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| Cecilia Payne-Gaposchkin, linocut 9.25" x 12.5" on ivory kozo paper by Ele Willoughby 2019 |
For the first quarter of twentieth century, astronomers believed that Earth and Sun were much the same, made of the same distribution of elements, differentiated only by temperature. Nuclear fusion, the source of solar energy had yet to be discovered, and scientists looked at the entire spectrum of light emitted to try and determine the nature of star stuff. Physicists were beginning to use spectroscopy to identify the elements of which things are made. It turns out that with stars, which are hot and full of excited atoms, rather than emission spectra, it is absorption spectra, like rainbows crossed with bar codes, which are the most useful. Light is emitted from stars at a broad range of frequencies (and those within the visible range we see as different colours), but there are specific stripes which are missing (or absorbed) because they exactly match the energy difference between two quantum mechanical states of their constituent atoms. Each element has its own ‘bar code’ of absorption lines. The lines of common metals like silicon and carbon are seen in the sun’s absorption spectrum which lead scientists to think it star stuff was the same as Earth stuff.
Born in Wendover, England, in 1900, Cecilia Payne was one of three children raised by her mother Emma Leonora Helena (née Pertz) after the death of her father, barrister and historian Edward John Payne, when she was only four. She attended St. Paul’s Girls’ School and won a scholarship to Newnham College at Cambridge to read botany, physics and chemistry in 1919. She was disappointed by botany, but found phyics a "pure delight". The department at Cambridge at this time included such luminaries as J.J. Thomson, Rutherford, C.T.R. Wilson, Chadwick and Bohr. This marked the year a lecture changed her life. So impressed, she later wrote out the lecture word for word correctly, comparing it against his published text. She wrote, “My world had been so shaken that I experienced something very like a nervous breakdown.” It was no everyday lecture. She had gone to hear Sir Arthur Eddington’s account of his 1919 expedition to the island of Principe off the west coast of Africa to photograph stars with apparent positions near the sun during the solar eclipse. Eddington had produced the first experimental evidence supporting Einstein’s revolutionary General Theory of Relativity, which predicted that large masses like the sun would bend spacetime itself and that gravity would bend light changing apparent position of stars. Cecilia Payne’s imagination was captured by astronomy. She completed her studies but Cambridge did not grant women degrees until 1948 and her only option in the UK would be to become a teacher. She met Arthur Shapley, Director of the Harvard College Observatory who had just established a graduate program. Thanks to a fellowship to encourage women to study at the observatory she left the US to pursue graduate school in the US. With Shapley’s encouragement she became the first PhD in astronomy at Radcliffe College (which is now part of Harvard).
Her 1925 thesis was "Stellar Atmospheres; a Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars." Indian physicist Meghnad Saha had recently developed his ionization theory, which relates the ionization state of a gas in thermal equilibrium to the temperature and pressure. That is, he explained how those stellar “bar codes” due to ionized gas in stars relates to their temperature and pressure. Astrophysicists use the phrase “to Saha correctly” now to describe the process of interpreting stellar atmospheres. Cecilia Payne was able to Saha correctly on the Harvard collection of stellar spectra; she showed that variations in absorption lines were related to ionization state and temperature, rather than the various amounts of elements. She found the abundances of silicon and carbon were just like here on Earth, as expected, but that hydrogen and helium were vastly more abundant. Hydrogen in fact was a million times more abundant! This meant it was the most abundant element in the universe. This seemed too astonishing to be true. When defending her thesis, astronomer Henry Norris Russell, swayed by the theories of American physicist Henry Rowland, convinced her that this result was spurious. But she was right and they were wrong. Within a few years astronomer Otto Struve described her work as "the most brilliant PhD thesis ever written in astronomy" and Russell himself found independent evidence of her result. Russell published his result and though he acknowledged Payne he was often wrongly credited with this discovery.
After her doctorate, she looked at the structure of the Milky Way by studying high luminosity supergiant stars, discovering many of their unusual properties including exotic ions in their spectra. Shapley suggested that she work on photographic stellar photometry which required meticulous work to establish standard stellar magnitudes and colours. She chafed under this time-consuming assignment but she knew the work was important. It lead to her best-known work on variable stars on which she spent many years. She used the millions of observations made with her assistants to investigate stellar evolution, and published the book 'Stars of High Luminosity' in 1930.
She became an American citizen in 1931 and then while on tour in Europe, met the stateless Russian-born astrophysicist Sergei I. Gaposchkin in Germany. He had flead the soviet purges in Russia and now feared for the future in Nazi Germany. She went to Washington to help him get an American visa. The two were married in 1934. She appended his last name to her own. They had three children and worked together on the observation and analysis of all variable stars bigger than magnitude 10 (a measure of brightness). Their paper on the subject became the standard reference. Their insight into variable stars served as means of elucidating the structure of the galaxy and the role of variable stars in stellar evolution. Payne-Gaposchkin worked at Harvard for her entire career. While she was barred from a professorship as a woman, and relegated to low-paid research positions, she nonetheless was able to publish several more books including 'Variable Stars' (1938), 'Variable Stars and Galactic Structure' (1954), and 'The Galactic Novae' (1957)'. Shapley worked to improve her position and in 1938 she was given the title Astronomer, later changed at her request to Phillips Astronomer. In 1943 she was elected Fellow of the American Academy of Arts and Sciences.
Thanks in part to the efforts of Donald Menzel who became Director of the Harvard College Observatory in 1954, she became the first woman full professor from within the faculty at Harvard's Faculty of Arts and Sciences in 1956 and was finally paid a salary commensurate with her stature. She trained several graduate students who went on to eminent careers in astronomy. Eventually she became the department Chair, the first woman Chair at Harvard. She retired in 1966, but continued working as an Emeritus Professor of Harvard, as a staff member at the Smithsonian Astrophysical Observatory, editing books and journals for the next twenty years. As well as the books mentioned she published more than 150 papers, popular science books and an astronomy textbook. In 1977, she was awarded the highest honour of the American Astronomical Society, the Henry Norris Russell Lectureship. Payne-Gaposchkin was a trail-blazer and role model for women in the male-dominated field of astronomy, and one of the great scientists of the twentieth century. Late in life, she wrote:
Young people, especially young women, often ask me for advice. Here it is, valeat quantum. Do not undertake a scientific career in quest of fame or money. There are easier and better ways to reach them. Undertake it only if nothing else will satisfy you; for nothing else is probably what you will receive. Your reward will be the widening of the horizon as you climb. And if you achieve that reward you will ask no other.
References
Wikipedia entries on Cecilia Payne-Gaposchkin (both in English and French), Meghnad Saha, the Saha ionization equation, and Absorption spectroscopy, accessed May, 2019.
Gingerich, O., Obituary - Payne-Gaposchkin Cecilia, Quarterly Journal of the Royal Astronomical Society, Vol. 23, P. 450, 1982
Smith, Elske V.P., Cecilia Payne-Gaposchkin, Physics Today 33, 6, 65 (1980); https://doi.org/10.1063/1.2914129