Cosmic ray

Cosmic rays are very high-energy radiation, mostly from outside the Solar System.
The term "ray" is a historical accident, as cosmic rays were at first, and wrongly, thought to be mostly electromagnetic radiation.
They are particles. Most are protons and alpha particles, which are the nuclei of helium atoms. Some are electrons (beta particles), gamma rays or photons and a tiny fraction are even heavier particles.
Origin.
Their origin is not exactly known. There is evidence that many primary cosmic rays come from the supernovae of massive stars. However, this is not thought to be their only source. Active galactic nuclei probably also produce cosmic rays. Galactic cosmic rays are Particles trapped in the magnetic field of Earth, Particles sent flying into space during a solar flare, and High-energy protons and heavy ions that originate outside our solar system
Secondary cosmic rays.
Cosmic rays can have as much energy as 1020 eV, far higher than the 1012 to 1013 eV that man-made particle accelerators can produce. 
When cosmic ray particles enter the Earth's atmosphere they hit other particles, like molecules of oxygen and nitrogen. This causes a shower of lighter particles, a so-called 'air shower'.
The number of particles made in an air shower event can reach billions. All of the particles in the shower stay within about one degree of the first particle's path. In a shower are x-rays, muons, protons, alpha particles, pions, electrons, and neutrons. 
Typical particles produced in such collisions are neutrons and charged mesons such as positive or negative pions and kaons. Some of these decay into muons. These are able to reach the surface of the Earth, and even penetrate for some distance into shallow mines. The muons can be easily detected by many types of particle detectors. The observation of a secondary shower of particles in different detectors at the same time shows that all of the particles came from that event.
Practical importance.
Effects on Earth chemistry.
Cosmic rays are responsible for the continuous production of certain unstable isotopes in the Earth's atmosphere, such as carbon-14, by the reaction:
Cosmic rays kept the level of carbon-14 in the atmosphere roughly constant (70 tons) for at least the past 100,000 years, until the beginning of above-ground nuclear weapons testing in the early 1950s. This is an important fact used in radiocarbon dating used in archaeology.
Effect on space vehicles.
Cosmic rays are of great practical interest, because they can damage microelectronics and life outside the protection of the Earth's atmosphere and magnetic field.