Cecilia Payne-Gaposchkin and the Thesis That Found What Stars Are Made Of

On April 22, 2026, English Heritage unveiled a blue plaque on the house at 70 Lansdowne Road in Notting Hill, West London. The charity puts these plaques on buildings where remarkable people lived, and this one honors a teenager who lived there in the years just after the First World War. While she lived in that house, she won the scholarship that took her to Cambridge. Six years later, at an observatory across the Atlantic, she worked out what the stars are made of.

Her name was Cecilia Payne, later Cecilia Payne-Gaposchkin. In 1925 her doctoral thesis showed that the Sun and the stars are made almost entirely of hydrogen and helium, the two lightest elements, and not, as nearly every astronomer then believed, of roughly the same stuff as the Earth. It is one of the most important results in the history of astronomy. It is also a result that one of the most powerful astronomers in America talked her out of believing, in print, until he reached the same conclusion himself four years later.

“Cecilia Payne-Gaposchkin was a scientist of exceptional brilliance and determination,” said Howard Spencer, a senior historian at English Heritage, when the plaque was announced. The unveiling capped more than a year of centenary events, from a long feature in Smithsonian magazine to celebrations at her old Cambridge college. The attention is overdue. Her discovery is now textbook basics, but for decades the person who made it was barely known outside her field.

A schoolgirl in a hurry

Cecilia Helena Payne was born on May 10, 1900, in Wendover, Buckinghamshire, northwest of London. Her father, a barrister, historian and musician, died when she was four, and her mother raised three children on her own. When Cecilia was twelve, the family moved to London so her younger brother, Humfry, could go to better schools. He later became an archaeologist and director of the British School at Athens.

Her own schooling was a fight for science. Her first London school didn’t offer mathematics or science to her at all. In 1918 she moved to St Paul’s Girls’ School, where her music teacher was the composer Gustav Holst, who encouraged her to become a musician. She chose science instead. Years later she recalled the moment she decided to do research, and the feeling that came with it: “I made up my mind to do research, and was seized with panic at the thought that everything might be found out before I was old enough to begin!”

In 1919 she won a scholarship to Newnham College, Cambridge, to study physics and chemistry. Cambridge would teach women but wouldn’t give them degrees. She finished her studies there, but the university didn’t award degrees to women until 1948. According to the Wolbach Library at the Center for Astrophysics, Harvard and Smithsonian, Ernest Rutherford made her sit by herself in the front row of his physics lectures.

Three nights without sleep

What turned her toward the sky was a single lecture. At Cambridge, Arthur Eddington spoke about his expedition to the island of Príncipe, off the west coast of Africa, where he had photographed stars near the Sun during the total eclipse of May 29, 1919. The positions of those stars had shifted by the amount Einstein’s general theory of relativity predicted. It was the result that made Einstein world-famous.

It changed everything for her. “The result was a complete transformation of my world picture,” she wrote later. “My world had been so shaken that I experienced something very like a nervous breakdown.” According to her autobiography, as quoted by Smithsonian, she went home and didn’t sleep for three nights. She turned to astronomy, and she came to know Eddington, whom she later called the greatest intellect she ever had the privilege to meet.

But Britain had little to offer a woman astronomer without a degree. The realistic option was teaching. Then Leslie Comrie, a young astronomer at Cambridge, introduced her to Harlow Shapley, the director of the Harvard College Observatory, who was lecturing in England. Shapley later recalled that “a tall young woman” in his audience had “just drank it in.” She told him she wanted to work for him. Harvard had a new fellowship meant to bring women to the observatory. Adelaide Ames had been its first holder in 1922. Payne became the second, and in 1923 she sailed for Massachusetts.

Reading the rainbow

To see why her result mattered, it helps to know how anyone could tell what a star is made of in the first place. In his Course of Positive Philosophy, the French philosopher Auguste Comte wrote that we would never learn the chemical composition of the stars. It seemed like a safe bet. You can’t scoop up a sample of a star. Within a few decades, though, Comte was proven wrong, and the tool that did it was the rainbow.

