Electromagnetic radiation left over from the original formation of the univiverse in the Big Bang.

Streams of high-energy particles from outer space, consisting of protons, alpha particles, and light nuclei, which collide with atomic nuclei in the earth's atmosphere, and produce secondary nuclear particles; primarily mesons (elementary particles responsible for the forces in the atomic nucleus); such as, pions and muons (types of subatomic particles), that shower the earth.

1. Nuclear and subatomic particles moving through space at high speeds; radiated from the sun and other stars.
2. Streams of ionizing radiations from space, largely of protons, alpha particles, and other atomic nuclei.
3. Very high energy nuclei moving at velocities close to that of light which are probably produced by supernova explosions.

On striking the earth's atmosphere, they produce cascades of other particles (by collision with nuclei in the atmosphere) called air showers.

The time it takes the sun to travel around the center of the galaxy, roughly 22.5 million years.