Hans Bethe explained how the stars shine

Person: Hans Bethe
Lived: 1906–2005
Field: Theoretical physics
From: German-born American

The story

Why the Sun keeps shining for billions of years was one of the great open questions of early twentieth-century science. By 1938 Hans Bethe, a refugee from Nazi Germany, was a professor at Cornell University and an expert on nuclear reactions, but the energy of the stars did not interest him much.

On 17 March 1938 the Carnegie Institution and George Washington University held their fourth annual Washington Conference on Theoretical Physics, devoted to the energy produced by stars. Only 34 people were invited, among them Subrahmanyan Chandrasekhar, George Gamow, John von Neumann and Edward Teller. Bethe at first declined, but Teller persuaded him to come. At the meeting the astronomer Bengt Strömgren set out what was known about the temperature, density and composition of the Sun and challenged the physicists to explain it.

By the end of the conference Bethe, working with Charles Critchfield, had worked out a sequence of nuclear reactions, the proton-proton chain, in which hydrogen fuses step by step into helium. Back at Cornell he examined the possible reactions and their rates and found a second route, the carbon-nitrogen-oxygen cycle, in which carbon acts as a catalyst. The Nobel Foundation later explained that the CNO cycle is the most important process in the brightest stars, while the Sun and fainter stars rely mostly on the proton-proton reaction. Crucially, Bethe also showed why other possible reactions could be ruled out.

Bethe's mother had fled Germany after Kristallnacht. When his graduate student Robert Marshak pointed out that the New York Academy of Sciences was offering a $500 prize for the best unpublished paper on solar and stellar energy, Bethe entered his paper. It won, and he used $250 of the prize to release his mother's furniture. The paper appeared in the Physical Review in 1939.

It was a breakthrough in the understanding of the stars. In 1967 Bethe received the Nobel Prize in Physics for his contributions to the theory of nuclear reactions, especially his discoveries concerning energy production in stars. Later measurements of the Sun's core temperature and luminosity confirmed that the proton-proton chain powers it, and in 2002, aged 96, Bethe wrote to the astrophysicist John Bahcall to congratulate him on using solar neutrino observations to measure the share of the CNO cycle.

Why it matters

Bethe's theory became the foundation of stellar astrophysics, explaining where starlight comes from and how stars live and evolve.

About Hans Bethe

Refugee physicist who explained how the stars shine, led theory at Los Alamos, cracked the Lamb shift and spent half a century campaigning against the nuclear arms race; Nobel Prize 1967.

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Hans Bethe

1906–2005 · Theoretical physics

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