Hans Bethe (1939): How Nuclear Fusion Makes the Stars Shine

Hans Bethe explained in 1939 why stars shine. His paper “Energy Production in Stars” identified the nuclear fusion reactions that turn hydrogen into helium: the proton-proton chain in the Sun and cooler stars, and the carbon-nitrogen (CNO) cycle in heavier ones. It won him the 1967 Nobel Prize in Physics.

The question

Why stars shine is one of the oldest questions in science. By the 1930s two pieces were in place. Cecilia Payne’s 1925 thesis had shown that stars are made mostly of hydrogen, and Einstein’s E = mc² said that a tiny loss of mass could release a large amount of energy. Turning four hydrogen nuclei into one helium nucleus loses a little mass. But which nuclear reactions actually do it inside a star, and do they produce the right amount of power?

What the paper found

• Main-sequence stars, the ordinary stars that are steadily burning hydrogen, are powered by the fusion of hydrogen into helium.
• In stars more massive than the Sun the dominant route is the carbon-nitrogen cycle, now called the CNO cycle, in which carbon acts as a catalyst: it helps four protons become helium and comes out unchanged at the end.
• In the Sun and cooler stars the dominant route is the proton-proton chain, which fuses hydrogen directly.
• The energy generation rates he computed matched the observed luminosities of stars.

How he did it

Bethe was a nuclear physicist, not an astronomer. He went systematically through the possible reactions between light nuclei, using laboratory knowledge of nuclear physics, and asked which could run fast enough at the temperatures and densities expected in stellar cores. Most were ruled out because they were too slow or would burn their fuel too quickly. Two sequences survived. He then estimated how much energy each would produce in stars of different masses and compared those numbers with real stars.

Why it mattered

Bethe answered the ancient question with physics that could be tested. Fusion explained how the Sun can shine steadily for so long, and why more massive stars burn so much brighter. His work set up the next question, how the heavier elements are made, which the B2FH paper of 1957 answered. Bethe received the 1967 Nobel Prize in Physics for his contributions to the theory of nuclear reactions, especially his discoveries concerning energy production in stars.

What we know now

The reactions Bethe proposed were later confirmed directly. Fusion in the Sun’s core produces neutrinos, ghostly particles that escape straight out. The Homestake experiment of 1968 caught them for the first time, though fewer than expected, and the Sudbury Neutrino Observatory in 2002 showed that the total number matches solar models once neutrinos changing flavor is taken into account. Fusion lifetimes also set how long the Sun has left, a question taken up in when will the Sun die, and they shape the whole life cycle of stars.

Who wrote it

Hans Bethe was a nuclear physicist who had left Germany in the 1930s and become a professor at Cornell University in Ithaca, New York, where he stayed for the rest of his career. He wrote the paper alone. It became the standard account of stellar energy, and the reactions he described are still the ones taught today.

Questions people ask

Why do stars shine?

Stars shine because nuclear fusion in their cores turns hydrogen into helium. The helium weighs slightly less than the hydrogen that made it, and the missing mass becomes energy. Hans Bethe worked out the specific reactions in 1939: the proton-proton chain and the CNO cycle.

What is the CNO cycle?

The CNO cycle is a chain of nuclear reactions in which carbon and nitrogen nuclei act as catalysts to fuse four protons into one helium nucleus. The catalysts come out unchanged. Hans Bethe showed in 1939 that it dominates energy production in stars more massive than the Sun.

What is the proton-proton chain?

The proton-proton chain is the set of fusion reactions that turns hydrogen directly into helium, without catalysts. Hans Bethe showed in 1939 that it dominates in the Sun and cooler stars. Neutrinos from these reactions were later detected on Earth, confirming that fusion powers the Sun.

Read the original

Bethe, H. A. (1939). “Energy Production in Stars.” Physical Review 55, 434-456. doi:10.1103/PhysRev.55.434

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