Cecilia Payne realised that the Sun and stars are primarily hydrogen and helium - only for a man to take credit for the discovery
Everyone knows that Isaac Newton discovered the law of gravity, and that Charles Darwin discovered evolution by natural selection. But how many people know that Cecilia Payne discovered what makes up the Universe?
This year marks a century since Payne, an English woman at Harvard university, submitted the most important astrophysics PhD of the 20th century. Her conclusion that the Sun – and by inference, the stars – is 98 per cent hydrogen and helium, two elements virtually non-existent in their free form on Earth, was initially dismissed by the most important astronomer in America. He later took credit for her discovery, which was only attributed many years later.
Born in Buckinghamshire, Payne moved with her family to London to be near her brother’s school – an early lesson that it was a man’s world. She nevertheless obtained a place at Cambridge, where she studied physics. But, on completing her studies, there was no opportunity for a woman to do a PhD. When a friend told her that Harlow Shapley, director of the Harvard Observatory, was giving a talk in London, she turned up and brazenly asked for a job.
Harvard was famous for its female “computers”, as they were called. Shapley paid a pittance to dozens of these women, of varying education levels, to do the labour-intensive work of classifying stars on thousands of glass photographic plates. They included Annie Jump Cannon, who found that the “spectra” of stars – how their light varied with colour – fit into a handful of categories; and Henrietta Swan Leavitt, who discovered that “Cepheid variable” stars could be used to measure the size of the Universe. But Payne declined a job as a “computer”, as she wanted to do something more fundamental. That was figuring out what the spectra of stars were actually telling us about their make-up. (To give you an idea of how much her work was valued, her salary was paid from a budget for “equipment expenses”.)
At the time, it was clear that each element in the Sun absorbed light at characteristic colours, causing dips in the spectrum that were like an elemental fingerprint. Each dip corresponded to the light absorbed when an electron jumped from one atomic orbit to another. But there was a complication. In the frenzy of the super-hot Sun, collisions between atoms stripped most of them of their electrons. Fortunately, in 1920 the Indian physicist Meghnad Saha wrote down an equation which, for any temperature, gave the fraction of each element that had lost zero, one, two, and so on, electrons. Payne adapted this equation to decode the solar spectrum. That was when she made her groundbreaking discovery.
Ever since Greek times, the Sun had been thought to be made of the same mix of elements as the Earth. Anaxagoras, in the fifth century BC, had declared that it was a mass of red-hot iron – and modern analysis of sunlight had backed him up, revealing a forest of dips due to iron. But Payne’s analysis told her something profoundly different.
The reason there are lots of dips due to iron is not because it is abundant but because an iron atom has 26 electrons. Consequently, electrons can occupy very many possible orbits and make a bewildering number of jumps between them. By contrast, hydrogen and helium have only one and two electrons, respectively. And at the 5,600 degrees Celsius of the solar surface, most have had their electrons knocked away, making them incapable of absorbing light.
Yet Payne could see that the spectral dips of the two elements were prominent. And she realised that there was only one explanation: she was seeing a tiny fraction of a huge number of atoms, because hydrogen and helium were in fact super-abundant. These two gases made up the majority of the Sun. However, when her dissertation was reviewed, the renowned astronomer Henry Norris Russell dismissed this conclusion. He made Payne insert a caveat, stating that she was almost certainly wrong. Then when he arrived at the same conclusion several years later he failed – surprise, surprise – to give prominence to her work.
Recognition for Payne came much later in life. In 1956, she was appointed the first woman professor at Harvard. Four years later, the Ukrainian-American astronomer Otto Struve referred to her early research as “the most brilliant PhD thesis ever written in astronomy”.
The twist in the tale is that visible stars and galaxies account for only one-sixth of the matter in the Universe. The lion’s share is in the form of invisible “dark” matter. And the person who confirmed its existence is another woman: Vera Rubin. But that’s another story.
his article is from New Humanist’s Spring 2025 issue. Subscribe now.
