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.
Cosmic rays are the universe's ultimate frequent flyers, some having traveled across the galaxy for millions of years before hitting Earth's atmosphere! These subatomic speed demons carry energies that dwarf anything we can create in particle accelerators - the most energetic cosmic rays pack the punch of a fastball concentrated into a single proton. They're constantly creating particle showers in our atmosphere and may have influenced the evolution of life by causing genetic mutations.
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