Which astronomer discovered that stars are mainly composed of hydrogen and helium?

Cecilia Payne-Gaposchkin discovered that stars are mainly composed of hydrogen and helium.

In her 1925 doctoral thesis, Payne used stellar spectra and the emerging science of ionization to show that hydrogen and helium were vastly more abundant in stars than heavier elements. Her conclusion challenged the prevailing assumption that stars had compositions similar to Earth.

Astronomer Henry Norris Russell initially discouraged the conclusion, but later work confirmed Payne’s interpretation. Russell published a paper in 1929 reaching a similar result, and Payne’s contribution was subsequently recognized as fundamental.

Payne later became a leading astronomer at Harvard University and the first woman to head a department there. Her work also included studies of stellar evolution and the Milky Way. Spectroscopy, the analysis of light separated into wavelengths, was essential to identifying stellar chemistry.