When sunlight is spread out into its colors by a prism, the spectrum isn’t smooth. In 1802 the English chemist William Hyde Wollaston noticed a few dark gaps in it. In 1814 the German optician Joseph von Fraunhofer, working independently, looked much more carefully and mapped 574 of these dark lines, labeling the strongest with letters that are still used today. Nobody knew what they were.

The answer came from Heidelberg. Working together in the late 1850s, the physicist Gustav Kirchhoff and the chemist Robert Bunsen showed that every chemical element, when heated, glows at its own set of colors, and that a cooler gas in front of a hot light source absorbs exactly those same colors. The dark lines in sunlight were fingerprints of elements in the Sun’s outer layers. In 1859 Kirchhoff showed that the Sun contains sodium. The same method let the pair discover two new elements on Earth, caesium and rubidium, from their spectral lines. Less than a decade later, astronomers watching an eclipse saw a yellow line that matched no known element, and the Sun gave its name to helium, the element first found in the Sun during the 1868 eclipse.

Spectroscopy could tell you which elements were in a star. What it couldn’t easily tell you was how much of each. The strength of a line depends on how much of an element there is, but it also depends on how hot the gas is and on what state the atoms are in. For decades astronomers treated line strength as a rough guide to quantity, and by that guide the Sun looked a lot like the Earth. In a 1914 paper, Henry Norris Russell of Princeton, one of the leading astronomers in America, compared the elements in the Sun with those in the Earth’s crust. “The agreement of the solar and terrestrial lists,” he wrote, was such as to confirm “very strongly” the older view that if the Earth’s crust were heated to the temperature of the Sun’s atmosphere, “it would give a very similar absorption spectrum.”

A sequence nobody could explain

Harvard had become the world’s great collector of stellar spectra. In 1872 the American astronomer Henry Draper had made the first photograph of a star’s spectrum showing distinct lines, of the bright star Vega. After his death his widow, Mary Anna Palmer Draper, paid for a vast Harvard project to photograph and classify the spectra of stars across the sky. The women of the observatory, the so-called Harvard computers whose work also produced Henrietta Leavitt’s law for measuring the distances to galaxies, sorted the results by hand. The Henry Draper Catalogue, published between 1918 and 1924, gave classifications for 225,300 stars. Annie Jump Cannon, who did most of the classifying, worked through about 350,000 stars in her lifetime.

Cannon’s scheme sorted stars into a sequence of letters: O, B, A, F, G, K, M. Generations of students have memorized it with the line “Oh Be A Fine Girl, Kiss Me.” The sequence worked beautifully as a filing system. Stars of the same class looked alike, and the classes ran in a smooth order. But nobody knew what the order meant. Were O stars made of different stuff from M stars? Were they at different stages of life? Or was something else going on?

The key came from India. In 1920 the physicist Meghnad Saha worked out an equation that described how the atoms in a hot gas lose their electrons, a process called ionization, depending on temperature and pressure. An atom that has lost an electron absorbs light at different colors than one that hasn’t. So a star’s spectrum depended heavily on its temperature, not just on its ingredients. The Indian astrophysicist Jayant Narlikar later counted Saha’s equation among the top ten achievements of 20th-century Indian science and said it “could be considered in the Nobel Prize class.” In Britain, Ralph Fowler and Edward Milne refined Saha’s approach, and shortly before Payne left for America, Milne told her that in her shoes he would use the Harvard plates to take Saha’s work from theory to practice.

Harvard, 1923

She arrived at an observatory that was, for a young woman scientist, both a gift and a trap. Shapley gave her the run of Harvard’s enormous collection of glass plates. She also learned that the observatory had a habit of treating women’s labor as cheap and plentiful. Projects there, she noticed, were measured in “girl hours.”

