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 star 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 interior of the Sun 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 combination of protons producing helium from hydrogen. This so-called PP-chain is an easy reaction to form stars.
For larger stars with hotter and denser cores, the most powerful reactive energy, called CNO-rotation, is the dominant source. This reaction uses hydrogen in the cycle of reactions with carbon, nitrogen and oxygen to produce helium.
The CNO cycle is one of the three components of the universe (with the exception of hydrogen and helium).
Neutrino detectors have proliferated effectively over the past decade. Modern inventors are able to discover 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 percent of the sun’s energy is generated by proton-proton fusion.
While the PP-chain dominates the convergence of the Sun, our star is large enough that the CNO cycle occurs at low levels. This should be the reason for the extra 1 percent of 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 burying their signal surface into 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 will 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.