Cecilia Payne-Gaposchkin (May 10, 1900 – December 7, 1979) was an English-American astronomer and astrophysicist who in 1925 proposed in her PhD thesis an explanation for the composition of stars in terms of the relative abundances of hydrogen and helium.
In 1925 she became the first person to earn a Ph.D. in astronomy from Radcliffe College (now part of Harvard). Her thesis was “Stellar Atmospheres, A Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars”. Astronomer Otto Struve called it “undoubtedly the most brilliant Ph.D. thesis ever written in astronomy”.
Payne was able to accurately relate the spectral classes of stars to their actual temperatures by applying the ionization theory developed by Indian physicist Meghnad Saha. She showed that the great variation in stellar absorption lines was due to differing amounts of ionization at different temperatures, not to different amounts of elements. She correctly suggested that silicon, carbon, and other common metals seen in the Sun’s spectrum were found in about the same relative amounts as on Earth, but that helium and particularly hydrogen were vastly more abundant (for hydrogen, by a factor of about one million). Her thesis thus established that hydrogen was the overwhelming constituent of the stars (see Metallicity).
When Payne’s dissertation was reviewed, astronomer Henry Norris Russell dissuaded her from concluding that the composition of the Sun is different from that of the Earth, contradicting the accepted wisdom at the time. However, he changed his mind four years later after deriving the same result by different means. After Payne was proven correct, Russell was often given the credit, although he himself acknowledged her work in his paper.