The Woman Who Found Out What Stars are Made Of

Readers may have seen the movie Hidden Figures. It’s a true story about three mathematicians who worked at NASA in the 60s. They were black women, and they succeeded despite facing systemic discrimination based on both race and gender. The lead character is Katherine Johnson. She calculated trajectories for Project Mercury and other missions. It also includes the stories of supervisor Dorothy Vaughan and engineer Mary Jackson. 

Katherine Johnson calculated trajectories, launch windows and emergency return paths for both Alan Shepherd and John Glenn. She also worked out the rendezvous paths of the command and lunar modules for the Apollo moon landings. Despite this, before the book and film about her life, she was unknown to the public.

That seems to be a pattern with female scientists. In an earlier story, we talked about Henrietta Leavitt. She was also a “computer”. Leavitt worked at the Harvard Observatory. She discovered the relationship between the luminosity and the period of Cepheid variable stars. This is a big deal because it allows astronomers to calculate the distance to faraway stars and galaxies. Without her formula, Edwin Hubble would not have discovered that there were other galaxies in the universe. Yet, like Katherine Johnson, nobody but a handful of space fans knows about her.

Our story today is about another unsung hero in the story of cosmology. Her name is Cecilia Payne. She grew up in England and went to Cambridge. She decided to become an astronomer after attending a lecture by Arthur Eddington. He described his famous expedition to watch the solar eclipse on the African island of Principe. His work proved Einstein’s Special Theory of Relativity. Despite completing her studies in physics, Cambridge did not award degrees to women at that time.

She realized that she would never find work as an astronomer in England, so she found a position at Harvard through networking. In contrast to Cambridge, Harvard had a fellowship to encourage women to study at the observatory. She wrote a doctoral dissertation in 1925. She called it Stellar Atmospheres; a Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars. It’s been called “the most brilliant PhD thesis ever written in astronomy”. Or, perhaps we should call it the most brilliant thesis the public has never heard of.

In Payne’s day, scientists believed that the sun and stars consisted of the same elements as the earth. The difference, in their minds, was that the sun was so much hotter. They thought that If the earth was heated to the same temperature, it would act like the sun. We can tell what objects consist of by looking at their spectrum, like the lines of colours we see with a prism. When we look at a star or a planet, the lines in their spectra show patterns that tell us the elements they contain.

The sun’s spectrum does include lines that represent the elements we find on earth. As a result, the conventional wisdom was that the sun’s composition was the same as the earth’s. Payne knew more about this than her peers. During her PhD research, she applied her knowledge of atomic spectra. She also used new discoveries about the thermal ionization of atoms. She learned about these from Indian physicist Meghnad Saha when he visited Harvard.

From this information, Payne concluded that, at the sun’s temperature, a mere 1 in 200 million hydrogen atoms would be visible in the sun’s spectral analysis. This meant that the quantity of hydrogen in the sun had been grossly underestimated. The same would be true for helium. Payne discovered that the sun does not consist of the same elements as the earth, although traces of them are present. The sun and all the stars, are 98% hydrogen and helium.

This is a vital discovery. We now know that the entire universe is as Payne predicted. It comprises 73% hydrogen and 25% helium. These ratios are one of the reasons that we know about the Big Bang. The Big Bang involved 12 nuclear interactions that led to the abundance of these two elements. The abundance of hydrogen and helium is one of the three central principles of cosmology, along with the expansion of the universe and the cosmic microwave background.

Payne spent her entire career at Harvard. At first, women were not allowed to be professors there. As a result, she languished in low paying, support positions. Despite her lower academic status, she published a series of critically acclaimed textbooks. These included The Stars of High LuminosityVariable Stars and Variable Stars and Galactic Structure

Her contributions were properly recognized more than 30 years after she received her PhD. She was named the first female professor and the first female department chair at Harvard. In her obituary, Payne was described as “the most eminent woman astronomer of all time”. In fact, she was arguably the most eminent astronomer of all time, period.

Women have won 19 Nobel Prizes in science, including last year’s Nobel Prize in Physics. Canadian scientist Dr. Donna Strickland won the prize for her work with lasers. She’s the chair of the Physics Department at the University of Waterloo. Yet, with the possible exception of Marie Curie, the public is unaware of their contribution to science.

We’ve published this story today in the hope that it will inspire readers to find out more about the women of science and the vital contributions they have made to our understanding of the universe.

There is always more to learn if we dare to know.

Learn More:
American Physical Society News

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