The answer to this fundamental question of astrophysics was discovered in 1925 by Cecilia Payne (1900-1979) & explained in her Ph.D. thesis. Payne showed how to decode the complicated spectra of starlight in order to learn the relative amounts of the chemical elements in the stars. In 1960 the distinguished astronomer Otto Struve referred to this work as “the most brilliant Ph.D. thesis ever written in astronomy.”
Cecilia Payne was born in Wendover, England. After entering Cambridge University she soon knew she wanted to study a science but was not sure which one. She then chanced to hear the astronomer Arthur Stanley Eddington (1882-1944) give a public lecture on his recent expedition to observe the 1919 solar eclipse, an observation that proved Einstein’s Theory of General Relativity.
She later recalled her exhilaration: “The result was a complete transformation of my world picture. When I returned to my room I found that I could write down the lecture word for word.” She realized that physics was for her.
Later, at Cambridge Observatory Cecilia told Professor Eddington, that she wanted to be an astronomer. He suggested a number of books for her to read, but she had already read them. Eddington then invited her to use the Observatory’s library, with access to all the latest astronomical journals.
"There is no joy more intense than that of coming upon a fact that cannot be understood in terms of currently accepted ideas." declared Cecilia Payne
Payne realized early during her Cambridge years, that a woman had little chance of advancing beyond a teaching role, & no chance at all of getting an advanced degree in England.
Women in the USA had only won the right to vote in national elections in 1920, just 3 years before Payne left England in 1923 for the United States. Here she met Professor Harlow Shapley (1885-1952), the new director of the Harvard College Observatory, who offered her a graduate fellowship.
Cecilia Payne became the 1st person to earn a PhD in astronomy from Harvard University. Her 1925 graduate thesis proposed that the Sun & other stars were made predominantly of hydrogen, & described as "the most brilliant PhD thesis ever written in astronomy." (Payne received the 1st Ph.D. in astronomy from Radcliffe College for her thesis, since Harvard did not grant doctoral degrees to women.)
But Harvard did have the world’s largest archive of stellar spectra on photographic plates. Astronomers obtain such spectra by attaching a spectroscope to a telescope. This instrument spreads starlight out into its “rainbow” of colors, spanning all the wavelengths of visible light. The wavelength increases from the violet to the red end of the spectrum, as the energy of the light decreases. A typical stellar spectrum has many narrow dark gaps where the light at particular wavelengths (or energies) is missing. These gaps are called absorption “lines,” & are due to various chemical elements in the star’s atmosphere that absorb the light coming from hotter regions below.
The study of spectra had led to the science of astrophysics. In 1859, Gustav Kirchoff & Robert Bunsen in Germany heated various chemical elements & observed the spectra of the light given off by the incandescent gas. They found that each element has its own characteristic set of spectral lines—its uniquely identifying “fingerprint.” In 1863, William Huggins in England observed many of these same lines in the spectra of the stars. The visible universe, it turned out, is made of the same chemical elements as those found on Earth.
Beginning in the 1880s, astronomers at Harvard College such as Edward Pickering, Annie Jump Cannon, Williamina Fleming, & Antonia Maury had succeeded in classifying stars according to their spectra into seven types: O, B, A, F, G, K, & M. It was believed that this sequence corresponded to the surface temperature of the stars, with O being the hottest & M the coolest. In her Ph.D. thesis (published as Stellar Atmospheres ), Payne used the spectral lines of many different elements & the work of Indian astrophysicist Meghnad Saha, who had discovered an equation relating the ionization states of an element in a star to the temperature to definitively establish that the spectral sequence did correspond to quantifiable stellar temperatures. Payne also determined that stars are composed mostly of hydrogen & helium. However, she was dissuaded from this conclusion by Princeton astronomer Henry Norris Russell (1877-1957), who thought that stars surely would have the same composition as Earth. (Russell conceded in 1929 that Payne was correct.)
In principle, it seemed that one might obtain the composition of the stars by comparing their spectral lines to those of known chemical elements observed in laboratory spectra. Astronomers had identified elements like calcium & iron as responsible for some of the most prominent lines, so they naturally assumed that such heavy elements were among the major constituents of the stars. In fact, Princeton's Henry Norris Russell at Princeton had concluded that if the Earth’s crust were heated to the temperature of the Sun, its spectrum would look nearly the same.
When Cecilia Payne arrived at Harvard, a comprehensive study of stellar spectra had long been underway. Annie Jump Cannon (1863-1941) whose cataloging work was instrumental in the development of contemporary stellar classification. Annie was nearly deaf throughout her career. She was a suffragist & a member of the National Women's Party.
Annie Jump Cannon (1863-1941)
Annie had sorted the spectra of several hundred thousand stars into seven distinct classes. She had devised & ordered the classification scheme, based on differences in the spectral features. Astronomers assumed that the spectral classes represented a sequence of decreasing surface temperatures of the stars, but no one was able to demonstrate this quantitatively.
Cecilia Payne, who studied the new science of quantum physics, knew that the pattern of features in the spectrum of any atom was determined by the configuration of its electrons. She also knew that at high temperatures, one or more electrons are stripped from the atoms, which are then called ions. The Indian physicist M. N. Saha had recently shown how the temperature & pressure in the atmosphere of a star determine the extent to which various atoms are ionized.