The plates seemed so precious that she hesitated to touch them. “When a little later I inadvertently sat down on a pile of plates, I rushed out of the observatory and did not dare to return for several days,” she wrote in her autobiography. “Fortunately no damage had been done.” One spectrum she used often, Science News notes, was a plate of the bright star Capella, catalogued at Harvard as C05256, with hydrogen’s lines labeled, probably in her hand. When Saha himself visited Harvard, she met him.

Her plan was to use Saha’s physics, in the form that Milne and Fowler had developed, to do what nobody had done: turn the strengths of lines in stellar spectra into actual temperatures and actual amounts of each element. To do that, she had to estimate how much of each element must be present for a line to be just barely visible on the plate, then correct for the fraction of atoms that were in the right state to absorb at that color. It was painstaking work across many stars and many elements, and she did it in about two years.

The photograph above was taken at the observatory in 1925, the year she finished. Payne stands in the back row, second from the left. Annie Jump Cannon is in the middle row, and Antonia Maury is in the front row. These were the women who had built the catalog her thesis would explain.

The numbers that made no sense

Her first big result was the one she was looking for, and it was a triumph. When she applied Saha’s equation across the spectral sequence, the O-B-A-F-G-K-M order fell into place as a sequence of temperatures. The differences between the classes came mostly from heat changing the state of the atoms, not from the stars being made of different things. She assigned temperatures to the classes, and she found that the chemical makeup of stars was strikingly uniform. In the American Astronomical Society’s summary, she derived abundances for 18 elements and found them nearly the same from one class of star to the next.

For the heavier elements, her results confirmed what Russell and others already thought: their proportions in the stars looked broadly like those on Earth. That part fit the old picture. Two elements did not. Helium came out far more abundant than anyone expected. Hydrogen came out about a million times more abundant than the metals.

The reason nobody had seen this before is one of the elegant points of her work. At the temperature of the Sun’s surface, around 5,700 kelvin, almost all hydrogen atoms are in their lowest-energy state, which doesn’t absorb visible light. As the American Physical Society explained in a 2015 history, only about one hydrogen atom in 200 million is in the excited state that produces the visible hydrogen lines astronomers saw. The lines looked ordinary, so astronomers assumed hydrogen was an ordinary ingredient. Once you correct for the tiny fraction of atoms that can absorb, the ordinary-looking lines imply an enormous amount of hydrogen.

She finished the thesis on January 1, 1925. It was a stunning claim: the stars were not like the Earth heated up. They were mostly hydrogen, with a good deal of helium, and only traces of everything else.

“Clearly impossible”

Harvard did not give doctorates to women, so her degree would come through Radcliffe College, Harvard’s college for women. On January 4 she spoke to Russell at Princeton about her draft, looking for advice. Ten days later he wrote back. “It is clearly impossible,” he said, “that hydrogen should be a million times more abundant than the metals.”

It is hard to overstate how much weight Russell carried. He was one half of the Hertzsprung-Russell diagram, the most famous chart in astronomy, and a gatekeeper for American astrophysics. Payne knew what that meant. Later that month she wrote to a friend that she would “not be afraid personally of him any more,” but added that his power was another matter: “the fate of such as I could be sealed by him with a word.”

She gave way. The final thesis presented her hydrogen and helium numbers and then disowned them. “The enormous abundance derived for these elements in the stellar atmosphere is almost certainly not real,” she wrote. Elsewhere it said “the actual values derived from the estimates of marginal appearance are regarded as spurious.” The thesis was accepted in April 1925, and Harvard Observatory published it that year as a book, Stellar Atmospheres, the first in its series of monographs.

Four years later, in 1929, Russell published his own long analysis of the Sun’s atmosphere. He found that hydrogen dominated. He cited Payne’s work and noted the “very gratifying agreement” between their results. For a long time afterward, it was Russell’s paper that got the credit. By 1965, according to Smithsonian, it had been cited more than three times as often as her thesis.

Was she silenced?

The story is often told as a clean morality tale: a great man crushed a young woman’s discovery and then took it for himself. Historians who have studied the letters say it’s more complicated, and the complications are interesting in their own right.

