Cecilia Payne-Gaposchkin

Cecilia Payne-Gaposchkin's 1925 doctoral thesis has been called "the most brilliant PhD thesis ever written in astronomy." In it, she demonstrated that stars are composed primarily of hydrogen and helium — a finding that contradicted the prevailing assumption that stars had roughly the same elemental composition as Earth. Born in England, Payne was inspired by Arthur Eddington's lectures on relativity at Cambridge. Frustrated by the lack of opportunities for women in British academia, she moved to Harvard College Observatory, where she became the first person to earn a PhD in astronomy from Radcliffe College. Applying the newly developed theory of ionization by Meghnad Saha, she analyzed the absorption lines in stellar spectra and showed that the vast differences between stellar spectra were due primarily to temperature, not composition. When her thesis advisor Henry Norris Russell initially dissuaded her from her hydrogen conclusion, she relented — but Russell later confirmed her result and, to his credit, acknowledged her priority.

• Proved that hydrogen and helium dominate stellar composition (1925) • Showed that spectral differences between stars are mainly due to temperature • First woman to earn an astronomy PhD from Harvard/Radcliffe • First female full professor at Harvard (1956) • Applied Saha ionization theory to stellar spectral classification