How Charlotte Moore Sitterly Wrote The Encyclopedia of Starlight

The “world’s most honored woman astrophysicist” worked tirelessly for decades to measure the makeup of the sun and the stars

Charlotte Moore Sitterly
Charlotte Moore Sitterly made huge strides in our understanding of how atoms are structured and what stars, especially our sun, are made of. Courtesy of Michael Duncan via the M.D. Moore Family / Photo Illustration by Shaylyn Esposito

Charlotte Moore could smell the coal burning in the furnace below her back-room workspace at the Princeton University Observatory. With a meager starting salary of $100 per month, she worked as a “computer” for the famous astronomer Henry Norris Russell, helping with calculations to describe how stars evolve and what kinds of materials burn inside them. Her boss’s mind seemed to run too quickly for anyone to follow, and the short, quiet woman he hired fresh out of college in 1920 was initially overwhelmed.

“I felt that he must think that I was the most ignorant person that ever showed up at his house,” she told space historian David DeVorkin in 1978.

At a time when few women had opportunities in the physical sciences, and fewer still received recognition for their efforts, Charlotte Moore Sitterly, as she was known after her marriage, was a pioneer in a field that has touched nearly all scientific disciplines: spectroscopy. The study of how matter interacts with light, including at wavelengths invisible to the human eye, transformed not only astronomy, but also physics, chemistry and optics, enabling a broad range of technologies.

Every element on the periodic table has a unique “fingerprint” of lines in the rainbow or “spectrum” that becomes visible when the element emits or absorbs light. Scientists can use these fingerprints to identify different materials, even in distant objects like stars. Moore Sitterly’s efforts led to a greater understanding of the structures of atoms and their spectral lines, giving scientists the tools to dissect the contents of the universe.

By organizing this information about elemental fingerprints into handy reference tables, the bashful employee, and later colleague, of Henry Russell provided invaluable tools for astronomers and scientists at large. Her 1945 publication “A Multiplet Table of Astrophysical Interest” is still cited today as a reference for astronomers exploring everything from the rocky stuff of meteors to the plasmas of giant stars.

“She certainly made an enormous contribution to stellar spectroscopy, and our understanding of stars and our ability to analyze them,” says Virginia Trimble, professor of astronomy at the University of California, Irvine.

Early Days

Charlotte Moore Sitterly was born in 1898 to a poor Quaker family in rural Pennsylvania, in a town called Ercildoun. She substitute taught while earning her undergraduate degree at Swarthmore College because, as she told DeVorkin, “Substitute teaching and tutoring were the two fields in which a woman could get some money toward working her way through college; almost everything else favored the men.”

The head of Swarthmore’s math department learned that Russell, at Princeton, was looking for an assistant to help measure and calculate the properties of stars. He recommended Moore Sitterly, still in her early 20s, and she agreed to work for Russell prior to meeting him—there was no interview.

How Charlotte Moore Sitterly Wrote The Encyclopedia of Starlight
A portrait of Charlotte Moore Sitterly taken in 1919, when she was 20 or 21 years old. Courtesy of Michael Duncan via the M.D. Moore Family

Russell had already distinguished himself as a world leader in astrophysics when Moore Sitterly arrived at Princeton. As he collected awards in the early 1920s, he had a lot of tasks for Moore Sitterly—enough for three people, it seemed. His first letter to her, now housed among other correspondence at the Princeton University Library's Department of Rare Books and Special Collections, mentions that she’d do “computations to be performed under my direction, including also the measurement of astronomical photographs. Determinations of the position of the Moon, in accordance with the plan developed by cooperation between this Observatory and Harvard, will form an important part of this work.”

In practice, she also worked on spectroscopy and calculating the masses of stars, especially double-star systems. Russell needed Moore Sitterly’s calculating and editing skills, and he was easily distracted and excited by many ideas, according to DeVorkin, who wrote the biography Henry Norris Russell in 2000. Russell could ramble for hours, and Moore Sitterly helped him focus.

“What she knew was that Russell was very consistently brilliant in his work, and that she saw herself as organizing that brilliance in the areas that she was involved in, and helping to make it more systematic,” says DeVorkin, who is now a senior curator in the Smithsonian Air and Space Museum’s space history department.

