Neutrinos prove the Sun is making a second type of fusion in its nucleus

Like all stars, our Sun is powered by the fusion of hydrogen into heavier elements. Nuclear fusion is not only what makes the 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: the neutrinos created in the Sun’s nucleus.

Whenever atomic nuclei undergo a fusion, they produce not only high energy gamma rays, but neutrinos as well. While gamma rays heat the Sun’s interior 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 proved 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 which produce helium from hydrogen. Known as the pp chain, this is the easiest reaction for stars to create. For larger stars with hotter, 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 why these three elements are among the most abundant in the universe (other than hydrogen and helium).

Over the past decade, neutrino detectors have become very efficient. Modern detectors are also capable of not only the energy of a neutrino, but also its flavor. We now know that the solar neutrinos detected from the first experiments do not come from common neutrinos in the pp chain, but from side reactions such as the decay of boron, which create higher energy neutrinos that are easier to detect. . Then in 2014, a team detected low-energy neutrinos directly produced by the pp chain. Their observations confirmed that 99% of solar energy is generated by proton-proton fusion.

While the pp chain dominates the fusion in the Sun, our star is large enough for the CNO cycle to occur at a low level. This should be what represents that additional 1% of the energy produced by the Sun. But since CNO neutrinos are rare, they are difficult to detect. But recently, a team observed them successfully.

One of the biggest challenges in detecting CNO neutrinos is that their signal tends to be buried in the noise of terrestrial neutrinos. Nuclear fusion does not occur naturally on Earth, but low levels of radioactive decay in Earth’s rocks can trigger events in a neutrino detector similar to detections of CNO neutrinos. So the team created a sophisticated analysis process that filters the neutrino signal from false positives. Their study confirms that CNO fusion is occurring in our Sun at predicted levels.

The CNO cycle plays a minor role in our Sun, but it 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 heaviest elements that make life on Earth possible.

Reference: The Borexino collaboration. “Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun.” Nature 587 (2020): 577