Like all stars, our sun is powered by the fusion of hydrogen into heavy elements. Nuclear fusion is not only what makes stars shine, it is also the primary source of the chemical elements that make up the world around us. Much of our understanding of stellar fusion comes from the theoretical models of the atomic nucleus dello, but for the star closest to us, we also have another source: neutrinos formed at the Sun’s core.
Whenever atoms go through nuclear fusion, they produce not only high energy radiation gamma rays, but also neutrinos. While gamma rays have been heating the interior of the sun for thousands of years, neutrinos zip out of the sun at almost the speed of light. Solar neutrinos were first discovered in the 1960s, but it was difficult to learn much about them unless they were emitted from the sun. This proved that atomic fusion takes place in the sun, but not the type of fusion.
In theory, the predominant form of fusion in the Sun should be the fusion of protons that produce helium from hydrogen. Known as the PP-chain, it is the easiest reaction to form stars. For larger stars with warmer and more dark cores, a more powerful reaction called a CNO-cycle is an effective source of power. This reaction uses helium in the cycle of reactions to produce carbon, nitrogen and oxygen. The CNO cycle is the reason why these three elements are the most abundant in the universe (except hydrogen and helium).
Neutrino detectors have become a lot more efficient over the past decade. Modern detectors are capable not only of neutrinos’ energy, but also of their taste. We now know that the solar neutrinos obtained from early experiments do not come from ordinary PP-chain neutrinos, but from secondary reactions such as boron decay, which makes high energy radiation neutrinos easier to detect. Then in 2014, a team discovered a low-energy neutrino produced directly by a PP-chain. Their observations have confirmed that 99% of the Sun’s energy is generated by proton-proton fusion.
While the PP-chain dominates fusion in the sun, our star is so large that the CNO cycle should take place at a lower level. It should be an additional 1% of the extra energy produced by the sun. But because CNO neutrinos are rare, they are hard to find. But recently a team successfully inspected it.
The biggest challenge in finding CNO neutrinos is that their signal is buried inside the sound of terrestrial neutrinos. Nuclear fusion does not occur naturally on Earth, but low-level radioactive decay from terrestrial rocks can trigger neutrino detector events that are similar to CNO neutrino probes. So the team created a sophisticated analysis process that filters the neutrino signal from the false positives. Their study confirms that CNO fusion occurs at the predictive level of our Sun.
The CNO cycle plays a small role in our Sun, but it is central to the life and evolution of larger stars. This work should help us to understand the cycles of large stars, and to better understand the origins of the heavy elements that make life possible on Earth.
Reference: Boraxino collaboration. “Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun.” Nature 587 (2020): 577
