Galactic cosmic rays

Cosmic rays are high-energy radiation, mainly originating outside the Solar System. Upon impact with the Earth's atmosphere, cosmic rays can produce showers of secondary particles that sometimes reach the surface. Composed primarily of high-energy protons and atomic nuclei, they are of mysterious origin. Data from the Fermi Space Telescope (2013) have been interpreted as evidence that a significant fraction of primary cosmic rays originate from the supernovae explosions of stars.Active galactic nuclei probably also produce cosmic rays.

The term ray is a historical accident, as cosmic rays were at first, and wrongly, thought to be mostly electromagnetic radiation. In common scientific usage, high-energy particles with intrinsic mass are known as "cosmic" rays, while photons, which are quanta of electromagnetic radiation (and so have no intrinsic mass) are known by their common names, such as gamma rays or X-rays, depending on their photon energy.

In current usage, the term cosmic ray almost exclusively refers to massive particles, as opposed to photons. Massive particles – those that have rest mass – can gain additional, kinetic, mass-energy when they are moving, due to relativistic effects. Through this process, some particles acquire tremendously high mass-energies. These are significantly higher than the photon energy of even the highest-energy photons detected to date. The energy of the massless photon depends solely on frequency, not speed, as photons always travel at the same speed. At the higher end of the energy spectrum, relativistic kinetic energy is the main source of the mass-energy of cosmic rays.

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