Like all stars, the Sun’s interior is constantly relying on nuclear fusion to produce heavier elements from hydrogen. This process not only emits high-energy gamma rays, but also produces neutrinos. Gamma rays are trapped inside the star for thousands of years, constantly heating up the Sun, while neutrinos escape outward from the Sun’s interior at the speed of light.
Scientists already know that there are at least two different types of nuclear fusion within stars. The Sun is considered a relatively small-mass star. This type of star produces helium from hydrogen, mainly through proton-proton chain reactions (pp-chain) inside the star.
For stars that are more massive than the Sun, with higher temperatures and higher core densities, fusion occurs mainly in a cycle called the carbon, nitrogen and oxygen cycle (CNO cycle). This cycle consists of hydrogen nuclei reacting with carbon, nitrogen and oxygen nuclei in turn to produce helium, with these three elements acting in turn as catalysts and regenerating. Scientists believe that this fusion reaction may also partially explain why these three elements are so abundant in the universe, second only to hydrogen and helium.
Both types of fusion produce neutrinos, but the energy levels of the resulting neutrinos are different.
Scientists first detected neutrinos from the Sun in the 1960s, but found nothing more special than the knowledge that they came from the Sun, proving only that nuclear fusion was occurring inside the Sun, but not which type.
Over the past decade, neutrino detectors have become more and more advanced, able to detect not only the energy of neutrinos, but also to distinguish their flavor state.
Scientists now finally know that early experiments detected neutrinos not only from proton-proton chain reactions, but also from a second type of fusion. in 2014, a research group detected low-energy neutrinos, produced directly from proton-proton chain fusion. Their observations confirm that 99% of the sun’s energy comes from proton-proton fusion.
The scientists believe that the Sun is also large enough to be capable of a fraction of carbon, nitrogen and oxygen cycle fusion, and estimate that 1% of the Sun’s energy is produced by this second type of fusion. But neutrinos produced of this type are rare and difficult to detect.
Recently an international team led by The Borexino Collaboration, an Italian neutrino detector project group, has finally succeeded in detecting neutrinos produced by this second type of fusion. The study was published Nov. 25 in the journal Nature.
One of the biggest difficulties in detecting whether neutrinos are produced by fusion in the carbon-nitrogen-oxygen cycle is that their signal is easily disturbed by neutrinos in the terrestrial environment, according to a report in UNIVERSE TODAY. Although nuclear fusion does not occur naturally on Earth, the decay of radioactive elements within certain rocks can trigger neutrino detectors, causing them to mistake neutrinos for neutrinos from fusion in the sun’s carbon-nitrogen-oxygen cycle.
This research group invented a sophisticated analysis method to filter these false signals. Their study confirmed that carbon-nitrogen-oxygen cycle fusion occurs in about 1 percent of the Sun’s interior.
Although the proportion of carbon-nitrogen-oxygen cycle fusion in the Sun’s interior is small, it is an important part of understanding how stars evolve to become larger, the study says. It also helps scientists understand the life cycle of massive stars and the origin of various elements on Earth.
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