Cecilia Helena Payne-Gaposchkin (née Payne; May 10, 1900–December 7, 1979) was a Britishborn 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 diffe…