What Stars Are Made Of

 

Donovan Moore, What Stars Are Made Of: The Life of Cecilia Payne-Gaposchkin

Harvard University Press, 2020, 320 pages 

 

Donovan Moore has given us a long overdue popular biography of the founding mother of astrophysics.

If you’re interested in the history of astronomy, you know the great names: Galileo, Newton, the Herschels, Lowell, Eddington, Hubble, Hertzsprung, Russell, Shapley, Hoyle.  The fact that I first heard of Cecilia Payne-Gaposchkin only a few years ago is a mark of the enduring sexism of the astronomical profession and those who document it.  Dr. Payne-Gaposchkin had to fight it her entire life.

Cecelia Payne was born in 1900 in England.  Fascinated by the night sky, she set her sights on becoming an astronomer. To that end, she enrolled at Newnham College, Cambridge.  Moore gives a good accounting of Payne’s early life, including many examples of the restrictions women faced.  For example: “We were discussing whether a Newnham student could be allowed to take an afternoon walk with an undergraduate,” recalled a student a year ahead of Cecilia.  Another student replied, “’Well . . . yes . . . if she is engaged to him.’  To which a friend of mine . . . replied that it seemed a heavy price to pay for a walk.”  Much less to work alongside men in a lab.

Further, it was a time when women studied botany and men studied physics.  It took a great deal of will-power for a woman to go against the grain.

On the other hand, Payne was in the right place at the right time.  We Americans tend to think of “Oxbridge” as a single entity, but in fact Oxford, founded first, has a focus on liberal arts.  Seeking to differentiate itself, the newer Cambridge focused on science.  At first, the emphasis was mostly on mathematics, but about 1870 Cambridge established the Cavendish Laboratory, the first specially designed experimental physics laboratory, and hired the great James Clerk Maxwell as its first director.  By 1919, when young Cecilia Payne arrived, Maxwell and his successors had built the world’s premier physics lab.  From 1904, when the Nobel prizes were established, to 1928, Cavendish scientists won nine of them.  Payne learned atomic theory and the art of spectroscopy from past and future Nobelists.

However, Cambridge would not award a physics degree to a woman.  Furthermore, there was the problem of finding work.  One of her mentors, “L.J. [Comrie] confirmed what Cecilia already suspected—that there was no chance for her to find work as a scientist in England.  He told her that he was soon going to take a teaching position at a college called Swarthmore in the United States.  He told her that if she wanted to be an astronomer, she, too, should go to America.”  Payne got a position at the Harvard College Observatory.

Another mentor, “[E.A.] Milne told Cecilia that if he were the one going to Harvard, he would use Harvard’s data to experimentally verify what was known as Saha’s equation.

“Meghnad Saha was a physicist in India who had developed an equation that related a star’s temperature and pressure to the lines in its spectrum.  He was attempting to link the spectrum of the sun and other stars to Bohr’s description of the atom.  But it was all still just theory; Saha could not demonstrate that his calculations were correct because he had no access to raw data.  Cecilia, on the other hand, had been taught by Bohr, had learned experimental atomic physics as the knee of Rutherford, and was headed to a facility that had a huge trove of data in the form of hundreds of thousands of stellar spectra.  Think about it, said Milne.

“She did think about it.  What Milne was describing was nothing less than the birth of astrophysics.”

Payne spent two years studying Harvard’s photographic plates and smoking a lot of cigarettes.  She had a problem.  The orthodoxy of the time was uniformitarian: it held that all bodies in the universe had roughly the same composition.  Following this logic, the sun and other stars must have roughly the same composition as the earth, composed predominantly of “metals”. (In astronomy, “metal” refers loosely to all elements heavier than helium.)  In fact, since there are a lot of iron lines in the solar spectrum, it was assumed the sun was mostly iron.

But Payne, using Saha’s theory and new results from high-temperature spectroscopy, concluded that the stars were almost entirely hydrogen.  She checked and rechecked her work, and accumulated a vast trove of evidence to back up her thesis.

Even so, Henry Norris Russell, the premier American astronomer of the time, refused to accept her results.  Without his imprimatur, Harvard would not award her a PhD.

“And so, instead of defying Eddington, Russell, and indeed the entire astronomical community, she defied herself.  ‘Although hydrogen and helium are manifestly very abundant in stellar atmospheres,’ she wrote in her thesis, Stellar Atmospheres, ‘the actual values derived from the estimates of marginal appearance are regarded as spurious.’  Her provocative results that hydrogen dominated the composition of stars, she concluded, were ‘almost certainly not real.’”

Payne got her PhD in 1925.  Despite the disclaimer, her dissertation was quickly recognized as important work and supporting results from other researchers flooded in.  After four years, Russell came around and took the credit.

CPG’s contributions did not end with her dissertation.  She went on to have a long and productive career in which she produced 284 publications.  She was belatedly appointed Harvard’s first female Professor of Astronomy in 1956 and continued to do important work until shortly before her death in 1979.

I have covered CPG’s professional life here, but there is much more in the book.  Moore covers her entire personal life, including childhood, early schooling, marriage, and family.  (The story of how she met her future husband and spirited him out from behind the iron curtain is a story in itself.)  Moore is very good at describing the places and culture, with extensive descriptions (and photos) of the schools attended, the town of Cambridge, and the physical facilities in which CPG worked.  He also gives a good feel for how difficult it was for a woman making her way in science at that time.  In this he is aided by family archives and extensive research.

The book is weaker when it comes to setting CPG’s work in the context of astronomical history.  It doesn’t tell us much about the work she built on, or what she and others went on to do after her dissertation.  For that, I’d recommend Dava Sobel’s The Glass Universe: How the Women of the Harvard Observatory Took the Measure of the Stars.  Sobel’s book, while it focuses on the female staff of the Harvard Observatory, also does a great job of describing the development of astronomy from the mid-19^th century to the mid-20^th, and how Harvard’s, and CPG’s, work fit into it.

Recommended, but if you aren’t familiar with the history of astronomy, read Glass Universe first.  Then, if you want to know more about the amazing Cecilia Payne-Gaposchkin, read What Stars Are Made Of.