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. 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" 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 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 on Einstein's equation governing the relationship between mass and energy—eventually provided strong confirmation of Payne's results.
What this question is testing
Anticipate
This is an Inference question about a particular group's assumption — the scientists who tried to "explain away" Payne's findings. The question is: what did they have to assume for their explain-away move to work?
Their move: For that to save the iron hypothesis, the surface (which Payne's data show is hydrogen-rich) and the interior (which they want to keep as iron) have to be made of different stuff. Otherwise their move doesn't rescue anything.
Goal
Look for the answer that says the sun's interior is not the same composition as its outer surface. Common traps:
Answers about nuclear fusion — those are about the P4 fix, not the explain-away
Answers about Payne's preconceptions — that's flipping the passage, since Payne is the one who suspected preconceptions distorted the data
Answers about spectroscopy's limits — not the assumption these specific scientists made
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