Posting this in response to a question I was asked. It is essentially an extract from my book 13.8
Cecilia Payne won a scholarship to Newnham College, Cambridge (the only way she could have afforded a university education) in 1919. She studied botany, physics and chemistry, but also attended a talk by Arthur Eddington about the eclipse expedition on which he had famously “proved Einstein right” by measuring the way light from distant stars was bent by the Sun. This fired her interest in astronomy, and she visited the university’s observatory on an open night, plying the staff with so many questions that Eddington took an interest, and offered her the run of the observatory library, where she read about the latest developments in the astronomical journals.
After completing her studies (as a woman, she was allowed to complete a degree course, but could not be awarded a degree; Cambridge did not award any degrees to women until 1948) she looked for a way to pursue this interest. There was no chance of a career in research in England, where the only job opportunities for women scientists were in teaching, but through Eddington she had met Harlow Shapley, from Harvard, on a visit to England. He offered her the chance to work for a PhD on a graduate fellowship (even though, technically, she was not a graduate) and in 1923 she left for the United States. Just two years later, she produced a brilliant thesis and became the first person to be awarded a PhD by Radcliffe College (also the first for work carried out at Harvard College Observatory). In it, she established that the Sun is mainly made of hydrogen. But, in a sign of the times, the idea was not fully accepted until two male astronomers independently came to the same conclusion.
Payne’s study of the solar spectrum made use of the then-recent discovery by the Indian physicist Meghnad Saha that part of the complication of the pattern of lines in a stellar spectrum (or the Sun’s Fraunhofer lines) was a result of different physical conditions in different parts of the atmosphere of a star. By the 1920s, physicists knew (as, of course, Bunsen and Kirchoff had not) that atoms are composed of a tiny central nucleus, with one or more electrons at a distance from the nucleus. Dark lines in a spectrum are produced when an electron absorbs a specific wavelength of light, moving to a higher energy level within the atom, and bright lines are produced when an electron drops down from one energy level to another and emits radiation (in the form, we would now say, of a photon). An atom which has lost one or more of its electrons is called an ion, and the spectra of ions are correspondingly different (in a way which can be calculated) from those of the “parent” atoms. Payne measured the absorption lines in stellar spectra and showed how the temperature (in particular) and pressure in the atmosphere of a star affects the ionisation of the atoms there. This makes for a more complicated pattern of lines than if all the atoms were in their un-ionised state. The spectra of stars differ from one another not because they are made of different things, but because of different amounts of ionisation in their atmospheres.
Payne’s great achievement was to unravel this complicated pattern of hundreds of Fraunhofer lines and work out what proportion of different elements in different stages of ionisation had to be present to account for the observations. Some idea of the difficulty of her task can be gleaned from the fact that her thesis was later described by the astronomer Otto Struve as “the most brilliant Ph.D. thesis ever written in astronomy”. She worked out the proportions of eighteen elements in the Sun and stars, discovering that they all had nearly the same composition. But the big surprise was that according to her analysis the Sun and stars are made almost entirely of hydrogen and helium. If she was correct, everything else put together made up only two per cent of the composition of our nearest star, and of all stars. Most of the matter in the Universe was in the form of the two lightest elements, hydrogen and helium. This was almost literally unbelievable in 1925. Payne believed her results were correct, but when Shapley sent a draft of her thesis to Henry Norris Russell at Princeton for a second opinion he replied that the result was “clearly impossible.” On Shapley’s advice, she added a sentence to the thesis saying that “the enormous abundance derived for these elements [hydrogen and helium] in the stellar atmospheres is almost certainly not real”. But with the thesis accepted and her doctorate awarded, she wrote a book, Stellar Atmospheres, which began to persuade astronomers that the results were, in fact, almost certainly real.
The change of mind was aided by the independent confirmation of Payne’s results by other astrophysicists. In 1928, the German astronomer Albrecht Unsöld carried out a detailed spectroscopic analysis of the light from the Sun; he found that the strength of the hydrogen lines implied that there are roughly a million hydrogen atoms in the Sun for every atom of anything else. A year later, the Irish astronomer William McCrea confirmed these results using a different spectroscopic technique. What this shows, more than anything, is that although Cecilia Payne was a brilliant researcher who got there first, this was a discovery whose time had come; given the technology oft the 1920s it was inevitable that the discovery would be made sooner rather than later. In 1929, having carried out a similar analysis using a different technique, Russell himself published a paper confirming these results, and giving due credit to Payne’s priority; unfortunately, because of Russell’s established position in the astronomical community, for some time he was often cited as the discoverer by people who should have known better (or at least, read his paper properly).
Payne went on to a distinguished career in astronomy; in 1934 she married the Russian-born astrophysicist Sergei Gaposchkin, and became known as Cecilia Payne-Gaposchkin. She remained at Harvard throughout her career, in spite of the low status and low pay she received as a woman. For many years, her official title was “technical assistant”, even though she carried out all the research and teaching duties expected of a Professor. It was not until 1956 that she was promoted to become a full Professor — the first female Professor at Harvard. But, like most scientists, she was not primarily motivated by status or salary. In 1976, three years before her death, she was awarded the prestigious Henry Norris Russell Prize by the American Astronomical Society. No doubt she appreciated the irony. In her acceptance lecture, she said, clearly referring to her early work on stellar spectra, “The reward of the young scientist is the emotional thrill of being the first person in the history of the world to see something or to understand something.” Even if someone else tells you it is “clearly impossible”.
 I am always careful to use the hyphen in the word “un-ionised” since Isaac Asimov once pointed out to me that the way to distinguish between a scientist and a politician is to ask them how to pronounce the word “unionised”:.
 Much later, McCrea was my PhD examiner.