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Like all stars, our Sun is powered by the fusion of hydrogen into heavier elements. Nuclear fusion is not only what makes stars shine, it is also a primary source of the chemical elements that make up the world around us. Much of our understanding of stellar fusion comes from theoretical models of atomic nuclei, but for our closest star, we also have another source: neutrinos created in the Sun’s core.
Whenever atomic nuclei undergo fusion, they produce not only high-energy gamma rays but also neutrinos. As gamma rays heat the interior of the Sun for thousands of years, neutrinos exit the Sun at almost the speed of light. Solar neutrinos were first detected in the 1960s, but it was difficult to learn much about them other than the fact that they were emitted by the Sun. This showed that nuclear fusion occurs in the Sun, but not the type of fusion.
According to the theory, the dominant form of fusion in the Sun should be the fusion of protons that produces helium from hydrogen. Known as the pp chain, it is the easiest reaction for stars to create. For larger stars with hotter and denser nuclei, a more powerful reaction known as the CNO cycle is the dominant energy source. This reaction uses helium in a cycle of reactions to produce carbon, nitrogen and oxygen. The CNO cycle is the reason these three elements are among the most abundant in the universe (besides hydrogen and helium).
Over the past decade, neutrino detectors have become very efficient. Modern detectors are also capable of not only a neutrino’s energy, but also its flavor. We now know that the solar neutrinos detected by early experiments do not come from common pp chain neutrinos, but from secondary reactions such as boron decay, which create higher energy neutrinos that are easier to detect. Then, in 2014, a team detected low-energy neutrinos produced directly from the pp chain. Their observations confirmed that 99% of the solar energy is generated by proton-proton fusion.
While the pp chain dominates the merger into the Sun, our star is large enough that the CNO cycle should take place at a low level. It should be what represents that 1% more of the energy produced by the sun. But because CNO neutrinos are rare, they are difficult to detect. But recently a team has successfully observed them.
One of the biggest challenges in detecting CNO neutrinos is that their signal tends to be buried in the noise of Earth’s neutrinos. Nuclear fusion does not occur naturally on Earth, but low levels of radioactive decay from Earth’s rocks can trigger events in a neutrino detector similar to CNO neutrino detections. Then the team created a sophisticated analysis process that filters the neutrino signal from false positives. Their study confirms that CNO fusion occurs within our Sun at the expected levels.
The CNO cycle plays a minor role in our Sun but is central to the life and evolution of more massive stars. This work should help us understand the cycle of large stars and could help us better understand the origin of the heavier elements that make life on Earth possible.
Reference: The Borexino collaboration. “Experimental tests of neutrinos produced in the CNO fusion cycle in the Sun”. Nature 587 (2020): 577
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