On this page
Ask most students what the Sun is made of and they will say hydrogen and helium. It sounds like something people always knew. They did not. Until the 1920s most astronomers assumed that stars had roughly the same composition as Earth's crust, because the spectrum of the Sun is crowded with lines of iron, calcium and other metals. The person who showed otherwise was a graduate student at Harvard, and the experts told her she was wrong. This post covers what she did, the physics behind it, and what a student can take from how she worked.
Who was Cecilia Payne?
According to the Science History Institute, Cecilia Payne lived from 1900 to 1979 and earned her PhD from Radcliffe College, now part of Harvard, in 1925. She had studied at Cambridge, where, as Harvard Magazine recounts, a lecture by Arthur Eddington on his recent solar expedition, which had tested Einstein's theory of relativity, turned her towards astronomy. The APS News history column notes that after meeting Harlow Shapley from Harvard, she moved to the United States to do a doctorate in astronomy.
She later published as Cecilia Payne-Gaposchkin. In 1956 she became the first woman promoted to full professor at Harvard and the first woman to chair a department there, both facts recorded by the Science History Institute.
Why did the Sun's spectrum mislead astronomers?
A star's spectrum shows dark absorption lines, each one the fingerprint of a particular atom or ion. Strong lines of a metal seemed to mean a lot of that metal. Payne's insight was that the strength of a line depends on two things: how much of the element there is, and what fraction of its atoms are in the right state to absorb that wavelength. In a hot atmosphere, atoms are ionised and excited to different degrees depending on temperature.
The tool for working out those fractions came from India. As APS News explains, she knew the 1920 work of Meghnad Saha on the thermal ionisation of atoms and applied his equations to the spectra of stars. Saha's equation tells you, for a given temperature and pressure, what fraction of an element is ionised. You can read more about him in our profile of Meghnad Saha and the ionisation equation.
A worked example: why hydrogen lines look weak
The visible hydrogen lines, the Balmer series, come from atoms whose electron starts in the second energy level, n = 2. You may have met these levels in our guide to hydrogen spectrum problems with Bohr's model. Lifting an electron from n = 1 to n = 2 takes 13.6 × (1 − 1/4) = 10.2 eV.
Now compare that with the thermal energy available at the Sun's surface, about 5800 K. The typical energy per particle is kT:
kT = (8.617 × 10⁻⁵ eV/K) × 5800 K ≈ 0.50 eV.
The Boltzmann factor for reaching n = 2 is e^(−10.2/0.50) = e^(−20.4) ≈ 1.4 × 10⁻⁹.
So only about one hydrogen atom in a billion (a few in a billion once you count the extra states at n = 2) is ready to absorb a Balmer line. The lines are modest not because hydrogen is rare but because almost all of it is sitting in the ground state, invisible at those wavelengths. Correct for that, as Payne did star by star, and hydrogen turns out to be by far the most abundant element. The Boltzmann factor is beyond the JEE and CBSE syllabus, but it uses only energy levels and exponentials that you already know.
Why was her result doubted?
Her conclusion went against the leading authority of the day. Harvard Magazine reports that Henry Norris Russell, head of the Princeton Observatory, wrote to her that the findings were "clearly impossible", and that her book Stellar Atmospheres therefore stated that her results were "almost certainly not real". The Science History Institute adds that Russell sat on her thesis committee and that his objection led her to include the cautionary statement.
Four years later the evidence came round. In On the Composition of the Sun's Atmosphere (1929), Russell used a different method and estimated that the solar atmosphere contains 60 parts of hydrogen by volume for every 2 of helium, 2 of oxygen and 1 of metallic vapours. He wrote that this great abundance of hydrogen helped to explain a number of previously puzzling facts. Payne had already published related results in Astrophysical Data Bearing on the Relative Abundance of the Elements in the Proceedings of the National Academy of Sciences in 1925.
What can a student take from how she worked?
Ask what a measurement really measures. A bright line looked like a lot of metal. Payne asked what else controls line strength. In your own problems, ask what an instrument reading actually depends on before you trust it.
Bring tools across fields. She took an equation from atomic physics and used it in astronomy. The Science History Institute quotes her describing the value of her work as bringing together facts that were previously unrelated and seeing a pattern in them.
Keep the evidence even when you have to soften the words. Her tables stayed in the thesis, so others could check them. Write down your working in full, even when you doubt the answer.
Expect resistance to a surprising result, and check it more, not less. The answer was settled by a second method, not by argument.
If energy levels and spectra still feel abstract, the Fermi Concept Library covers every concept in physics and chemistry, each with its own lesson, so you can revise the Bohr model before going further.
Try the worked example again for a hotter star at 10 000 K and see how much larger the fraction of hydrogen at n = 2 becomes.
