Which astronomer showed in 1925 that hydrogen and helium dominate the composition of stars?

Cecilia Payne-Gaposchkin showed in 1925 that hydrogen and helium dominate the composition of stars.

Her doctoral dissertation at Harvard examined stellar spectra and applied newly developing ideas about ionization. She concluded that stars contained far more hydrogen and helium than heavier elements, overturning the then-common assumption that stars had roughly Earth-like chemical proportions.

The result was initially treated cautiously, partly because influential astronomer Henry Norris Russell doubted the conclusion. Russell later reached a similar result by a different method and helped establish it as correct.

Payne-Gaposchkin became a leading Harvard astronomer and, in 1956, the first woman to head a department there. Her work is sometimes confused with Annie Jump Cannon’s stellar classification system, but Payne-Gaposchkin used spectroscopy to infer stellar composition rather than creating the classification scheme itself.