By Emily Levesque, University of Washington
Cecilia Payne arrived at Harvard in 1923, traveling from England after completing her studies in physics, chemistry, and astronomy at Cambridge University. She had not been officially granted a degree—Cambridge didn’t begin awarding degrees to women until 1948—and she left the United Kingdom in the hopes of pursuing a professional scientific career.
In 1920, Indian physicist Meghnad Saha had made an exciting discovery. He discovered the exciting connection between temperature and the elements seen in stellar spectra.
The equation that he created, which now bears his name, linked the ionization state of various elements to the temperature of the gas that these elements were in.
Saha’s reasoning went something like this: a hotter gas would produce more high-energy photons, a result known as blackbody radiation originally discovered by physicist Max Planck. Those energetic photons would in turn collide with the electrons in the atom’s gaseous state, exciting those electrons until they were ejected from the atom entirely. The loss of an electron left behind a positively charged atom known as an ion and those hot ionized gases would have very different spectral signatures than the cooler non-ionized gases.
Saha computed mathematical formulas that predicted where different lines would appear in a spectrum as a function of temperature, but his work invoked temperatures far too high to be achieved in a laboratory. Instead, he turned to the stars and found that his predictions matched the appearance of the spectral types in Annie Jump Cannon’s classification scheme.
Standing on the Shoulders of Saha
Cecilia Payne used Saha’s equations as a starting point for her doctoral thesis at Harvard, convinced that if she could identify spectral lines and mathematically describe their intensity, she could develop a system for precisely measuring the temperatures of stars. Her work established the first temperature scale for stars based on their classifications and spectral appearances.
Still, Payne pushed her work further. The strength of a spectral line in a star can depend on many things. One is temperature, but the abundance of the element that is absorbing light is also important.
A star with very little silicon, for example, may display only weak absorption from silicon or even none at all, simply because there aren’t many atoms in the star to do the absorbing. Yet, the silicon is there, comprising part of the star.
In addition to measuring temperature, Payne wanted to measure the relative abundance of the elements in stars: in short, she literally wanted to determine what stars were made of.
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The Elements of a Star
Payne’s results proved controversial at first. Most astronomers of the time assumed that the stars must have roughly the same composition as the Earth: elements like silicon, oxygen, aluminum, and calcium had all been seen in stellar spectra, and those elements were common on Earth.
Payne’s research suggested something else. Most of the heavier elements in the periodic table seemed to have similar abundances in stars and on Earth, but the two lightest elements, hydrogen and helium, were wildly different. Hydrogen, in fact, appeared to be a million times more abundant in stars than it was here on Earth.
Payne included this result in her thesis but was discouraged from trusting her results by other astronomers; she included a caveat in her write-up that the hydrogen abundance was “almost certainly not real”.
Still, even with this qualification, her results were explosive. Payne had discovered that all stars had very similar compositions: all with similar abundances of heavy elements, and all with seemingly huge amounts of hydrogen and helium. Payne concluded that Annie Jump Cannon’s spectral type sequence was entirely driven by stellar temperature and was almost completely independent of variations in element abundance.
First to Receive a Ph.D. from Harvard
Payne’s thesis was widely lauded as brilliant work. She became the first person to receive a Ph.D. in astronomy from Harvard. She went on to spend the rest of her academic career at Harvard, studying varying stars with her husband and fellow astronomer Sergei Gaposchkin, and generating an enormous catalog of stellar variables that is still in use today.
Today, we know that Cecilia Payne’s discovery was correct: the stars, and indeed most of the universe, are composed almost entirely of hydrogen. In our own Milky Way, hydrogen makes up about 75% of normal matter. Helium comprises another 23%, leaving just 2% for all of the heavier elements in the entire periodic table.
Payne’s research paved the way for several other incredible discoveries on the chemistry and evolution of stars.
Payne and her contemporaries recognized at the time that their work was focused on the outer layers of stars, where light was able to escape from the star’s interior and reach Earth with the stellar atmosphere’s chemical fingerprint preserved in its spectrum. However, the deep inner workings of stars, including the source of their luminous energy churning away in their cores, would remain a mystery until 1938.
Payne’s research also highlighted how stars impact the chemistry of their surroundings. Thanks to the speed of light, peering at distant galaxies in the universe is like looking back in time. When we observe a galaxy one billion light-years away, we’re seeing light that was emitted one billion years ago, in an earlier era of our universe. Since Cecilia Payne’s discovery, we’ve found that the relative abundance of heavy elements in our universe has increased with time.
Payne revolutionized our understanding of stellar temperatures and composition, and many women in the years since have indelibly shaped our picture of the stars.
Common Questions about Cecilia Payne’s Major Contributions to Astronomy
In her doctoral thesis, Cecilia Payne used Saha’s work as a starting point, aiming to ultimately develop a system for measuring the temperatures of stars.
Most astronomers of the time assumed that the stars must have roughly the same composition as the Earth. However, Cecilia Payne’s research suggested something else. She found that the two lightest elements, hydrogen and helium, were wildly different in stars and on Earth. Hydrogen, in fact, appeared to be a million times more abundant in stars than it was here on Earth.
Cecilia Payne’s doctoral thesis was viewed as a brilliant work and she became the first person to receive a Ph.D. in astronomy from Harvard University. She went on to catalog a great number of stellar variables. Her work paved the way for future important discoveries in astronomy.