An anonymous reader quotes a report from SyFy: For the first time, scientists have detected neutrinos coming from the Sun’s core that got their start via the CNO process, an until-now theorized type of stellar nuclear fusion. […] The Borexino neutrino observatory is 1400 meters under the rock below the Gran Sasso mountain in Italy. It has an 8.5 meter wide nylon balloon filled with 280 tons of pseudocumene, surrounded by a tank of water, surrounded by over 2200 very sensitive photon detectors. They turned everything on, then waited. Over the course of July 2016 – February 2020 (1072 days), they painstakingly recorded all the events, and had to go through heroic efforts to prevent all manners of other reactions that also create little light flashes from interfering with their experiment. They also had to distinguish proton-proton chain neutrinos from ones made in the CNO cycle, but the neutrinos have different energies, which makes it possible to separate them out. They just announced their results: They detected the CNO neutrinos! About 20 per day interacted with the pseudocumene — 20 per day, when sextillions of them had passed through! — about what you’d expect from theory.
This is an important discovery for a lot of reasons. For one thing, while the proton-proton chain dominates in the Sun, in stars with more than about 1.3 times the Sun’s mass the CNO cycle dominates (it kicks in strongly at higher temperatures), so knowing how it works in the Sun tells us about other stars. Also, the presence of heavier elements (what astronomers misleadingly call metals, meaning any element heavier than hydrogen and helium) can affect the fusion rate in the Sun’s CNO cycle, and the amount of these metals isn’t perfectly well known; different methods to measure them yield slightly different amounts, but enough to mess up what we know about the fusion in the core. This experiment agrees with ones that find a lower metal content. That has a ripple effect on a lot of other ideas, including details on how we think the Sun and planets formed, how the Sun ages, and how it will die. All that, from less than two dozen neutrinos a day, while countless more go undetected.
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