In the early 1900s, most astronomers mistakenly believed that 66 percent of the sun's substance was iron. As a graduate student at Harvard University in the 1920s, Cecilia Payne—later a professor of astronomy there—argued pioneeringly that the sun is instead composed largely of hydrogen and helium. Her claim, though substantiated by the evidence and later uniformly accepted, encountered strong resistance among professional astronomers.

The orthodox view that the sun was mainly iron was buttressed by the knowledge that Earth and all known asteroids contain iron. Also, the evidence from spectroscopy—a technique used to identify chemicals by the distinctive spectral properties of the light patterns they emit when heated to incandescence—was generally taken to show that iron was the predominant element in the sun. But how could a body composed largely of iron generate the huge energy output of the sun? The eminent British physicist Lord Kelvin had hypothesized that the sun was continuously contracting and that the resulting compression had raised the temperature of the sun's materials sufficiently to account for its enormous heat. But, given the usual assumptions about the sun's size and rate of contraction, it followed that the sun's age would be about 20 million years; evidence from the fossil record, however, strongly suggested that the sun had warmed Earth for billions of years. For Payne, this meant that the "iron" hypothesis had to be reexamined, together with the extensive spectroscopic data alleged to support it.

Preliminary examination of the spectroscopic data convinced Payne that they lent themselves to multiple readings. She suspected that preconceptions about the sun's makeup as being mainly iron might have led to skewed interpretations of that data, and this led her to subject the data to rigorous critical scrutiny and review. Analyzed without preconceptions, she found, the data could be consistently read as indicating that, while it does indeed contain iron (along with other elements found on Earth), 90 percent of the sun is hydrogen and most of the remainder is helium. Most astronomers at the time dismissed Payne's interpretation, and some sought to explain it away simply by claiming that what she had examined was data about the sun's outer surface rather than its interior.

Absent a generally accepted explanation of how hydrogen and helium could produce the sun's energy, Payne's findings could not easily override her contemporaries' preconceptions. We now know that the sun's heat is generated through nuclear fusion: the sun's gravitational force compresses together atoms of hydrogen, causing a nuclear reaction. This reaction produces enormous amounts of energy, while forming helium and other elements. But this process—so well charted today that even elementary physics textbooks discuss it—was inadequately understood in the 1920s. The emergence of that understanding—which relied on Einstein's equation governing the relationship between mass and energy—eventually provided strong confirmation of Payne's results.