astronomy community was divided. A centuries-old question — whether ours was the only galaxy, or if there were others — still had no definitive answer.
Some argued that all the stars and clouds of dust and gas seen in the night sky were a part of the Milky Way. Others believed that some of those dusty, gaseous formations — called spiral nebulae — were, in fact, other galaxies. That, despite their hazy appearance, there were stars within spiral nebulae. They were just too far away to be seen clearly with the telescopes of the time.
Indeed, the latter hypothesis turned out to be true, as Edwin Hubble demonstrated at Mount Wilson Observatory in 1923. There are other galaxies beyond our own. We would never know this, if not for a type of pulsating star called the Cepheid variable. At a time when women astronomers were few and most often relegated to working as assistants, it was one such assistant who made a crucial and fundamental discovery about the nature of Cepheid variables. In doing so, Henrietta Swan Leavitt provided the key to solving one of the most significant celestial uncertainties of her time.
Henrietta
Henrietta Swan Leavitt was born on July 4, 1868, in Lancaster, Mass. She was the eldest daughter of George Roswell Leavitt — a minister — and his wife, also named Henrietta Swan. “Miss Leavitt inherited, in a somewhat chastened form, the stern values of her puritan ancestors. She took life seriously,” wrote her colleague, Solon.I. Bailey, upon her death. "Her sense of duty, justice and loyalty were strong." He describes her as being religious, devoted to her family and a considerate friend. "She had the happy faculty of appreciating all that was worthy and lovable in others and was possessed of a nature so full of sunshine that all of life became beautiful and full of meaning," Bailey added.
In 1888, at the age of 20, she enrolled in Radcliffe College (known then as the Society for the Collegiate Instruction of Women and nicknamed the “Harvard Annex”). There were not many science courses offered there at the time, Leavitt’s biographer, George Johnson, notes. She took physics, analytical geometry and differential calculus. And in her fourth year, she took a course in astronomy, earning an A. But, we do not know what drew her to the subject and led her to make it her life’s work. “No diary has been found recording what it was about the stars that moved her,” Johnson writes. “One of history’s small players, her story has been allowed to slip through the cracks.”
Leavitt completed her studies in 1892. A man finishing the very same course of study would have earned a B.A, but Leavitt came away with only a certificate.
The Computer
Shortly after that, Leavitt began working at the Harvard College Observatory (HCO), first as a volunteer (1894-96) and then as a full-time employee (in 1902). She was one of a corps of women assistants known as “computers.” Her job was to study a vast number of photographs of the night sky and gather data about the nature of the stars.
Leavitt's position was relatively novel for the time. The director of the observatory, Edward Charles Pickering, had recently begun using photographic technology to capture images of stars as seen through a telescope. The resulting images of tens of thousands of stars brought about the need for several tenacious and precise minds to sift through multitudes of data.
According to science historian Margaret W. Rossiter, Pickering was an advocate for the higher education of women and thought that they might fit the bill exactly. His first hire was his housekeeper, Williamina Fleming. Pleased with Fleming's work, Pickering tasked her with hiring more women assistants. Between 1885 and 1900, Fleming had twenty assistants, including Leavitt. These "computers" were also rather irreverently referred to as "Pickering's Harem."
The practice of hiring women computers became popular in other observatories in the country as well. Rossiter notes that between 1875 and 1920, 164 women worked in observatories for at least a year or more; 12 at Mount Wilson. They were mostly high school graduates, though college graduates were well sought after.
These women were essential to the success of research endeavors. Harlow Shapley, who worked at Mount Wilson Observatory before taking over at HCO in 1921 following Pickering’s death, used the term “girl-hours” to describe the time it took to work on a research problem. “I introduced the term “girl hours,” and it would take a lot of girl hours to solve some of the problems. In fact, one job took several kilo-girl hours to get it through. So I should say that I had help on a lot of the details,” he said. He also noted that while there were only a few assistants at Mount Wilson Observatory, HCO was “swarming with these assistants.” “That’s why we got somewhere, got it done,” he added.
Click right or left to see some of the correspondence between Mt. Wilson and the computers it sought to hire:
August 2, 1920
Miss Charlotte Abbott,
5 Providence Road,
Charlotte, North Carolina
Dear Miss Abbott:
We wish to find someone to fill a vacancy here as soon as possible. If later you find yourself free, I should be glad to hear from you, for we may have something that you could do.
Yours very truly,
Superintendent Computing Division
August 24, 1920
Mr. F.H. Seares,
Mount Wilson Solar Observatory
Pasadena, California
Dear Mr. Seares,
I have your letter of August second and have given much thought to the matter. Please tell me whether conditions there are such that the salary you offer would be a living wage, and whether I could be immediately useful in the observatory or would need training. Would we live on the mountain or in Pasadena?
I may be able to accept a position in January but not before.