No matter how hard she worked, Princeton would not consider Moore Sitterly for a PhD—in fact, a woman would not enter any Princeton graduate program until 1961. “I was used to prejudice against women because Princeton was a man's stronghold, and a woman was really out of step there,” she said in a 1981 interview with the National Bureau of Standards.

Within her first couple of years at Princeton, Moore Sitterly became chronically ill, perhaps because of the coal fumes and the strenuous nature of the work—though she wrote in an appreciative letter to Russell from a hospital in 1922: “Please do not think for one moment you that you are in any way responsible for this illness.”

For the sake of her health, in 1925, Moore Sitterly asked Russell for a leave of absence and headed westward to work with astronomers on spectra of the sun at Mount Wilson Observatory near Los Angeles. She continued collaborating with Russell, who visited once a year. In February 1926, she wrote to him, “If there is any work I can do for you, please don’t hesitate to ask me.” In July, she was surprised that he had left his own name off of a publication about the relative position of the moon—the results of her very first Princeton assignment—but kept hers. “When I think of the many difficulties you helped me out of, I don’t think you have been quite fair to yourself and have given me far too much credit,” Moore Sitterly wrote.

When her position at Mount Wilson was up for renewal, Moore Sitterly delicately sought Russell’s approval to stay in a handwritten letter dated September 6, 1926:

You probably have realized that the time has come to consider a position for next year, as my appointment here expires on January 1, 1927. I feel that it would be very unfair for me to make any plans without consulting you and also I value your advice very much indeed.

The solar spectrum work is just well started and I hate to start something I cannot finish. Also I feel that it would be very unwise to consider coming east in January, for these two reasons I think I should stay here a while longer, at least. The position is open to me and I think I shall stay at present.

I feel in an awkward position here, as you can imagine, and I think [Mount Wilson director] Dr. [Walter] Adams does, also. On the other hand, I should feel very badly if they flatly refused to consider a reappointment, after you had recommended me, for I have tried my very best not to be any discredit to you, although of course I am entirely unable to measure up to you in ability.

Later, when Russell took his family for a sabbatical in Europe in 1929, Moore Sitterly saw an opportunity to complete a PhD—and unlike Princeton, the University of California, Berkeley, would admit women for doctorate degrees. Using data from the Mount Wilson 150-foot solar telescope, she studied the atomic spectral lines of sunspots, deducing that the temperature of these features was about 4,700 degrees Kelvin.

Russell still visited. According to a Carnegie Observatories yearbook from 1929-1930: “Dr. Henry Norris Russell, Research Associate, spent two months in Pasadena during the autumn of 1930 and devoted especial attention to a study of the results of an analysis of the line-spectrum of sun-spots made by Miss Moore.”

How Charlotte Moore Sitterly Wrote The Encyclopedia of Starlight
Charlotte Moore Sitterly (third from the right) meets with President Johnson and a group of people in the White House after earning the Career Service Award, April 30, 1966. Courtesy of Michael Duncan via the M.D. Moore Family

Moore Sitterly sent Russell the program for her final oral examination in 1931, as well as a telegram saying she’d passed, and Russell heartily congratulated her on the achievement. But Moore Sitterly wrote back that she was nervous “as usual” and that a professor expressed annoyance at her during the exam: “He said he could not make me state the simplest facts because I always went to something much more complex and complicated than they wanted. He said it was very obvious that I had never taught.”

Although the Mount Wilson astronomers highly regarded Moore Sitterly, Russell badly wanted her back at Princeton. He wrote to her in October 1930: “By all means don’t do anything about any other position until I see you. We have been expecting all along to have you come back here and I don’t see how we can get on without you.”

She did return to Russell in 1931, and although their decades-long correspondence reflects a cordial relationship, she once said that the best thing about going to Princeton was meeting her husband, according to a relative of hers through marriage, Michael Duncan. And Moore Sitterly told DeVorkin in 1978 that if she had met Russell in an interview, she might not have gone to work for him in the first place. But in her characteristic modest way, she clarified the remark by saying her work at Princeton “might have been done better by somebody else.”

Although Moore Sitterly became a close friend of the Russell family, she addressed the famous astronomer as “Dr. Russell” in all of her letters to him, while by 1940 he had started writing “Dear Charlotte.”

