"There is no joy more intense than that of coming upon a fact that cannot be understood in terms of currently accepted ideas."
What are the stars made of? The answer to this fundamental question of astrophysics was discovered in 1925 by Cecilia Payne and explained in her Ph.D. thesis. Payne showed how to decode the complicated spectra of starlight in order to learn the relative amounts of the chemical elements in the stars. In 1960 the distinguished astronomer Otto Struve referred to this work as “the most brilliant Ph.D. thesis ever written in astronomy.”
Cecilia Payne (1900–1979) was born in Wendover, England. After entering Cambridge University she soon knew she wanted to study a science, but was not sure which one. She then chanced to hear the astronomer Arthur Eddington give a public lecture on his recent expedition to observe the 1919 solar eclipse, an observation that proved Einstein’s Theory of General Relativity. She later recalled her exhilaration: “The result was a complete transformation of my world picture. When I returned to my room I found that I could write down the lecture word for word.” She realized that physics was for her.
Later, when the Cambridge Observatory held an open night for the public, she went and asked the staff so many questions that they fetched “The Professor.” She seized the opportunity and told Professor Eddington that she wanted to be an astronomer. He suggested a number of books for her to read, but she had already read them. Eddington then invited her to use the Observatory’s library, with access to all the latest astronomical journals. This simple gesture opened the world of astronomical research to her.
England, though, was not in Payne’s professional future. She realized early during her Cambridge years that a woman had little chance of advancing beyond a teaching role, and no chance at all of getting an advanced degree. In 1923 she left England for the United States, where she lived the rest of her life. She met Harlow Shapley, the new director of the Harvard College Observatory, who offered her a graduate fellowship.
Harvard had the world’s largest archive of stellar spectra on photographic plates. Astronomers obtain such spectra by attaching a spectroscope to a telescope. This instrument spreads starlight out into its “rainbow” of colors, spanning all the wavelengths of visible light. The wavelength increases from the violet to the red end of the spectrum, as the energy of the light decreases. A typical stellar spectrum has many narrow dark gaps where the light at particular wavelengths (or energies) is missing. These gaps are called absorption “lines,” and are due to various chemical elements in the star’s atmosphere that absorb the light coming from hotter regions below.
The study of spectra had in fact given rise to the science of astrophysics. In 1859, Gustav Kirchoff and Robert Bunsen in Germany heated various chemical elements and observed the spectra of the light given off by the incandescent gas. They found that each element has its own characteristic set of spectral lines—its uniquely identifying “fingerprint.” In 1863, William Huggins in England observed many of these same lines in the spectra of the stars. The visible universe, it turned out, is made of the same chemical elements as those found on Earth.
In principle, it seemed that one might obtain the composition of the stars by comparing their spectral lines to those of known chemical elements observed in laboratory spectra. Astronomers had identified elements like calcium and iron as responsible for some of the most prominent lines, so they naturally assumed that such heavy elements were among the major constituents of the stars. In fact, Henry Norris Russell at Princeton had concluded that if the Earth’s crust were heated to the temperature of the Sun, its spectrum would look nearly the same.
When Payne arrived at Harvard, a comprehensive study of stellar spectra had long been underway. Annie Jump Cannon had sorted the spectra of several hundred thousand stars into seven distinct classes. She had devised and ordered the classification scheme, based on differences in the spectral features. Astronomers assumed that the spectral classes represented a sequence of decreasing surface temperatures of the stars, but no one was able to demonstrate this quantitatively.
Cecilia Payne, who studied the new science of quantum physics, knew that the pattern of features in the spectrum of any atom was determined by the configuration of its electrons. She also knew that at high temperatures, one or more electrons are stripped from the atoms, which are then called ions. The Indian physicist M. N. Saha had recently shown how the temperature and pressure in the atmosphere of a star determine the extent to which various atoms are ionized.
Payne began a long project to measure the absorption lines in stellar spectra, and within two years produced a thesis for her doctoral degree, the first awarded for work at Harvard College Observatory. In it, she showed that the wide variation in stellar spectra is due mainly to the different ionization states of the atoms and hence different surface temperatures of the stars, not to different amounts of the elements. She calculated the relative amounts of eighteen elements and showed that the compositions were nearly the same among the different kinds of stars. She discovered, surprisingly, that the Sun and the other stars are composed almost entirely of hydrogen and helium, the two lightest elements. All the heavier elements, like those making up the bulk of the Earth, account for less than two percent of the mass of the stars.
Most of the mass of the visible universe is hydrogen, the lightest element, and not the heavier elements that are more prominent in the spectra of the stars! This was indeed a revolutionary discovery. Shapley sent Payne’s thesis to Professor Russell at Princeton, who informed her that the result was “clearly impossible.” To protect her career, Payne inserted a statement in her thesis that the calculated abundances of hydrogen and helium were “almost certainly not real.”
She then converted her thesis into the book Stellar Atmospheres, which was well-received by astronomers. Within a few years it was clear to everyone that her results were both fundamental and correct. Cecilia Payne had showed for the first time how to “read” the surface temperature of any star from its spectrum. She showed that Cannon’s ordering of the stellar spectral classes was indeed a sequence of decreasing temperatures and she was able to calculate the temperatures. The so-called Hertzsprung-Russell diagram, a plot of luminosity versus spectral class of the stars, could now be properly interpreted, and it became by far the most powerful analytical tool in stellar astrophysics.
Payne also contributed widely to the physical understanding of variable stars. Much of this work was done in association with the Russian astronomer Sergei Gaposchkin, whom she married in 1934.
From the time she finished her Ph.D. through the 1930s, Payne advised students, conducted research, and lectured—all the usual duties of a professor. Yet, because she was a woman, her only title at Harvard was “technical assistant” to Professor Shapley. Despite being indisputably one of the most brilliant and creative astronomers of the twentieth century, Cecilia Payne was never elected to the elite National Academy of Sciences. But times were beginning to change. In 1956, she was finally made a full professor (the first woman so recognized at Harvard) and chair of the Astronomy Department.
Her fellow astronomers certainly came to appreciate her genius. In 1976, the American Astronomical Society awarded her the prestigious Henry Norris Russell Prize. In her acceptance lecture, she said, “The reward of the young scientist is the emotional thrill of being the first person in the history of the world to see something or to understand something.” As much as any astronomer, she had fully experienced that most important of all scientific rewards.