Humans have observed the stars for millennia, ever since our ancestors looked toward the sky. Throughout our history, the sparkling patterns of light upon the inky black sky have acted as compasses and calendars, told stories of gods and goddesses, and inspired poets and artists. But humanity’s relationship with the stars changed dramatically in 1925, when a young graduate student figured out what the twinkly lights were made of—and laid the foundation of stellar astrophysics.
This intuitive and brilliant astronomer was Cecilia Payne, and at 24 years old, she showed that stars were not like our Earth, but instead were glowing spheres of mostly fiery hydrogen and helium, the two lightest and simplest elements in the universe.
“It is such a fundamental piece of understanding for humanity,” says Anna Frebel, an astrophysicist at the Massachusetts Institute of Technology.
But like many hypotheses or discoveries that counter the leading theories of the time, Payne’s thesis was questioned and argued. The fact that she was a young female astronomer pushing against the status quo at a time when all academic experts in astronomy were men heightened tensions.
Today, Payne’s graduate thesis remains a staple on the bookshelves of stellar astrophysicists, its more than 200 pages yellowed from age and use. Today’s scientists see it as a masterpiece of astronomical writing, of putting the pieces together. “It was attention to detail,” says University of Wyoming stellar astrophysicist Meridith Joyce of Payne’s thesis. “It was accurate, and it was really brave.”
Investigating the stars above
In the early 1600s, humans began building and using telescopes. Observers realized just how prevalent stars are in the night sky, and that they clump in cloudlike “nebulas.”
In the early 1800s, observers first used prisms to spread incoming sunlight into a rainbow of colors. Later that century, astronomers determined that by placing a prism between a telescope’s lens and a detector they could record starlight. The prism broke apart the incoming starlight, which rained onto a photographic glass plate coated in an emulsion. As the photons of light interacted with that emulsion, it created a dark mark. Upon that plate lay a fanned-out pattern of dark regions and empty spaces each marking a specific color of light—the signature of a far-away star.
By the mid-1800s, researchers found that light from heated gaseous elements in laboratories produced opposite spectral patterns—mostly empty regions separated by bright lines of color. Physicists realized they could use those bright lines to interpret that gas’s chemistry.
The materials in Earth’s crust, scientists found, showed similar patterns to those seen in the stars, and so astronomers thought that the Sun and all the stars were made of the same stuff as Earth. “We had no real reason to believe that anything in the universe was different than what we could observe on Earth,” says Joyce.
Who was Cecilia Payne?
Cecilia Payne’s light came into existence May 10, 1900, in the English town of Wendover. According to her autobiography, while in her teens, she studied science and music before earning a scholarship to attend Newnham College at the University of Cambridge in 1919.
Payne initially studied botany but switched to physics within her first year. She learned about atomic physics from Ernest Rutherford, the man who discovered that every atom had a positively charged core called a nucleus, and Niels Bohr, who studied how electrons behave surrounding that positive nucleus.
Late that year, Payne by chance attended a lecture at Trinity College by Arthur Eddington, where he announced the results of his expedition during the total solar eclipse of 1919. He captured images of stars’ positions, which appeared to shift, due to the Sun’s pulling on their starlight, altering the lights’ pathways. During that lecture, Eddington confirmed Albert Einstein’s brand-new general theory of relativity, and Payne fell in love with astronomy.
In 1923, she sailed to America to begin her graduate studies at Harvard College Observatory and Radcliffe College in Cambridge, Massachusetts. “She got herself to the only place that women could succeed” in astronomy, says Thom Burns, the observatory’s curator of astronomical photographs.
When Payne joined the observatory, all astronomers and students were men. Some 10 to 20 women also worked at the observatory, but they were the “computers”—a term used for lab assistants who performed calculations. In this case, the women looked for patterns in the starlight and recorded changes to the visible stars. Payne, under a graduate fellowship, had a different role from the other women.
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Her mentor Harlow Shapley initially urged Payne to continue the work of a computer, Henrietta Swan Leavitt, who had realized that variations in some stars’ light could be used to measure distances. But Payne wasn’t interested in Leavitt’s project. “Instead, she wanted to focus on the decades of mostly untouched spectroscopy plates,” says Burns.
No institution had more of those emulsion plates preserving star signatures than Harvard. Another computer, Annie Jump Cannon, had started to classify stars based on their signatures. Payne wanted to build on this work, combining what she learned about the inner workings of atoms at Newnham with the recent scientific theories of physics and chemistry, to understand stars.
The physics of the stars
Physics research was accelerating at the time, and the discoveries and scientific theories were traveling across the globe.
Researchers had figured out the source of the patterns on spectroscopy plates—electrons shifting energy levels as they zipped around an atom’s nucleus, an action that emits or absorbs light. The color of that light was specific to a particular atom, meaning an electron in a carbon atom would always absorb or emit the same amount of light to reach a zippier state or drop back to a relaxed state. Within a few years, experiments determined most of energies that kick electrons to different levels for many of the atomic elements.
By looking for those characteristic lines in the starlight spectra, scientists could see the missing colors or empty spaces on the plates matched perfectly with atomic elements. They could now identify the constituent elements. The stars in the sky showed blank spaces where their gas absorbed the energy streaming from the nuclear furnace burning at their cores.
Most of these lab studies looked at neutral elements. Stars are giant balls of superhot, pressurized gas, and no one had yet figured out how those extremes might change the light patterns of different elements.
Payne combined the current understandings of atomic physics with a “brilliant idea” from Indian physicist Meghnad Saha, she wrote in her autobiography . Saha had just determined how gas behaves in differing temperatures and densities, and specifically how the contained electrons move in extreme environments.
Based on the high temperatures and pressures of stars, Payne calculated the strengths of the starlight spectral lines in the Harvard plates. “The different lines always have a certain strength relation to each other,” says Frebel. From that, Payne could calculate the abundance of the elements in the stars.
Payne’s work showed hydrogen and helium—the two lightest chemical elements—are incredibly abundant in stars, while heavier elements are much less prevalent. She also described what creates the observed shapes of the lines: how the interior pressures and temperatures of the gaseous material affects the light signature. Understanding those shapes, says Iowa State University stellar astrophysicist Steven Kawaler, “is essential for using them, the spectra, to understand the dynamics of the atmospheres.” Payne used the absorption lines not just for abundances or temperatures, he adds, but to understand what’s physically happening within stars.
Stars are everything
Payne completed her thesis in 1925, and with it she earned her doctoral degree in astronomy from Radcliffe. Initially her contemporaries were skeptical. Renowned Princeton astronomer Henry Norris Russell, one of the leading stellar astronomers of all time, was one of her loudest critics. As a nod to his concerns, she wrote of hydrogen and helium, “The enormous abundance derived for these elements in the stellar atmosphere is almost certainly not real.”
While the rest of the text oozes confidence, says Kawaler, this statement was a “temporary diffusion of an otherwise very exciting result.” Just four years later, however, Russell confirmed Payne’s findings.
This work “gave us the starting point of understanding what is measured in stellar spectroscopy,” says Frebel, who uses starlight to search for the oldest stars. Payne’s discoveries have helped future researchers piece together what is occurring below a star’s surface over its lifetime, how energy produced at the center of stars moves through their outer layers, and how stars die in explosive blasts or instead feebly fade into the inky black backdrop.
“Stars are everything,” says Joyce. “Everything else we know about the universe comes from stars.”
