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Like all stars, our sun is powered by combining hydrogen into heavier components. Atomic fusion is not only the brightness of the stars, but also the 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 atoms, but as for our nearest star, we have another source: the neutrinos formed in the center of the Sun.
Whenever atoms undergo fusion, they produce not only high-energy gamma rays but neutrinos as well. As gamma rays heat the Sun’s interior for thousands of years, neutrinos whiz across the Sun at the speed of light. Solar neutrinos were first discovered in the 1960s, but it’s hard to learn much about them other than the fact that they were ejected from the sun. This showed that nuclear fusion occurs in the sun, but not the type of fusion.
Theoretically, the dominant form of fusion in the sun would be the fusion of protons that produce helium from hydrogen. This so-called PP chain is an easy reaction to form stars. For larger stars with hotter and denser nuclei, the CNO rotation is the dominant source of the most powerful reactive energy. This reaction uses helium in the cycle of reactions to produce carbon, nitrogen and oxygen. The CNO cycle is responsible for the abundance of these three elements (except hydrogen and helium) in the universe.
Neutrino detectors have become increasingly effective over the past decade. Modern inventors are able to create not only the energy of a neutrino, but also its taste. The solar neutrinos detected by the first experiments are not derived from common PP chain neutrinos, but from secondary reactions such as the decay of boron, which produce easily identifiable high-energy neutrinos. Then, in 2014, a team of low-energy neutrinos produced directly from the PP chain was detected. Their observations confirmed that 99% of the solar energy is generated by proton-proton fusion.
Since the PP chain dominates the Sun’s fusion, our star is large enough that the CNO cycle occurs at low levels. This should be due to the 1% more energy produced by the sun. But CNO neutrinos are rare and difficult to detect. But recently a team noticed them successfully.
One of the biggest challenges in detecting CNO neutrinos is that their signal surface is buried in neutrino noise. Atomic 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. The team then developed a sophisticated analytical process that filters the neutrino signal from false positives. Their study confirms that CNO fusion occurs at predicted levels within our Sun.
The CNO cycle plays a small role in our Sun, but is also central to the life and evolution of more massive stars. This work should help us understand the rotation of large stars and better understand the origin of the heavier elements that make life on Earth possible.
Note: Borexino Collaboration. “Experimental evidence of neutrinos produced during the CNO fusion cycle in the sun. ” Natural 587 (2020): 577
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