From the time of Greek thinker Anazagoras in the 5th century BC until the 1920s, people thought the sun had the same composition as the Earth. Then came a woman who turned the received wisdom on its head. She wrotes the greatest astronomy PhD of the 20th century. Yet hardly anyone knows her name. SHe was the first woman proffeser at Harvard yet her salary for many years was classed as “equipment expenses”. She made one of the greatest astonomical discoveries of all time yet another astronomer gor most of the credit. Undoubtedly, Cecilia Payne suffered for being a woman in a male-dominated profession. But her lot would have been worse had she not left Cambridge, England – where women were barred from research – and moved to Cambridge, Massachusetts, At Harvard, she found more freedom – and the vast collection of stellar spectra on which she would cut her scientific teeth.
THE big problem in early 20th-century astronomy was to understand the spectra of stars. In 1859, the German physicist Gustav Kirchhoff had found that certain elements, when heated in a flame in his laboratory, gave out light with a wavelength identical to light in the spectrum of sunlight. He concluded that the same elements existed on the sun as on Earth, a conclusion later extended to other stars by the English amateur astronomer William Huggins.
However, without any idea of how spectral lines are created, it was impossible to say which elements were common in stars and which rare. Everything changed with the advent of quantum theory and the realisation by the Danish physicist Niels Bohr in 1913 that an atom’s electrons circle its nucleus in discrete orbits and emit or absorb light when they jump from one orbit to another. Crucially, an electron can absorb enough energy to be kicked out of an atom altogether.
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The connection with hot bodies like stars was made in 1920 by the Indian physicist Meghnad Saha, who combined quantum theory with the theory of heat – thermodynamics. He proposed that collisions between atoms in a gas could also kick out electrons and devised an equation that predicts, for any temperature and density, what fraction of an element’s atoms will be neutral, stripped of one electron, two electrons, and so on. The final step in determining how many atoms of an element were needed to generate an observed spectral line was taken by the English physicists Arthur Milne and Ralph Fowler. But they did not apply their method to real stars. This was the task that Cecilia Payne took up with gusto on her arrival at Harvard in September 1923.
Payne was greatly helped in her task by the vast Harvard spectral archives, which had already been combed by an army of back-room women employed as human computers. One of their number, Wilhelmina Fleming, had put stars into alphabetical categories according to the strength of the spectral line produced by hydrogen. Annie Jump Cannon had then rearranged the sequence to make a smoother transition between the categories, ending up with O, B, A, F, G, K, M, R, N, S – which generations of astronomy students have remembered with the mnemonic “Oh, be a fine girl and kiss me right now, sweetie”.
Applying the methods of Fowler and Milne, Payne found that Cannon’s sequence was actually a temperature sequence, with O the hottest and S the coolest. It was temperature, not composition, that was responsible for most of the differences between stars. Just because a star’s spectrum showed no lines from, for example, neutral iron, did not mean it had no iron – only that the star was so hot that there was no iron left with a full complement of electrons to emit the spectral lines characteristic of neutral iron. All stars, Payne found, had roughly the same composition.
This crucially important observation that there was a universal mix of elements later led the British cosmologist Fred Hoyle and his team to look for, and find, universal nuclear processes that have forged all the universe’s elements. But the discovery of a universal abundance of elements was only the first of Payne’s great discoveries. In the spectrum of the sun was a bombshell.
At the solar temperature of 5600 kelvin, Payne’s calculations showed that nearly all the atoms of the two lightest elements, hydrogen and helium, should be stripped of an electron. Nevertheless, the sun’s spectrum has strong signature lines for these elements. How come? There was only one explanation: the tiny fraction of hydrogen and helium atoms not completely stripped of electrons was a tiny fraction of a stupendously huge number. According to Payne’s calculations, hydrogen and helium were ridiculously abundant – they made up 98 per cent of the mass of the sun.
This was not what she had wanted to find. Everyone since Anaxagoras had thought the sun was made of the same elements as the Earth, principally iron. But there was hardly any hydrogen or helium on Earth. Thinking that her method must be giving a false conclusion, when Payne came to write her doctoral thesis in 1925 she described her result as “spurious”. In one published paper, she even wrote: “The abundance of both hydrogen and helium in stars is improbably high and is almost certainly not real.”
In rejecting her greatest discovery, Payne was heavily influenced by Henry Norris Russell, the American astronomer who had discovered red giant stars. In a letter to Payne uncovered by Harvard historian of astronomy Owen Gingerich, Russell wrote: “I am convinced that there is something seriously wrong with the present theory. It is clearly impossible that hydrogen be a million times more abundant than the metals” – “metals” being astronomy-speak for all elements other than hydrogen and helium.
Russell was one of the most influential astronomers of his day and his opinion carried tremendous weight. However, the evidence that hydrogen in particular was superabundant steadily mounted. By 1929 it was overwhelming, and Russell published his own calculations in The Astrophysical Journal. Because he used a different method to Payne, he spelled out his own approach first, and only later mentioned that his result matched hers. This gave an unfortunate impression. As Russell’s biographer, David DeVorkin, puts it: “Only at the end of his 71-page paper did Russell give full credit to Payne’s 1925 conclusions, saying nothing of his original rejection of them.”
Payne was the first to find evidence that almost 90 per cent of the atoms in the universe are hydrogen, with most of the remaining 10 per cent helium. All the other elements, from oxygen to calcium to iron, are no more than minor contaminants of the cosmos.
A hydrogen sun immediately opened up the possibility of a hydrogen-powered sun. The details were worked out in 1929 by the Englishman Robert Atkinson and the German Fritz Houtermans. The sun, as we now know, is a giant, controlled H-bomb. But this was only one spin-off of Payne’s discovery. The huge abundance of helium would turn out to be totally inexplicable. The universe simply had not existed for long enough for stars to have forged the quantity of helium observed. There had to be an alternative cosmic furnace. We now know that furnace existed at the beginning of time. Payne’s discovery was crucial evidence for the hot big bang.
Astronomer Otto Struve, referring to Payne’s discovery that stars all had roughly the same composition, called her PhD “undoubtedly the most brilliant thesis ever written in astronomy”. But it was Russell who won most of the acclaim for the discovery of the superabundance of hydrogen. “With what we know today, we could wish that it were otherwise,” says Gingerich. “But it is the person who persuades his colleagues of a new result who gets the credit.” In 1977, a few years before she died, the American Astronomical Society awarded Cecilia Payne one of its highest honours. Ironically, it was the Henry Norris Russell prize.