cosmic rays
/ KOZ-mik /
Despite the name, cosmic rays are not rays of light at all — they are tiny particles, mostly the bare nuclei of atoms, racing through space at very nearly the speed of light. They rain down on Earth constantly, day and night, passing harmlessly through your body by the dozens every second, having traveled from violent events scattered across the galaxy and beyond.
Most cosmic rays are single protons (hydrogen nuclei), with a smaller fraction of heavier nuclei and electrons. What makes them remarkable is their energy: the fastest carry the punch of a well-hit tennis ball packed into a single subatomic particle — millions of times more energy than human accelerators can reach. The lower-energy ones come from the Sun, but the higher-energy galactic cosmic rays are thought to be accelerated by the shock waves of supernova explosions, which can fling particles back and forth across the blast front, boosting their speed each time. Because they are electrically charged, the galaxy's magnetic field bends their paths into tangled spirals, so by the time they arrive their original direction is scrambled and we cannot simply trace them back to their source.
Cosmic rays are an active ingredient of the interstellar medium, not just passing traffic. They ionize and heat the cold gas deep inside dense clouds where starlight cannot reach, driving the chemistry that builds molecules. They also help set the pressure that puffs up the galactic gas. The very highest-energy cosmic rays, far rarer and likely from outside our galaxy, remain a genuine mystery — we still are not certain what natural engine can accelerate a single particle to such staggering energies.
When a cosmic-ray proton slams into the upper atmosphere, it shatters into a cascade of secondary particles — an 'air shower' raining down over a wide area — and detectors on the ground catch this debris to reconstruct the original particle's energy.
A single cosmic ray triggers an air shower of secondary particles.
Cosmic 'rays' are particles, not electromagnetic radiation, and because their charged paths are bent by magnetic fields, the direction they arrive from is not the direction of their source. This is why pinning down where they come from is so hard.