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Cecilia 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. Independent observations eventually proved she was correct. Her work on the nature of variable stars was foundational to modern astrophysics.Photograph credit: Science Service; restored by Adam Cuerden