Like all stars, our sun is powered by the fusion of hydrogen into heavier elements. Nuclear fusion is not only what makes stars glow, it is also a major source of the chemical elements that make up the world around us. Much of our understanding of star fusion comes from theoretical models of atomic nuclei, but for our next star we also have another source: neutrinos, which are created in the core of the sun.
Whenever atomic nuclei fuse, they not only produce high-energy gamma rays, but also neutrinos. While the gamma rays heat the interior of the sun for thousands of years, neutrinos emerge from the sun at almost the speed of light. Solar neutrinos were first discovered in the 1960s, but much was difficult to learn about them other than that they were emitted by the sun. This proved that nuclear fusion takes place in the sun, but not the nature of the fusion.
The CNO cycle starts at higher temperatures. Photo credit: RJ Hall
Theoretically, the dominant 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 simplest response that stars can produce. For larger stars with hotter and denser nuclei, a more powerful reaction known as the CNO cycle is the dominant source of energy. This reaction uses helium in one reaction cycle to produce carbon, nitrogen, and oxygen. The CNO cycle is the reason why these three elements are among the most abundant in the universe (besides hydrogen and helium).
In the past decade, neutrino detectors have become much more efficient. Modern detectors can use not only the energy of a neutrino, but also its taste. We now know that the solar neutrinos detected in early experiments do not come from the usual pp-chain neutrinos, but from secondary reactions such as boron decay, which generate neutrinos with higher energies that are easier to detect. Then, in 2014, a team discovered low-energy neutrinos that are produced directly by the pp chain. Their observations confirmed that 99% of solar energy is generated through proton-proton fusion.
The energy level of various solar neutrinos. Photo credit: HERON, Brown University
While the pp chain dominates fusion in the sun, our star is large enough that the CNO cycle should occur at a low level. It should be what that extra 1% of the sun's energy makes up. However, since CNO neutrinos are rare, they are difficult to detect. But recently a team successfully observed them.
One of the biggest challenges in detecting CNO neutrinos is that their signal tends to be buried in terrestrial neutrino noise. Nuclear fusion does not occur naturally on Earth, but low level radioactive decay of terrestrial rocks can trigger events in a neutrino detector that are similar to CNO neutrino detections. Therefore, the team has developed a sophisticated analysis process that filters the neutrino signal from false positive results. Their study confirms that CNO fusion in our sun is taking place at predicted levels.
The CNO cycle plays a subordinate role in our sun, but is of central importance for the life and development of more massive stars. This work should help us to understand the cycle of the great stars and to better understand the origin of the heavier elements that make life on earth possible.
Reference: The Borexino collaboration. "Experimental evidence for neutrinos produced in the sun in the CNO fusion cycle." Nature 587 (2020): 577