A PhD thesis proved stars are made almost entirely of hydrogen

In 1925, Cecilia Payne-Gaposchkin's Harvard doctoral thesis showed hydrogen and helium vastly outweigh every other element in stars, against the assumption that stars share Earth's rocky composition. The result was so unexpected that a senior astronomer talked her into calling it 'spurious' in print — he confirmed she was right four years later. It has been called the most brilliant PhD thesis in the history of astronomy.

— Cecilia Payne-Gaposchkin, Stellar Atmospheres — PhD thesis, Radcliffe College (Harvard), 1925