On the morning of 26 May 1925, in a small examination room on the second floor of Radcliffe College in Cambridge, Massachusetts, a twenty-five-year-old British graduate student named Cecilia Payne defended a doctoral thesis titled Stellar Atmospheres: A Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars.
The examining committee consisted of Harlow Shapley, then director of the Harvard College Observatory, and two Harvard physicists. The defence ran for about ninety minutes. Payne was passed without revisions.
The thesis, published later that year by the Harvard Observatory under the title Stellar Atmospheres, was for a generation the most important doctoral dissertation in twentieth-century astrophysics. Otto Struve described it, in 1962, as the most brilliant Ph.D. thesis ever written in astronomy.
It contained, among many other results, the following claim. Stars are made overwhelmingly of hydrogen and helium. The heavier elements that make up the bulk of the Earth, and the bulk of the human body, exist in stellar atmospheres only at the level of fractions of one percent.
The claim, by 1925, was implicit in the spectroscopic record. No one had pushed it through to a quantitative answer because no one had had Payne's combination of training in the new quantum theory and access to the Harvard plate library.
Payne had been born in Wendover, Buckinghamshire, in 1900. She had read botany at Newnham College, Cambridge, and had been moved permanently toward astronomy by Arthur Eddington's lectures, in 1919, on the results of the solar-eclipse expedition that had confirmed Einstein's prediction of light-bending.
She had completed the natural-sciences tripos at Cambridge but had not been awarded a degree, because Cambridge did not award degrees to women until 1948. She had applied to Harvard for graduate study in 1922 and had been accepted by Shapley, who was at that point trying to build up the observatory's research staff with people whose ideas were larger than its budget.
Her tools at Harvard were the same plate library that Annie Cannon had used to construct the Henry Draper Catalogue and the same quantum-theoretical apparatus that the Indian physicist Meghnad Saha had developed in Calcutta and London between 1920 and 1923.
Saha's equation, published in 1920 in the Philosophical Magazine, related the degree of ionisation of a gas to its temperature and pressure. The implication for stellar spectroscopy was direct. The strength of an absorption line in a stellar spectrum depended not only on the abundance of the element responsible but on the temperature and pressure at which the absorbing layer existed.
Without Saha's equation, the strength of a hydrogen Balmer line could not be translated into a hydrogen abundance. With it, the translation became, in principle, tractable.
Payne did the translation. She worked her way, line by line, through the absorption spectra of about a dozen well-observed stars across the temperature sequence. She applied Saha's equation. She derived abundances.
The abundances of the heavier elements, derived this way, agreed with the abundances of the same elements in the Earth's crust to within an order of magnitude or so. The agreement was widely expected. It was the conventional view, in 1925, that the Sun and the Earth had formed from the same material and should have, broadly, the same composition.
The abundance of hydrogen, derived this way, came out larger than the abundances of all the other elements combined by a factor of about a million.
Payne wrote, in the second-to-last chapter of the thesis, that the result was almost certainly not real.
She did not write that because she doubted her own data. She wrote that because Henry Norris Russell, then the most senior stellar astrophysicist in the United States, had written to her, on receiving an advance copy of the thesis chapter, to say that the hydrogen result was almost certainly spurious and that she would be wise to say so in print.
Russell's reasoning was based on the same conventional view that had led to the equal-composition assumption. The Sun and the Earth ought to be made of the same things. The hydrogen result contradicted that. Therefore something must be wrong with the analysis of the hydrogen lines.
Payne added the disclaimer. The relevant sentence runs: The enormous abundance derived for these elements in the stellar atmosphere is almost certainly not real. It is the only sentence in the thesis that subsequent astrophysics has unambiguously contradicted.
Four years later, in 1929, Russell himself, working with the same Saha-equation methods and a different sample of stars, reached the same conclusion. He published it as his own result in The Astrophysical Journal. He cited Payne, in a footnote, as having reached a similar conclusion earlier.
The hydrogen-dominant composition of the Sun is now sometimes credited to Russell, sometimes to Payne, and sometimes to both. The priority is, on the documentary record, Payne's. The disclaimer she was persuaded to insert has not protected her position in the priority dispute. The disclaimer has been used, on the contrary, as evidence that she was not certain herself.
She was certain. The disclaimer was a piece of professional self-protection from a young woman who had been advised by her senior in the field, in writing, that the alternative was the destruction of her career.
The career was not destroyed. Payne stayed at Harvard. She married the Russian-born astrophysicist Sergei Gaposchkin in 1934. She raised three children. She became, in 1956, the first woman to be appointed a full professor in the Faculty of Arts and Sciences at Harvard, and shortly afterward the first woman to chair a Harvard department.
The chair was Astronomy. The appointment was made not by promotion but by the simple device of moving the existing departmental chair to a different post and waiting to see whether anyone would object. No one, on the documentary record, did.
She continued to observe and to write into her seventies. Her late papers on variable stars in the Magellanic Clouds, written with Sergei, are still cited in the variable-star literature. Her autobiography, The Dyer's Hand, was completed in 1979 and published posthumously in 1984.
She died on 7 December 1979, of lung cancer, at her home on Garden Street in Cambridge. She was seventy-nine.
Her thesis is still in print, in a reproduction edition. The disclaimer about hydrogen is still in it. A reader who comes to the book today, knowing what the next century did, will find the disclaimer harder to look at than any other sentence in the volume.
The harder thing is that Payne herself, when asked about it later in life, would not denounce Russell. She would say, with the dry brevity that her colleagues remembered, that he had given her the advice he thought was correct, and that she had taken it, and that she had been wrong to take it, and that this was, in her experience, how most things in science went.