David DeVorkin, historian emeritus at the Smithsonian’s National Air and Space Museum and a biographer of Russell, doesn’t think Russell’s caution was about her being a woman. Her conclusion was so radical, he told Smithsonian, that Russell felt it needed more evidence. In 1925 Saha’s ideas were still new and not universally accepted, and Payne herself had called her hydrogen and helium figures “improbably high.” To a careful theorist, the most likely explanation was that something about those particular lines was misleading, not that the universe was made of something nobody expected. DeVorkin also noticed something about how Payne handled the result. She kept the numbers in the thesis, he wrote, “in a manner that was designed to record for posterity that she was the first” to find them.

A history page maintained at the Center for Astrophysics, Harvard and Smithsonian, even warns readers against the “THE MAN” version of the story, in which Russell is simply the villain. Russell’s 1929 paper did acknowledge her work, and the agreement between them.

None of that changes what happened to her. She had the answer first, she was told it was impossible, and for decades the citation counts followed the more famous name. In a later interview she put it plainly: “Henry Norris Russell talked me out of” it.

“The most brilliant Ph.D. thesis ever written in astronomy”

The astronomers who mattered understood. In their 1962 history of 20th-century astronomy, Otto Struve and Velta Zebergs called her work “undoubtedly the most brilliant Ph.D. thesis ever written in astronomy.” She was the first person to earn a PhD in astronomy from Radcliffe. In 1927 she became the youngest astronomer to be given a star, the mark of distinction, in the reference book American Men of Science. In 1934 she received the first Annie J. Cannon Award in Astronomy.

She wrote the thesis itself, according to the Wolbach Library, in a “six week ecstasy.” Late in life, giving the American Astronomical Society’s Henry Norris Russell Lectureship, the honor named for the man who had told her she was wrong, she described what science had given her.

“The reward of the young scientist is the emotional thrill of being the first person in the history of the world to see something or understand something. Nothing can compare with that experience. The reward of the old scientist is the sense of having seen a vague sketch grow into a masterly landscape.”

Girl hours

The thesis made her famous among astronomers, but Harvard was slow to make it official. For years her job title was a variant of “technical assistant” to Shapley. The courses she taught weren’t listed in the Harvard catalogue until 1945. Dorrit Hoffleit, who worked at the observatory for years, called her “the most brilliant, and at the same time, the person most discriminated-against, at Harvard College Observatory.”

She kept working at a pace that is hard to believe. After her thesis she turned to the brightest and most massive stars, and then to variable stars, the ones that change in brightness. With her assistants she made more than 1,250,000 observations of variable stars, and then extended the work to the Magellanic Clouds, the two small galaxies near the Milky Way where Henrietta Leavitt had found her famous pulsing stars, adding about 2,000,000 more. She wrote books including The Stars of High Luminosity (1930) and Variable Stars (1938).

In 1931 she became an American citizen. On a tour of Europe in 1933 she met a Russian-born astronomer, Sergei Gaposchkin, and helped him get a visa to the United States. They married in March 1934 and settled in Lexington, Massachusetts. She added his name to her own, and they raised three children. Her variable-star work with Sergei laid the groundwork for later studies of such stars.

Shapley finally got her the title of “Astronomer” in 1938, after assuring the university that it wouldn’t make her the equal of a professor. Real change came under his successor, Donald Menzel. In 1956 she became the first woman promoted to full professor from within Harvard’s Faculty of Arts and Sciences, and then the first woman to chair a department at Harvard. Her students included Frank Drake, who went on to pioneer the radio search for other civilizations, the hunt that later produced the Wow! signal from Sagittarius, as well as Helen Sawyer Hogg, Frank Kameny and Owen Gingerich. She retired from teaching in 1966, kept working at the Smithsonian Astrophysical Observatory, and died at her home in Cambridge, Massachusetts, on December 7, 1979. Shortly before her death she had her autobiography privately printed under the title The Dyer’s Hand.

From recipe to fire

Her discovery did more than settle an argument about ingredients. It turned out to be the piece that made the rest of stellar physics work.