Payne began a long project to measure the absorption lines in stellar spectra, & within two years produced a thesis for her doctoral degree, the first awarded for work at Harvard College Observatory. In it, she showed that the wide variation in stellar spectra is due mainly to the different ionization states of the atoms & hence different surface temperatures of the stars, not to different amounts of the elements. She calculated the relative amounts of eighteen elements & showed that the compositions were nearly the same among the different kinds of stars. She discovered, surprisingly, that the Sun & the other stars are composed almost entirely of hydrogen & helium, the two lightest elements. All the heavier elements, like those making up the bulk of the Earth, account for less than two percent of the mass of the stars.
Most of the mass of the visible universe is hydrogen, the lightest element, & not the heavier elements that are more prominent in the spectra of the stars! This was indeed a revolutionary discovery. Harlow Shapley sent Payne’s thesis to Professor Russell at Princeton, who informed her that the result was “clearly impossible.” To protect her career, Payne inserted a statement in her thesis that the calculated abundances of hydrogen & helium were “almost certainly not real.”
She then converted her thesis into the book Stellar Atmospheres, which was well-received by astronomers. Within a few years it was clear to everyone that her results were both fundamental & correct. Cecilia Payne had showed for the first time how to “read” the surface temperature of any star from its spectrum. She showed that Cannon’s ordering of the stellar spectral classes was indeed a sequence of decreasing temperatures & she was able to calculate the temperatures. The so-called Princeton Hertzsprung-Russell diagram, a plot of luminosity versus spectral class of the stars, could now be properly interpreted, & it became by far the most powerful analytical tool in stellar astrophysics.
From the time she finished her Ph.D. through the 1930s, Payne advised students, conducted research, & lectured—all the usual duties of a professor. Yet, because she was a woman, her only title at Harvard was “technical assistant” to Professor Harlow Shapley.
In 1933, Payne traveled to Europe to meet Russian astronomer Boris Gerasimovich, who had previously worked at the Harvard College Observatory & with whom she planned to write a book about variable stars. In Göttingen, Ger., she met Sergey Gaposchkin, a Russian astronomer who could not return to the Soviet Union because of his politics. Payne was able to find a position at Harvard for him. They married in 1934 & often collaborated on studies of variable stars. She was named a lecturer in astronomy in 1938, but even though she taught courses, they were not listed in the Harvard catalog until after World War II.
Despite being indisputably one of the most brilliant & creative astronomers of the 20C, Cecilia Payne was never elected to the elite National Academy of Sciences. But times were beginning to change. In 1956, she was finally made a full professor (the 1st woman so recognized at Harvard) & chair of the Astronomy Department.
Her fellow astronomers certainly came to appreciate her genius. In 1976, the American Astronomical Society awarded her the prestigious Henry Norris Russell Prize. In her acceptance lecture, she said, “The reward of the young scientist is the emotional thrill of being the 1st person in the history of the world to see something or to understand something.”
Collections of Cecilia Payne- & Sergei Gaposchkin. Wolbach Library, Harvard & Smithsonian Center for Astrophysics, Cambridge, Mass.
Papers of Harlow Shapley, 1906-1966; HUG 4773.10 Box 89. Harvard University Archives, Harvard University, Cambridge, Mass.
Papers of Cecilia Helena Payne-Gaposchkin, 1924, circa 1950s-1990s, 2000; HUGB P182.5, P182.50. Harvard University Archives, Harvard University, Cambridge, Mass. Link.
Project PHaEDRA. Wolbach Library, Harvard & Smithsonian Center for Astrophysics, Cambridge, Mass. Link.
Radcliffe College Alumnae Association Records, ca.1894-2004; RG IX, Series 2, box 241. Schlesinger Library, Radcliffe Institute, Harvard University, Cambridge, Mass.
Wilbur Kitchener Jordan Records of the President of Radcliffe College, 1943-1960; RG II, Series 3, boxes 27, 60. Radcliffe College Archives, Schlesinger Library, Radcliffe Institute, Harvard University, Cambridge, Mass.
Bartusiak, Marcia. 1993. “The Stuff of Stars.” The Sciences, no. September/October: 34–39.
Boyd, Sylvia. 2014. Portrait of a Binary : The Lives of Cecilia Payne & Sergei Gaposchkin. Penobscot Press.
DeVorkin, David. 2010. “Extraordinary Claims Require Extraordinary Evidence: C.H. Payne, H.N. Russell & Standards of Evidence in Early Quantitative Stellar Spectroscopy.” Journal Od Astronomical History & Heritage 13 (2): 139–44.
Gaposchkin, Cecilia Helena Payne. 1984. Cecilia Payne-Gaposchkin: An Autobiography (“The Dyer’s Hand”) & Other Recollections. Cambridge ; New York: Cambridge University Press.
Gaposchkin, Sergei. 1970. The Divine Scramble. Self-Published.
Gingerich, Owen, Katherine Haramundanis, & Dorrit Hoffleit. 2001. The Starry Universe: The Cecilia Payne-Gaposchkin Centenary. L. Davis Press.
Popova, Maria. 2017. “Stitching a Supernova: A Needlepoint Celebration of Science by Pioneering Astronomer Cecilia Payne.” Brain Pickings (blog). May 10, 2017.
Woodman, Jennifer. 2016. “Stellar Works: Searching for the Lives of Women in Science.” Dissertations & Theses, June.