High-energy particles, primarily protons and atomic nuclei, that travel through space at nearly the speed of light and constantly bombard Earth's atmosphere. These particles originate from various cosmic sources including supernovae, active galactic nuclei, and solar flares.
Coined by physicist Robert Millikan in 1925, combining 'cosmic' from Greek 'kosmos' (universe) and 'rays' referring to radiation. Initially thought to be electromagnetic radiation like gamma rays, they were later discovered to be energetic particles, though the name 'rays' stuck.
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