Cecilia Payne-Gaposchkin wrote what is considered to be "the most brilliant Ph.D. thesis ever written in astronomy," in which her detailed her theory that stars are made mainly of hydrogen and helium. It was contradictory to the beliefs at the time that there was no difference in the elements between the sun and the earth. Payne accurately applied the ionization theory of Meghnad Saha to relate the spectral classes of stars to their actual temperatures.

She studied millions of stars throughout her career in order to try and understand the structure of the milky way.