Neutrinos prove that the Sun’s core is doing a second type of fusion

Like all stars, our sun is driven by fusion into the heavy elements of hydrogen. Not only does nuclear fusion brighten the stars, it is a primary source of the chemical elements that make up the world around us. Most of our perceptions of stellar fusion come from theoretical models of atomic nuclei, but we have another source for our nearest star: the neutrino formed in the core of the Sun.

Whenever nuclear fusion takes place, they produce not only high-energy gamma rays, but also neutrinos. Gamma rays have been heating the Sun’s interior for over a thousand years, with neutrinos ziping out of the Sun at almost the speed of light. Solar neutrinos were first detected in the 1960s, but it was difficult to know much more about them than they were emitted from the sun.

The CNO cycle kicks in at high temperatures. Credit: RJ Hall

According to the theory, the dominant form of solar synthesis should be the fusion of protons producing helium from hydrogen. Known as the PP-chain, it is the easiest reaction to create stars. For larger stars with warmer and denser cores, the more powerful response, known as the CNO-cycle, is the dominant source of energy. This action uses a cycle of helium to produce carbon, nitrogen and oxygen. The CNO cycle is why these three elements are the most abundant in the universe (excluding hydrogen and helium).

Neutrino detectors have become much more efficient in the last decade. Modern detectors are capable of not only neutrino energy, but also its taste. We now know that the solar neutrinos detected from the initial experiments do not come from ordinary PP-chain neutrinos, but from secondary reactions such as boron decay, which produces higher energy neutrinos that are easier to detect. Directly identified the low-energy neutrinos produced. Their observations confirmed that 99% of the Sun’s energy was produced by proton-proton fusion.

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Different solar neutrino energy levels. Credit: Heron, Brown University

The PP-chain dominates the synthesis in the Sun, but our star is large enough that the CNO cycle should be at a low level. This should be due to the extra 1% of energy produced by the sun, but these are difficult to detect because CNO neutrinos are rare. But recently a team has successfully observed them.

One of the biggest challenges in CNO neutrino detection is that their signals tend to be buried in terrestrial neutrino sounds. Nuclear fusion does not occur naturally on Earth, but low-level radioactive decay from terrestrial rocks can trigger events in neutrino detectors such as CNO neutrino detection. The team has thus developed a sophisticated analysis process that filters the neutrino signal from false positives. Their study confirms that CNO fusion occurs at the forecast level in our Sun.

The CNO cycle plays a minor role in our Sun, but it is also the center of life and evolution of larger stars. This work can help us understand the cycles of the larger stars and better understand the source of the heavy elements that make life possible on Earth.

References: Borexino collaboration. “Experimental evidence of neutrinos produced in the Sun’s CNO fusion cycle.” Nature 587 (2020): 577

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