how we figured out why the sun shines

The Mechanism

Hans Albrecht Bethe was born on 2 July 1906 in Strasbourg, then part of the German Empire, the son of a Prussian professor of physiology and a musically gifted Jewish mother. He took his doctorate at Munich under Arnold Sommerfeld in 1928, at 22, then held junior posts at Frankfurt and Stuttgart. In April 1933, seven months after Hitler was appointed Chancellor, Bethe — half-Jewish under the newly-enacted *Gesetz zur Wiederherstellung des Berufsbeamtentums* — was dismissed from his teaching position at Tübingen. He emigrated first to Manchester and then, in 1935, to Cornell University in Ithaca, New York, as assistant professor of physics; he was 28. The question of what powered the stars was one of the oldest open questions in modern physics. In 1854 Hermann von Helmholtz and William Thomson (Kelvin) had proposed that the sun's heat came from slow gravitational contraction — a mechanism that, given the sun's mass and luminosity, could power it for a few tens of millions of years but no more. In the 1860s Kelvin used this to estimate the age of the sun at between twenty and fifty million years and set off a bitter dispute with Charles Darwin and the geologists, who wanted much longer. In 1920 Arthur Eddington, in his address "The Internal Constitution of the Stars" to the British Association at Cardiff, pointed out that Aston's new measurements of atomic weights showed helium was 0.8 % lighter than four hydrogens; if hydrogen could be fused into helium inside a star, the mass defect would supply energy for far longer than gravitational contraction could — long enough for evolution. He offered no mechanism, and closed with the memorable line: "If the answer is 'No, this is not possible,' we can only say to the physicists, 'Please go and find a hotter place.'" By the mid-1930s cross-sections for nuclear reactions were finally being measured; a young Cornell colleague of Bethe's, Charles Critchfield, had just calculated in early 1938 that at solar-core temperatures two protons could collide, one could β-decay into a neutron, and the resulting deuteron could stick — a rate slow but not impossible. On the morning of Thursday, 17 March 1938, Bethe took the train down from Ithaca to Washington, D.C., for the fourth annual Washington Conference on Theoretical Physics — jointly hosted by George Washington University and the Carnegie Institution, and organised by George Gamow, Edward Teller, and Merle Tuve. Only 34 physicists were invited. The conference topic that year was the internal constitution of the stars; the astrophysicist Bengt Strömgren opened the meeting with what was known about the sun's composition, temperature, density, and luminosity. Bethe had almost turned the invitation down — nuclear astrophysics was not his subject — and Teller had had to persuade him to come. During the three-day conference (17–19 March), Bethe and Critchfield discussed their proton-proton chain calculation in detail. The mechanism accounted for the sun and cooler stars but broke down for heavier stars whose interiors are hotter than about 18 million kelvin. On the train ride back to Ithaca, and in the weeks that followed at Cornell, Bethe worked out a second mechanism that could power hotter stars: a catalytic cycle in which a carbon-12 nucleus captures a proton to become nitrogen-13, which β-decays into carbon-13, which captures another proton to become nitrogen-14, which captures another to become oxygen-15, which β-decays into nitrogen-15, which captures a fourth proton and splits into helium-4 and carbon-12 — regenerating the original carbon and releasing an α-particle's worth of binding energy. Four protons in, one helium out, and the carbon nucleus recovered unchanged as a catalyst — the ***carbon-nitrogen-oxygen (CNO) cycle***. Bethe submitted the completed 23-page paper, *"Energy Production in Stars,"* to the *Physical Review* on 7 September 1938; it was published on 1 March 1939 in Volume 55, pages 434–456. In the paper, Bethe walked systematically through every nuclear reaction between light nuclei that could occur at stellar temperatures and worked out which combinations could plausibly account for the observed luminosities and lifetimes of stars of different masses. The answer that emerged was clean and stayed clean: cool stars like the sun burn by the proton-proton chain; hot stars burn by the CNO cycle. The paper made the sun and every star like it a hydrogen-to-helium nuclear reactor. Astronomy became nuclear physics. Bethe was later awarded, for this single paper, the 1961 Eddington Medal of the Royal Astronomical Society and, in 1967, the Nobel Prize in Physics — the citation reading "for his contributions to the theory of nuclear reactions, especially his discoveries concerning the energy production in stars." He continued to work on the physics of stars until his death on 6 March 2005 at the age of 98, still publishing on solar-neutrino problems in his ninety-eighth year. Every photon striking your face on a sunny afternoon began four hydrogen nuclei ago, in a helium nucleus that was made in a chain of reactions worked out in fewer than six months by a young half-Jewish refugee at Cornell in the spring of 1938.