They are subatomic particles, units of matter smaller than an atom. Astronomers believe that cosmic rays fill our Milky Way Galaxy and other galaxies. Scientists study cosmic rays because these particlesare the only matter that reaches the earth from outside the solar system. Scientists also learn about processes that occur when a star explodes as a supernova. Most cosmic rays have energies between a few million and a few billion electronvolts.
Secondaries form when primaries collide with atoms at the top of the atmosphere.
The collision changes the primary and the atom into a shower of secondaries. Many secondaries then collide with other atoms, making more secondaries. Some secondaries reach the surface and even penetrate deep into the ground. No measurable amount of primaries reaches the earth's surface. Most primaries that reach the earth's atmosphere have traveled through the Milky Way for millions of years.