Cosmic rays are subatomic particles that are flung through space by various astronomical events, including supernovae and "superflares"—more powerful versions of solar flares that happen on other stars.

(See "Hundreds of Superflares Seen on Sunlike Stars.")

Earth is constantly being bombarded by a relatively steady amount of cosmic rays. When the high-energy particles collide with atoms in our atmosphere, they shake loose subatomic particles called neutrons.

Free-floating neutrons can then attach to atmospheric nitrogen, creating carbon 14—a heavier, radioactive version of the element.

Carbon 14 gets incorporated into trees and plants during photosynthesis and into the bodies of animals that eat the vegetation. Scientists can trace its rate of decay in wood and bone to tell the ages of ancient remains.

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