Stars have roughly the same elemental composition as Earth.

Astronomers can learn what a star contains by spreading its light into a spectrum.

Dark and bright lines appear at particular wavelengths because atoms and ions absorb or emit specific energies of light. Early stellar spectra contained recognizable lines from iron, calcium, sodium, hydrogen, and many other elements also found on Earth.

It was tempting to assume that the strongest-looking lines came from the most abundant elements and that stars therefore had a roughly Earth-like chemical mixture.

Cecilia Payne showed why that inference failed.

In her 1925 doctoral work, she applied the new physics of ionization to stellar atmospheres. Temperature determines how many electrons an atom has lost and which energy states are populated. That, in turn, determines which spectral lines become strong or weak.

A weak line can therefore come from an abundant element in the wrong physical state, while a strong line can come from a much less abundant element whose atoms happen to produce that transition efficiently.

Once temperature and ionization were taken into account, hydrogen and helium emerged as overwhelmingly dominant in ordinary stars.

Heavier elements are still crucial. Their smaller abundances vary among stars and preserve information about earlier generations of stellar evolution and nucleosynthesis. What changed was the idea that a stellar spectrum could be read as a simple abundance chart resembling Earth’s composition.