She explained why some atomic nuclei are oddly stable, then won a Nobel for it
Physicists had noticed that nuclei with exactly 2, 8, 20, 50, 82 or 126 protons or neutrons were unusually stable, but nobody knew why. In 1949, Maria Goeppert Mayer showed that nucleons arrange themselves into shells inside the nucleus, much like electrons do around it, and that these particular counts fill a shell completely. She shared the 1963 Nobel Prize in Physics for it, becoming only the second woman ever to win that prize, after Marie Curie.