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neutrino astronomy, study of stars by means of their emission of neutrinos, fundamental particles that result from nuclear reactions and are emitted by stars along with light. Approximately 100 billion neutrinos have raced through your body since you began reading this article. The light received from a star is emitted by the surface layers, which in turn absorb the light coming from the interior. Neutrinos, on the other hand, are absorbed only very weakly by matter and, once created by nuclear reactions in the stellar core, pass directly through the outer parts of the star. Thus neutrinos permit astronomers to look directly into the energy-producing core of a star. Their weak tendency to interact with matter also makes them very difficult to detect. Neutrino "observatories" are located in deep mines, where hundreds of feet of rock shield out the cosmic rays that would completely swamp the tiny effects due to neutrinos. The neutrinos pass as easily through the rock as they pass through the star. They react with chlorine in the detector to produce a radioactive isotope of argon, which is detectable. Because of its proximity, the sun is expected to be by far the most intense source of neutrinos and has been the initial object of study. However, several neutrino detectors observed a rush of neutrinos from Supernova 1987A in a nearby galaxy called the Large Magellanic Cloud. Although their journey from the exploding star began at the moment its core collapsed, they did not move quickly at first since the gravity of the core was so strong. When the shock wave from the explosion reached the neutrinos, it freed them to travel between galaxies, and they arrived on earth about three hours before the first visible light of the explosion appeared.
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