Cosmic rays are a form of electromagnetic radiation.

The word “ray” did not originally settle what cosmic rays were. Early experiments showed that penetrating ionizing radiation increased with altitude, pointing to an extraterrestrial source, but physicists still had to decide whether the incoming radiation behaved more like very energetic gamma rays or like charged particles.

That distinction became experimentally testable. Cloud chambers revealed tracks and secondary showers, while studies of how the radiation responded to Earth’s magnetic field and laboratory magnetic fields showed charge-dependent behavior that electromagnetic waves could not explain. By the 1930s, cosmic-ray research was also uncovering new particles, including the positron, through tracks produced in detectors. The primary radiation was increasingly understood as energetic matter arriving from space.

Today most primary cosmic rays are atomic nuclei—especially protons—with smaller contributions from heavier nuclei and electrons. High-energy photons also arrive from space, and cosmic rays generate electromagnetic radiation secondarily, but the historical name survived after the physical classification changed.