Cosmic rays. Artwork of high-energy particles and radiation from a star in deep space (cosmic rays) impacting molecules and atoms in the Earth's atmosphere. These primary impacts cause a secondary cascade of subatomic particles. Detection and analysis of these particles, which include protons, neutrons, light nuclei, neutrinos, pions, and muons, can reveal the source of the cosmic rays. These sources may include gamma ray bursts (GRBs), active galactic nuclei, supernovae and quasars. Such research is carried out with atmospheric balloons, or with advanced detectors built underground or underwater, to shield them from other radiation.