Did Cecilia Payne Discover What Stars Are Made Of?
Ask what the Sun is made of and the answer today is quick: mostly hydrogen, with helium second and everything else a trace. A century ago almost nobody believed that. The person whose numbers first said it was a young graduate student at Harvard College Observatory named Cecilia Payne. The short answer to our question is yes, largely, but with a twist she wrote herself. Our sticker Cecilia Payne-Gaposchkin: Almost Certainly Real answers one sentence in her thesis.
Who was Cecilia Payne?
Cecilia Payne (later Payne-Gaposchkin) came from England to Harvard College Observatory, where she completed her doctoral thesis in 1925 (Harvard Wolbach Library[1]). It was published that year as Stellar Atmospheres, the first Harvard Observatory Monograph (Payne 1925[2]).
What did her 1925 thesis show?
A new tool: Saha’s ionisation theory
Stellar spectra differ enormously from star to star. Earlier astronomers had sorted them into classes but could not say why they differed. In 1920 the Indian physicist Meghnad Saha published a theory of how atoms in a hot gas lose electrons (ionise) as the temperature rises (Saha 1920[3]). Payne applied it systematically to the spectra Harvard had collected (Payne 1925[2]).
Stars differ mainly in temperature
Her central result: the great differences between stellar spectra are mostly differences of temperature, not of ingredients. Different temperatures leave different atoms in different states of ionisation, and that changes which lines appear. Underneath, the stars looked strikingly uniform in composition (Payne 1925[2]).
And hydrogen and helium dominate
Her numbers also said something far stranger: hydrogen and helium vastly outnumbered every other element in stellar atmospheres. At the time, many astronomers assumed the Sun had roughly the same make-up as the Earth’s crust.
Why this was a surprise
The surprise was not only the size of the numbers. Hydrogen lines are prominent in many stellar spectra, but how strong a line looks depends on temperature and ionisation as much as on how much of an element is present. That is exactly the effect Saha’s theory let her correct for. Once she corrected for it, the abundances of most heavier elements came out similar from star to star. Hydrogen and helium were the glaring exceptions. A result that agreed so well for most elements and disagreed so wildly for two was easy to blame on the method rather than on the stars, and in 1925 that is how she, following Russell, framed it.
Why did she call it “almost certainly not real”?
On page 188 of the thesis, Payne wrote: “The enormous abundance derived for these elements in the stellar atmosphere is almost certainly not real” (Payne 1925[2]). She suggested that hydrogen’s figure might reflect its abnormal behaviour in the spectrum rather than a true abundance.
The context matters. Henry Norris Russell of Princeton, the leading authority on the subject, had written to her that it was “clearly impossible that hydrogen should be a million times more abundant than the metals,” and her caution echoes and cites him (DeVorkin 2010[4]).
Did Russell take the credit?
This is where the popular story often goes too far. By 1929, Russell had found hydrogen dominant in the Sun’s atmosphere by other routes (Russell 1929[5]). His paper does credit her: he called her thesis “the most important previous determination of the abundance of the elements by astrophysical means” (Russell 1929[5]).
The historian David DeVorkin, looking at the letters and the thesis, reads the episode as a matter of standards of evidence rather than suppression: an extraordinary claim needed extraordinary support, and Russell approved her thesis, abundance table and all (DeVorkin 2010[4]). Others emphasise how much pressure a young woman scientist faced from the field’s most powerful figure. How much pressure she felt, and how much credit she lost, is still argued over. We label that point open, rather than picking a side.
What is not in doubt: her thesis made the first strong quantitative case, from stellar spectra, that stars are mostly hydrogen, and she put her numbers in print even while doubting them. For other “who really discovered it?” cases in physics, see Who Really Discovered It? Physics Credit Myths.
So what are stars made of?
By number of atoms, ordinary stars like the Sun are overwhelmingly hydrogen, with helium a distant second, and all the heavier elements together a small fraction. That hydrogen and most of the helium date back to the first few minutes after the Big Bang; the heavier elements were made later, inside stars. Our pillar post How Did the First Elements Form? tells that story, and it starts with the fact Payne’s thesis first put on the page.
APS News retells how her result changed astronomy (APS News[6]).
Wear the result
The Almost Certainly Real sticker is also on the Cosmic Weights & Measures sticker sheet, alongside Rømer, Du Châtelet and Chandrasekhar. Meet the rest of the portraits in Physicist Stickers.
FAQ
What did Cecilia Payne discover?
That differences in stellar spectra are mainly differences in temperature, and that stellar atmospheres are dominated by hydrogen and helium.
Why did she doubt her own result?
It contradicted the accepted view, and Russell had told her such a hydrogen abundance was impossible. She wrote that it was “almost certainly not real.”
Did Russell credit her?
Yes. His 1929 paper called her thesis the most important previous astrophysical determination of the abundances.
When was her thesis published?
1925, as Harvard Observatory Monographs No. 1, Stellar Atmospheres.
References
- Harvard Wolbach Library, “Cecilia Payne-Gaposchkin: education and doctoral thesis”. https://library.cfa.harvard.edu/cecilia-payne-gaposchkin/education-and-doctoral-thesis
- C. H. Payne (1925), Stellar Atmospheres, Harvard Observatory Monographs No. 1 (thesis; ADS scan). https://articles.adsabs.harvard.edu/pdf/1925HarMo...1.....P
- M. N. Saha (1920), “Ionization in the solar chromosphere,” Phil. Mag. 40, 472. https://doi.org/10.1080/14786441008636148
- D. H. DeVorkin (2010), “Extraordinary claims require extraordinary evidence: C. H. Payne, H. N. Russell and standards of evidence,” J. Astron. Hist. Herit. 13, 139. https://articles.adsabs.harvard.edu/pdf/2010JAHH...13..139D
- H. N. Russell (1929), “On the Composition of the Sun’s Atmosphere,” ApJ 70, 11. https://doi.org/10.1086/143197
- R. Williams (2015), APS News, “January 1, 1925: Cecilia Payne Gaposchkin and the day the universe changed”. https://www.aps.org/apsnews/2015/01/cecilia-payne-gaposchkin-universe-changed
Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.