Cecilia Payne and the Ingredients of Stars

A Puzzle in Starlight

Cecilia Payne, later known as Cecilia Payne-Gaposchkin, wanted to know what stars are made of. Born in England in 1900, she became fascinated by science. In 1923, she moved to the United States to study stars at Harvard College Observatory. At the time, many scientists thought stars contained roughly the same mixture of elements as Earth. Elements are basic materials, such as iron and hydrogen. Payne would find evidence that challenged this idea.

Her research did not require a journey into space. Instead, she studied starlight that had been spread into a band of colors called a spectrum. Dark lines crossed these bands where certain colors were missing. Different elements could leave different patterns of lines, somewhat like identifying marks. A diagram showing these bands could have the caption, "Missing colors offer clues about a star's materials." Scientists could compare the patterns with those produced by elements studied on Earth.

A New Way to Read the Lines

Reading those patterns was not simple. A strong line did not necessarily mean that a star contained a large amount of an element. Temperature also affected the lines. Payne drew on other scientists' work about how heat changes atoms, the tiny pieces that make up matter. In very hot conditions, atoms can lose electrons, even tinier parts. Such changes affect the lines an element produces. Payne used this knowledge to make a better interpretation, or explanation, of the patterns. She could then compare the amounts of different elements more accurately.

In her 1925 thesis, a long research paper for an advanced degree, Payne reached a surprising conclusion. Hydrogen and helium were far more abundant, or plentiful, in stars than heavier elements such as iron. The stars were not simply hot versions of Earth. Their ingredients were mixed in very different amounts. Her careful calculations revealed something that a quick look at the lines could miss.

Following the Evidence

At first, an experienced astronomer questioned her conclusion about hydrogen. Payne included a warning in her paper that this result might not be correct. Yet she still presented her calculations. Later research by other astronomers supported her finding. The evidence gradually helped change scientists' understanding of stars. A surprising answer had become stronger because researchers could check it, not because everyone immediately agreed.

Payne continued studying stars for decades and died in 1979. Her work showed why collecting clues is just the beginning of science. By explaining how temperature affected those clues, she uncovered the main ingredients of stars. Careful interpretation had turned familiar patterns of light into a new picture of the universe.