Like all stars, our sun is powered by the combination of hydrogen into heavier components. Atomic fusion is not only the shining of 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 far as our nearest star is concerned, we have another source: neutrinos formed at the center of the Sun.
Whenever atoms undergo fusion, they produce not only high-energy gamma rays but also neutrinos. As gamma rays heat the Sun’s interior for thousands of years, neutrinos zip the Sun at the speed of light. Solar neutrinos were first discovered in the 1960s, but it is difficult to learn much about them other than the fact that they were ejected from the Sun. This proved that nuclear fusion occurs in the sun, but not the type of fusion.
Theoretically, the dominant form of fusion in the sun should 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 cores, 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 more and more effective in the last decade. Modern inventors are able to make not only the energy of a neutrino, but also its taste. Solar neutrinos detected from early experiments are not derived from common PP-chain neutrinos, but from secondary reactions such as boron decay, which produce easily identifiable high-energy neutrinos. Then in 2014, a team Low-energy neutrinos produced directly by the PP-chain were detected. Their observations confirmed that 99% of the sun’s energy is generated by proton-proton fusion.
As the PP-chain dominates the fusion of the Sun, our star is large enough that the CNO cycle occurs at low levels. This should be due to the 1% extra 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 within neutrino noise. Atomic fusion does not occur naturally on Earth, but low levels of radioactive decay from terrestrial rocks can trigger events in a neutrino detector similar to CNO neutrino detections. The team therefore 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 it is also central to the life and evolution of more massive stars. This work should help us to understand the rotation of the big stars and to better understand the origin of the heavier elements that make life possible on Earth.
Note: Borexino Collaboration. “Experimental evidence of neutrinos produced during the CNO fusion cycle in the sun. ” Natural 587 (2020): 577