In 1920, Eddington had proposed that the stars shine by fusing hydrogen into helium. Once astronomers accepted that stars are mostly hydrogen, his idea had all the fuel it needed. In 1939 the physicist Hans Bethe worked out the nuclear reactions that power the Sun, including the carbon-nitrogen-oxygen cycle, a chain in which those heavier elements help hydrogen turn into helium. The same physics, applied on Earth, became the hydrogen bomb tested at Castle Bravo in 1954.

Her numbers also fed the question of where elements come from in the first place. In April 1948, Ralph Alpher, Hans Bethe and George Gamow published “The Origin of Chemical Elements” in Physical Review, the paper that began the modern theory of how the lightest elements were forged in the hot early universe. Why the universe is about three-quarters hydrogen and a quarter helium by mass, with a dusting of everything else, is now one of the best-tested predictions of Big Bang cosmology.

In 2020, an experiment buried under the Gran Sasso mountains in Italy closed one of those loops. The Borexino detector reported in Nature the first evidence for neutrinos from the Sun’s carbon-nitrogen-oxygen cycle, the reaction chain Bethe described in 1939. The CNO cycle makes only about one percent of the Sun’s energy, with the proton-proton chain making the rest. The researchers noted that the CNO neutrino flux depends on how much carbon, nitrogen and oxygen sits in the Sun’s core, opening a new way to measure the very thing Payne set out to measure: what the Sun is made of.

A century later, still arguing about the recipe

Payne’s basic answer has held up completely. A 2025 review chapter on the Sun’s chemical composition by Maria Bergemann, Katharina Lodders and Herbert Palme gives the hydrogen in the Sun’s visible surface as about 73.9 percent by mass, helium as about 24.5 percent, and everything else, all the elements astronomers call “metals,” at about 1.6 percent.

The details, though, are still a live fight, which she probably would have enjoyed. Since the early 2000s, improved models of the Sun’s atmosphere had lowered the estimated amounts of carbon, nitrogen, oxygen and neon. That created a problem. Helioseismology, the study of sound waves ringing through the Sun, measures the Sun’s interior directly, and models built with the lower values didn’t match it, because the heavier elements control how easily energy passes through the solar interior. The 2025 review notes that newer analyses, including Magg and colleagues in 2022 and Lodders and colleagues in 2025, come out higher and closer to the older values, and that with them “the conflict between helioseismic data and solar models may be resolved.” Not everyone agrees yet. A 2024 study in Nature Communications that used solar oscillations to infer the opacity of the Sun’s interior opened by noting “the current debate on its chemical composition.”

Her method also grew into a whole field. Astronomers now measure the chemical makeup of stars by the thousands. Natalie Hinkel, who built the Hypatia Catalog of stellar compositions for more than 13,000 stars, told Smithsonian, “It was really Cecilia Payne who started this field.” Those measurements now help astronomers guess what rocky planets around other stars might be made of, since a star and its planets grow from the same cloud.

A plaque, a century on

The house on Lansdowne Road joins a list of London plaques that also honors Eddington, the man whose eclipse lecture lit the fuse. At the April unveiling, speakers included the astrophysicist Jocelyn Bell Burnell, who discovered the first pulsars as a graduate student, and the planetary scientist Michele Dougherty. Cambridge, which once refused her a degree, marked the thesis centenary at Newnham in December 2025.

Cecilia Payne-Gaposchkin spent her career being underestimated and kept working anyway. Science News closes its centenary profile with a line of hers that reads like an answer to all of it, and to that letter from Princeton in January 1925.

“The truth will prevail in the end. Nonsense will fall of its own weight, by a sort of intellectual law of gravitation.”

If you look up on a clear night, nearly every star you see is what she said it was: mostly hydrogen and helium, the same simple recipe she read off a stack of glass plates a century ago. If you want to see more, our guide to the best home telescopes is a good place to start, and the galaxies named after sombreros, owls and other everyday things are made of the same stuff.

Further reading