Cecilia Payne's Thesis (1925): Stars Are Made of Hydrogen

Cecilia Payne discovered in 1925 that stars are made overwhelmingly of hydrogen and helium. In her Harvard PhD thesis she used new atomic physics to read stellar spectra, showing that stars differ mainly in temperature, not composition. The result told astronomers what most of the visible universe is made of.

The question

By the 1920s Harvard astronomers had sorted vast numbers of stars by their spectra, the barcode of dark lines that atoms imprint on starlight. The classes looked very different from one another, and the natural reading was that stars were made of different mixtures of elements. Leading astronomers, including Princeton’s Henry Norris Russell, held that the Sun and Earth had the same composition. The question was whether the lines reflected what stars were made of, or something else.

What the thesis found

• The great variety of stellar spectra is mainly due to differences in temperature, not chemical makeup.
• Stars have broadly similar compositions.
• Hydrogen and helium are vastly more abundant in stars than any other element.
• Under pressure from Russell, an outside examiner of her dissertation, Payne described those hydrogen and helium abundances as “almost certainly not real.”

How she did it

Payne applied a new theory of ionization, developed by the physicist Meghnad Saha, to the Harvard collection of stellar spectra. Saha’s theory predicts how many atoms of an element are ionized, stripped of electrons, at a given temperature. A hot star can show weak lines of an element simply because most of its atoms are in the wrong state to absorb light, not because the element is rare.

By correcting for that effect, Payne could separate the role of temperature from the role of composition. Once temperature was accounted for, most stars looked chemically alike, and the strength of the hydrogen lines pointed to an enormous excess of hydrogen.

Why it mattered

Payne had found what stars, and therefore most of the matter we can see, are made of. The result went against the consensus, and Russell convinced her to downplay it. In 1929 Russell reached the same conclusion by a different method, and the hydrogen-rich picture was accepted.

Everything later in stellar physics builds on it. A hydrogen-rich Sun is what makes Bethe’s 1939 theory of nuclear fusion in stars work, and the abundances of heavier elements became the puzzle that the B2FH paper on how stars make the elements set out to solve in 1957. Our guide to stars traces how those ideas fit together.

What we know now

Payne’s central result has held up completely: hydrogen and helium dominate the composition of stars, and the cosmic abundances measured today rest on the method she pioneered. The thesis itself is now in the public domain and was released on Project Gutenberg in 2024, so anyone can read it a century after it was written.

Who wrote it

Cecilia Payne, who had studied physics at Cambridge before moving to Harvard College Observatory, earned the first PhD in astronomy at Radcliffe College, the women’s college at Harvard. She later published as Cecilia Payne-Gaposchkin. The astronomer Otto Struve later called her thesis the most brilliant PhD thesis ever written in astronomy. Harvard College Observatory is now part of the Center for Astrophysics | Harvard & Smithsonian.

Questions people ask

What did Cecilia Payne discover?

Cecilia Payne discovered that stars are made mostly of hydrogen and helium. In her 1925 PhD thesis she showed that the differences between stellar spectra come mainly from temperature, and that once temperature is accounted for, stars share a similar composition dominated by the two lightest elements.

Why was Cecilia Payne’s discovery doubted?

Many astronomers in 1925, including Henry Norris Russell of Princeton, believed the Sun and Earth had the same composition. Russell, an examiner of her thesis, persuaded Payne to call her hydrogen and helium abundances “almost certainly not real.” Russell reached the same conclusion himself in 1929, and her result was then accepted.

What are stars made of?

Stars are made overwhelmingly of hydrogen and helium, with small amounts of heavier elements such as carbon, oxygen and iron. Cecilia Payne first showed this in 1925 by analyzing stellar spectra with Saha’s ionization theory. The heavier elements were later shown to be forged inside earlier generations of stars.

Read the original

Payne, C. H. (1925). “Stellar Atmospheres: A Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars.” Harvard Observatory Monographs No. 1 (PhD thesis, Radcliffe College). Project Gutenberg edition

The full thesis is in the public domain and free to read or download from Project Gutenberg.