Very truly yours,
Charlotte Bushnell Abbott
Vassar College, Poughkeepsie, New York
Mr. F.H Seares
Mount Wilson Solar Observatory
Pasadena, California
My dear Mr. Seares,
Miss Sheldon of your staff writes me that there is to be a vacancy on the staff of computers in the near future. I am very anxious to come to California for work that is not teaching – after my graduation in June. Do you think it would be possible that I might be able to fill the vacancy? I have had the following courses in mathematics:
- Solid and spherical geometry
- Algebra and plane trigonometry
- Analytic geometry
- Advanced algebra
- Elementary differential and integral calculus
- Theory of equations
- Curve tracing
I am taking a course on general astronomy with Miss Furness next semester, which consists of a mathematical treatment of the principal branches of astronomy. Miss Sheldon is a very dear friend of mine and has assured me that I would be much interested in the work of the observatory — and also much pleased with Pasadena itself.
For further reference, please apply to Ella [not legible], Secretary of Vassar College, [not legible] Professor of Mathematics, Vassar College, and Miss Eleanor Furness, with whom I shall be studying after February the fifth. If it is necessary, I am willing to do some summer school work in both astronomy and mathematics, allowing me to reach Pasadena in September.
Very sincerely,
Ruth E. Barnett
January 13, 1912
Many of the assistants were considered noted astronomers even at the time. Some, including Fleming, was even listed in editions of "American Men in Science." Margaret Harwood, who started her career at HCO, went on to become the director of the Maria Mitchell Observatory in Nantucket. Yet, according to Rossiter, for the most part, there was no real room for career advancement. The women remained as assistants for decades on end.
When they were paid, it was not very much — just 25 to 35 cents an hour. When Leavitt finally joined the staff of HCO in 1902, she was paid 30 cents an hour, five cents higher than what was typical for an entry-level position. In Pickering words, this was "in view of the quality" of her work.
Click right or left to see one particularly revealing letter of application:
January 21, 1918
Mrs. Betty Trier Berry
1929 No. Western Avenue,
Los Angeles, California
Dear Mrs. Berry,
We have a position in our Computing Division now vacant that I can offer you at the beginning salary of $825 per year, the appointment to take effect on February 1. As annual vacation of one month is granted to the staff, and there is no work on Saturday afternoons. The amount offered is probably much less than you have been earning, but I am hoping that you will wish to try the work under these conditions. With your interest in astronomy, I am under the impression that you will not regret such an acceptance.
Very sincerely yours,
Superintendent Computing Division
January 23, 1918
My dear Mr. Seares,
Your kind communication of the 21st referring to a vacancy in your Computing Department at $16 per week wage reached me today; I trust you will pardon the intrusion on your time if I write you at some length on the matter.
I am dependent on my own efforts for my support; and desirous as I am of entering again the field of astronomical work, I am afraid of the ugly practical questions that would inevitably arise were I to limit my earning capacity to that rather pathetic amount. So far therefore as the position of which you write is concerned, I am regretfully obliged to say that it will be impossible for me to accept it.
It occurs to me, however, that you speak of my “interest in astronomy” as something quite apart from the work of the position you offer me. If I am correct in assuming from this that you have in mind for me, at a later time, some better position at a less mechanical, hence far more interesting, branch of the work, then it would seem that I am being given an opportunity to “try out,” for which I am certainly grateful — but with all the risk of non-success resting upon me. As I say, I am scarcely in a position to assume this risk, and a salary which actually supports me during the probationary period would be obviously quite essential before I dared make a radical change of profession.
I am so very anxious to “come home” to the work I love and so confident that I can be of actual assistance to you in it that I sill venture to hope for an opportunity to join your staff.
Respectfully yours,
Betty T. Berry
January 30, 1918
Dear Mrs. Berry:
I am afraid that under existing conditions we can do nothing more than repeat the offer of my former letter; but from your reply I appreciate that that would be useless. I am sorry, for I had hoped that we might have you with us.
Very sincerely,
Variable Stars
Pickering wanted to know more about stars than just their positions and patterns of movement. One of his primary interests was the photometry of stars, that is measuring their brightness or magnitude.
He tasked Leavitt with working on the photometry of variable stars, or variables. These stars pulsate, becoming bright and dim by turns. Brighter stars appear as bigger dots on photographic plates, and so Leavitt examined images of the skies, taken at various points in time, to see which stars had changed in their magnitudes. She would also use these images to calculate their periods—the time that it takes for a variable star to go from bright to dim to bright again.
By the spring of 1904, Leavitt began searching for variables in the Large and Small Magellanic Clouds. We now know these to be distant satellite galaxies, which orbit the Milky Way in a slow crawl.
At first, she discovered 57 faint variables in the Small Magellanic Cloud when comparing two photographs. When more photographs were taken — two to three days apart and with exposure times of three to four hours — Leavitt discovered 969 new variables. She then began to scan the Large Magellanic Cloud, with equally staggering results — 808 new variables.
In just about two years, Leavitt had discovered a whopping 1,777 new variable stars in the Magellanic Clouds.
She then carefully calibrated and tabulated their magnitudes and positions.