The Multiplet Tables

Promoted at Princeton from “computer” to “research assistant,” and later “research associate,” Moore Sitterly worked on a project in the 1930s that would become part of her legacy: the multiplet tables. A multiplet table is like an encyclopedia of atomic elements, their possible electron states as well as the wavelengths that correspond to the elements emitting or absorbing single particles of light.

“Russell was fascinated with the various rules and theories about how atoms emit and absorb light, and what you can learn physically about a star that has those kinds of spectral lines in it,” DeVorkin says. “She was the one who could take some of those ideas, but turn them into actual practice.”

Inspired by Russell’s own limited spectral tables, Moore Sitterly saw the need for a comprehensive index, and she took pride in its execution. “I refuse to let anyone else assemble the table for me, because it is a mean job and one that requires infinite care,” she wrote to Russell in October 1931.

This “mean job” turned out to be of great consequence for the advancement of science. “The multiplet table is a key step between what you see and what you’re going to analyze,” Trimble says. “What she did was essential for stellar astronomy.”

Yet Russell was reluctant to publish such an ambitious effort. Moore Sitterly said in 1981 that Russell thought this comprehensive spectral index was “probably not the right thing to do. … I think he worried about keeping it up to date, but he never told me exactly why.”

Moore Sitterly persisted, and she persuaded Russell to agree to a “trial edition” in the early 1930s, DeVorkin writes in his biography of Russell. The 250 copies went quickly. Still, while scientists used the multiplet table to make progress in unpacking the sun’s composition, Russell thought there were too many unidentified lines in the solar spectrum to justify a second printing.

The revised edition finally came out in 1945 “on very poor paper and very cheaply,” Moore Sitterly said in 1981. But it was in very high demand. “Everybody wanted it, and I remember that one day when Dr. Russell came in, he was the most surprised person; and it was sincere,” she said.

Russell also had initial misgivings about Moore Sitterly’s marriage to astronomer Bancroft Sitterly, she said in her 1978 interview with DeVorkin. “Banny,” as he was known, had been a student of Russell’s, and he knew Charlotte since the 1920s. Russell was “upset” about their 1937 marriage, possibly because he thought it would mean losing Moore Sitterly as a researcher.

But she continued working part-time for Russell, even while living in Cambridge so her husband could work at MIT on “top secret” assignments during World War II. Moore Sitterly commuted to Princeton often and worked on the spectra of elements such as neutral iron during this time, although officially she “wasn’t allowed to know” why. While her work's specific applications to the war effort are unknown, atomic spectroscopy was important for a wide range of applications, including the Manhattan Project.

A New Era for Measuring Light

With Russell about to retire, Moore Sitterly was invited in 1945 to begin a new position at the National Bureau of Standards, which now the National Institute of Standards and Technology, or NIST, then headquartered in Washington, D.C. Russell acknowledged her resignation from Princeton in a reverential letter dated October 23, 1945:

During the twenty years and more of your connection with us, you have done excellent work which has added greatly to the distinction of the Observatory, in addition to securing for yourself a permanent place in the history of American astrophysics. Not only the quality of this work deserves formal record, but your indefatigable energy and faithful devotion to the work. You have habitually gone well beyond the line of duty and the quality of the work has been greatly the gainer. Your exceptional accuracy in detail has been invaluable in the preparation of our spectroscopic work, above all in the great Revised Multiplet Table, but I gratefully acknowledge the immense help which you have given me in the preparation of papers of my own in earlier years.

In her new job, Moore Sitterly insisted that she continue working on solar spectra as a condition of employment. Her work on the composition of the sun “is really what I enjoy most,” she said in 1981.

How Charlotte Moore Sitterly Wrote The Encyclopedia of Starlight
Charlotte Moore Sitterly working at her desk at the National Bureau of Standards in Washington, D.C. Courtesy of Michael Duncan via the M.D. Moore Family

Until this point, all Moore Sitterly could learn about the sun’s composition came from telescopes on the surface of the Earth. She did not have access to tools that could dissect all of the sun’s ultraviolet rays, which are mostly blocked by the atmosphere. But in 1946, scientists led by Richard Tousey at the Naval Research Laboratory captured the sun’s ultraviolet spectra using a V-2 rocket. This experiment sparked a decades-long collaboration between Moore Sitterly and Tousey. She published the “Ultraviolet Multiplet Table” in 1950.

