cosmic rays

Extremely energetic elementary particles travelling through the universe at practically the speed of light. They were discovered by V. F. Hess in 1912 during a balloon flight. Particles beyond the Earth's atmosphere are known collectively as primary cosmic rays. When they encounter the atmosphere, their collisions with atomic nuclei produce air showers of elementary particles, know as secondary cosmic rays. The chemical composition of atomic nuclei found among cosmic rays mirrors the cosmic abundance as found in stars like the Sun, although there are small differences at the highest energies. Cosmic rays are the only particles we can detect that have traversed the Galaxy. The ones of the highest energy may even have come from quasars and active galactic nuclei. Lower-energy cosmic rays are generated within the Galaxy in supernova explosions, supernova remnants and pulsars. Solar flares are a source of the lowest-energy cosmic rays, which increase in intensity at times of maximum solar activity.