The Cepheids
When Leavitt finally published her findings in 1908, she made special note of 16 variables in the Small Magellanic Cloud. She had calculated their periods and had noticed an interesting pattern. “It is worthy of notice that in Table VI, the brighter variables have the longer periods,” she wrote.
These stars had other characteristic properties. Their periods were constant and did not change over time. They would also dim gradually, quickly flare up in brightness and begin dimming again. These traits are the hallmarks of variable stars known as Cepheids (or Cepheid variables).
In 1912, a second paper was published, but under Pickering's name. He did, however, note in the first paragraph that the manuscript was, in fact, prepared by Leavitt.
Leavitt had added more stars to her special roster, bringing it to a total of 25. For each of these Cepheids, Leavitt's observation held: The brighter the variable, the longer the period.
This seemingly simple observation set the stage for a deeper understanding of our universe.
A Stellar Yardstick
Leavitt’s findings came to be known as the “period-luminosity” relationship. In her 1912 paper, she noted another important point. “Since the variables are probably at nearly the same distance from earth, their periods are apparently associated with their actual emission of light [...],” she wrote. From this we can infer that, if two different Cepheids in two different locations have the same period, they are actually equally bright.
But what if one of those Cepheid appears to be brighter than the other? Leavitt's relationship suggests that the Cepheid that appears to be brighter is closer to Earth and the one that appears dimmer, is farther away.
However, we cannot calculate exactly how far these Cepheids are, without knowing the distance to one of them. Then, it becomes possible to calculate the distance from Earth to the other.
In 1915, Harlow Shapley observed a Cepheid in a globular cluster (a dense spherical cluster of stars). This Cepheid was close enough that Shapley could use well-established methods such as parallax — which allows us to measure distances to nearby objects — to measure the exact distance to the star. With this information, it became possible to calculate the distance to any Cepheid in the universe.
Leavitt’s period-luminosity relationship had helped to fashion a powerful new stellar yardstick. “It essentially allowed us to start measuring distances to much, much, further away objects,” says Laura Whitlock, an astrophysicist and director of the planetarium at Georgia College and State University. “So, the universe became more measurable because of her method.”
Shapley then used this information to chart the Milky Way Galaxy. Though he didn’t get everything right, he made one fundamental discovery. The sun was not at the center of our galaxy. It was about two-thirds of the way, away from the center.
What Shapley got wrong, was the extent of our galaxy. It did not encompass everything seen in the night sky.
In 1923, with the construction of a powerful new telescope at Mount Wilson Observatory, Edwin Hubble discovered the presence of a Cepheid, dubbed V1, in a spiral nebula (then thought to be just a spiral shaped cloud of gas and dust). When he calculated the distance, it turned out to be a million light-years away. It was so far from the outer reaches of our own Milky Way that it couldn't be anything but a neighboring galaxy with its own stars. And so, Andromeda emerged as the second of the estimated billions of galaxies beyond our own.
Endings
Leavitt didn’t live long enough to see how far her discovery had taken the world of astronomy.
She was often ill, and this kept her away from her work for multiple years. According to her biography, she had experienced trouble with her eyes and hearing and was deaf towards the later part of her life. She died of stomach cancer on December 12, 1921, at the age of 53.
And yet, Leavitt accomplished so much. She became the head of stellar photometry at HCO. She discovered 2,400 variable stars, nearly half of all those known in her lifetime.
“Edwin Hubble’s work could never have been done if Henrietta Swan Leavitt had not done her work and that’s just a statement of fact,” says Whitlock. “Harlow Shapley’s work, of understanding where our solar system is in the Milky Way Galaxy, could never have been done without Henrietta Swan Leavitt’s work. So, a lot of our understanding of our place in the universe is really based on her work.”
Sources
Bailey, Solon.I. 1922. “Henrietta Swan Leavitt.” Popular Astronomy, volume.30: 197-198.
Johnson, George. 2005. “Miss Leavitt’s Stars.” New York: W.W.Norton and Company.
Rossiter, Margaret W. 1980. “"Women's Work" in Science, 1880-1910.” Isis, volume 71, no. 3: 381-398.
Interview of Harlow Shapley by Charles Weiner and Helen Wright on 1966 June 8, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4888-1
Bailey, Solon. I. 1931. “The History and Work of Harvard Observatory, 1839 to 1927; An Outline of the Origin, Development, and Researches of the Astronomical Observatory of Harvard College Together with Brief Biographies of Its Leading Members.” New York: McGraw-Hill Book Company Inc.
Leavitt, H.S. 1908. “1777 Variables in the Magellanic Clouds.” Annals of Harvard College Observatory, volume 60:87-108.3.
Pickering, E.C. 1912. “Periods of 25 Variable Stars in the Small Magellanic Cloud.” Harvard College Observatory Circular, volume 173:1-3.
Top Image: NASA's Hubble Space Telescope maps the undisturbed Triangulum galaxy (M33) in an image that spans 14,500 light-years | NASA, ESAand M. Durbin, J. Dalcanton, and B.F. Williams (University of Washington)
