Cecilia Payne found out what the Sun is made of. This seems like it should be a bigger deal.
I learned about Cecilia Payne from two books: “15 Million Degrees: A Journey to the Centre of the Sun” by solar physicist Lucie Green, and “13.8: The Quest to Find the True Age of the Universe and the Theory of Everything” by veteran science writer John Gribbin. This post is based mainly on information extracted from those two books. Both authors described her as probably the most underrated astrophysicist of the twentieth century. Her story illustrates the struggle of female scientists to win recognition.
Born in England, Payne studied sciences at Cambridge University but was not awarded a degree- she was disqualified by her fatal lack of a “Y” chromosome. So she made the move to America. Following in the footsteps of Annie Jump Cannon and Henrietta Swan Leavitt, she arrived at Harvard College Observatory in 1923 to join their new graduate program in astronomy. Her PhD thesis was to quietly make history.
When sunlight or the light of a distant star is spilt through a prism, the resulting rainbow spectrum contains spectral lines. A thin, dark line interrupting the colours shows where atoms in the outer atmosphere of the star are absorbing light. Different chemical elements have a strong preference for particular wavelengths of light: for example, hydrogen produces dark lines in four very precise places in the visible spectrum.
The absorption lines in sunlight show mostly the same chemical elements that are common on Earth. So astronomers naturally assumed that the Sun and the Earth are made of similar stuff in similar proportions. The relative strength of the different lines seemed to back this up. Payne showed that this was an illusion.
Her PhD work was a fiendishly complex analysis of the effects of temperature on light absorption by ionised atoms of different elements. She showed that these effects were confusing the picture. In fact, the bulk of the Sun- 99 percent of it- was made up of the two lightest elements, hydrogen and helium. All the rest, including elements like iron and silicon that are very common on Earth, were merely traces- a thin foam of heavy elements on a vast boiling sea of hydrogen and helium. And the other stars were much the same.
When it came to seeking publication, Payne was told that her results were too implausible to be true. She was strongly discouraged from pressing home her conclusions. Instead, she included a caveat saying that her new approach had produced results that were almost certainly wrong. Still, her colleagues knew of her radical new model of the sun, and the results were out there in print.
The idea of a hydrogen/helium Sun percolated gradually through the astronomical community, but not everyone who took it up seems to have known who originated it. It helped provide the answer to one of the burning scientific questions of the day: how has the Sun managed to keep shining for billions of years?
Astrophysicists thought the newly-discovered nuclear reactions might supply the power, but couldn’t get the theory to work for a Sun of Earthlike composition. German physicist Hans Bethe eventually cracked the problem: he found a chain reaction where hydrogen nuclei can fuse together to make helium nuclei, releasing huge amounts of energy. This could only happen naturally in the heart of Cecilia Payne’s “implausible” hydrogen/helium Sun.
Happily, Payne’s career did not end with her marriage as it might well have done a generation previously. She married the astronomer Sergei Gaposchkin (changing her name to Cecilia Payne-Gaposchkin), with whom she collaborated with in some of her later work. But she never did get the recognition that was her due.