“She recalled a day long ago in Princeton, when she and Russell and others were discussing what the ultraviolet solar spectrum would look like,” Vera Rubin, an astronomer who found groundbreaking evidence of dark matter’s existence, wrote of Moore Sitterly in 1991 in the Journal of Astronomical History and Heritage. “They had fun guessing, but concluded that they would never live to see it, because no one could build a spectrograph stable enough.”

But in Moore Sitterly’s first years at the Bureau of Standards, the new ultraviolet work had to remain a side project. She received the massive assignment of assembling a comprehensive index of how different atoms and their electron orbitals are structured. The publication, “Atomic Energy Levels,” attracted the attention of Niels Bohr, who wrote her a congratulatory letter on the 1949 publication, as well as Linus Pauling, who asked her for information about the ionization potentials of heavy elements, and also thanked her for the volume, in 1950. “When may we expect to have Volume 2?” he asked. The answer turned out to be 1952—and then 1958 for Volume 3.

“Without climbing into a space capsule, indeed without even leaving her desk, tiny Dr. Charlotte Moore Sitterly is blazing a path to the fiery sun,” a 1951 Associated Press article declared. She was described as a “spare, bespectacled lady with probing gray eyes,” although the reporter incorrectly states that Moore Sitterly earned her PhD at Princeton.

Moore Sitterly’s correspondence with Russell continued frequently long after her move to the National Bureau of Standards. Each extended invitations to the other, including to stay at each other’s homes while in town to discuss scientific work. They wrote of their spouses and their vacations. Russell gave news of his children and grandchildren, and reported when someone was ill. Moore Sitterly even brought Russell to Washington to consult on questions of atomic spectra. Later, in a 1977 speech, she joked: “When he came it was I who presented him with his schedule of work.”

Their long professional relationship lasted until Russell died in 1957. “Only three weeks before his death, H.N.R. was discussing with me the electron configurations of the rare-earth elements,” she wrote 20 years later.

Moore Sitterly went to work with Tousey’s group at the Naval Research Laboratory from 1971 to 1978, while still maintaining an office at the Bureau of Standards. She continued analyzing new data from V-2 rockets and later Skylab, the first U.S. space station, to further her understanding of the solar spectrum.

Moore Sitterly’s painstaking cataloging has served countless scientists over the last several decades. Her work influenced the development of lasers and the beginnings of quantum mechanics, says Michael Duncan, professor of chemistry at the University of Georgia and a relative of Moore Sitterly through marriage.

To Duncan, Moore Sitterly was always “Aunt Charlotte.” He would stay with her when he visited Washington in the 1980s, and he remembers her as a short, friendly grandmother figure who made great cherry pie, and would sometimes ask him to trim the bushes.

She was also frugal, Duncan says—even reusing coffee grounds from one day to the next—which likely stemmed from her humble beginnings in Pennsylvania. With no children of her own, she would spend holidays with Duncan’s wife’s family, and give everyone the soap that she had made in her basement for Christmas. She was also very interested in the history of her family.

“Like she did with the atomic tables, she was just meticulous at getting all the details and everything organized and worked out, and so she made and published a genealogy of the Moore family,” Duncan says.

DeVorkin still remembers the day in 1978 when he visited Moore Sitterly for an American Institute of Physics interview. He went around her craftsman cottage on Brandywine Avenue in Washington to find her on the back porch. After he moved to the city in 1980, he would occasionally take Moore Sitterly to meetings.

“I certainly had a feeling that she was very dynamic, very persistent and very clear-headed,” he says.

Moore Sitterly published her last collection of atomic tables in 1985, just shy of her 87th birthday. She died in 1990 before she could receive the prestigious Bruce Medal for lifetime achievement from the Astronomical Society of the Pacific in person. This last accolade came with the title of “The world’s most honored woman astrophysicist.”

As academic institutions reckon with what MSNBC’s Rachel Maddow dubbed “dude walls”—rows of portraits of white male scholars, particularly scientists—Moore Sitterly remains a rare counterexample, a woman who pushed science forward and received many awards in her lifetime. At Princeton, the astrophysics building has been moved downhill and built anew since young Charlotte’s days working above the coal furnace. Still, her face remains absent from the pictures of luminaries, including Russell, that watch over new generations of students chasing secrets hidden in the light of the universe.