In the early 20th century, a major astronomical puzzle was: How is a star's temperature affected by what it's made of? It was solved in 1925 in a young woman's doctoral thesis. Her analysis was a major breakthrough in astrophysics. Otto Struve, one of the most prominent astronomers of the 20th century, described her work as “the most brilliant Ph.D. thesis ever written in astronomy.”

Family and early education
Cecilia Payne was born in England in 1900 into a cultured intellectual family. But her father – a lawyer, musician and historian – died when she was four, leaving her mother with three young children. Despite the difficulties, Emma Payne was determined that her children would get a good education.

The secondary school that Cecilia attended didn't approve of science, and that's what she wanted to learn. Happily, one of her teachers taught her botany and chemistry, and the head teacher of St Paul's Girls' School in London accepted her transfer. At St Paul's she was able to study science.

From Cambridge, England to Cambridge, Massachusetts
In 1919 Cecilia got a scholarship and was accepted to Newnham College, a women's college in the University of Cambridge. Although her main subject was botany, she grew more interested in physics. But what changed her life was hearing Arthur Eddington's account of his eclipse expedition to test one of the predictions of Einstein's general theory of relativity. She was fascinated, and completely won over to astronomy.

Cecilia couldn't include astronomy in her program of study. It was in the faculty of mathematics, not physics. However Eddington encouraged her interest, and also let her use the observatory library. While still a student, she carried out work that was published in the Monthly Notices of the Royal Astronomical Society. Eddington then proposed her for membership in the society.

Yet Cecilia realized that she had no future in Britain. Having completed the Cambridge degree requirements, she wasn't awarded a degree, and graduate study was a closed door for women. (The university didn't award degrees to women until 1948.)

Fortunately, Harlow Shapley, director of the Harvard Observatory in Cambridge, Massachusetts, not only wanted to start a graduate program in astronomy, but a fellowship was available to encourage women to study there. When Shapley came to London in May 1922, Cecilia was introduced to him. Several months later she wrote to Shapley expressing her keen desire to study at the observatory, and her mentors in Cambridge – including Eddington – wrote letters of support.

The doctorate
Shapley persuaded Payne to produce a doctoral dissertation, even though the observatory didn't yet have a PhD program. She researched and completed it in two years, earning the first astronomy PhD from Radcliffe (now part of Harvard University).

Back to the big question: how did the chemical composition of a star's atmosphere affect its temperature? Payne had studied what was then the new science of quantum physics. She applied Indian physicist Meghnad Saha's work on atomic structure to the question of temperature.

Using Harvard Observatory's vast collection of stellar spectra, she calculated the abundances of eighteen elements. Stars were classified by their light spectra, which in turn was an indicator of temperature. However Payne discovered that the chemical compositions of the stars was roughly similar – and it didn't determine the temperature. The spectra were produced by the way in which temperature and pressure affected the energy levels of the electrons in the atoms.

But Payne's calculations also produced the result that hydrogen was vastly more abundant than heavier elements, over a million times more. This was so amazing that Henry Norris Russell, director of Princeton University Observatory, declared that it was “clearly impossible” and persuaded Payne to tone down that conclusion. We now know this superabundance of hydrogen is real. A few years later Russell also accepted its reality, having obtained the same results by a different method. Although he acknowledged Payne's work, he was generally credited with the discovery.

Payne-Gaposchkin
In 1933, on a tour of European observatories, Cecilia Payne met Sergei Gaposchkin, a Russian astronomer in exile in Germany and in danger from the Nazi regime. With Payne's help he was able to get a visa to the USA and a job at Harvard Observatory. He came to the USA later that year and the following year they got married.

The couple collaborated on several projects. Sergei was a competent astronomer, but Shapley maintained that Cecilia was “the Gaposchkin”. Along with their assistants, the Gaposchkins made over three million observations of variable stars. This created a data base for studying the evolution of stars.

Career and honors
Despite the praise for her thesis, Payne didn't get a faculty post. She was employed as Shapley's technical assistant. It wasn't until 1938 that she was officially hired as an astronomer. But she would go on to become the first woman tenured in Harvard's Faculty of Arts and Sciences, the first promoted to full professorship from within the faculty, and the first woman to head a department at Harvard. Finally, from her retirement from teaching in 1966 to her death in 1979, she was an Emeritus Professor, and also remained on the staff of the Smithsonian Astrophysical Observatory.

In 1936 she had become a member of the American Philosophical Society, America's equivalent to Britain's Royal Society. She was elected a fellow of the American Academy of Arts and Sciences in 1943. In 1977, two years before her death, the American Astronomical Society awarded Cecilia Payne-Gaposchkin their highest honor, the Henry Norris Russell lectureship.