Cecilia Helena Payne-Gaposchkin (1900 – 1979) was a British-born American astronomer and astrophysicist who proposed in her 1925 doctoral thesis that stars were composed primarily of hydrogen and helium. Her groundbreaking conclusion was initially rejected because it contradicted the scientific wisdom of the time, which held that there were no significant elemental differences between the Sun and Earth.

Payne realized that her only career option in the U.K. was to become a teacher, so she looked for grants that would enable her to move to the United States. After being introduced to Harlow Shapley, the Director of the Harvard College Observatory, where he had just established a graduate program in astronomy, she left England in 1923. This was made possible by a fellowship to encourage women to study at the observatory. Adelaide Ames had become the first student on the fellowship in 1922; the second was Payne.

Payne wrote a doctoral dissertation in 1925 then became the first person to earn a PhD 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."

Payne-Gaposchkin retired from active teaching in 1966 but continued her research as a member of staff at the Smithsonian Astrophysical Observatory, as well as editing the journals and books published by Harvard Observatory for twenty years.

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