Cosmic rays are electrically charged, high-energy particles that travel through space. They are subatomic particles, units of matter smaller than an atom. Astronomers think that cosmic rays fill galaxies, including our own Milky Way, and cross vast stretches of intergalactic space (the space that separates the galaxies).
Scientists study cosmic rays because these particles include the only matter that reaches Earth from outside the solar system. Studies of cosmic rays reveal conditions in intergalactic space and interstellar space (the space that separates the stars). Scientists also study cosmic rays to learn about supernovae (exploding stars).
Physicists measure the energy of cosmic rays in units called electronvolts (eV). One electronvolt is the energy needed to move an electron between two points with a potential difference of 1 volt (see Volt). Most cosmic rays have energies between a few million and a few billion electronvolts. Scientists abbreviate 1 million electronvolts as 1 MeV and 1 billion electronvolts as 1 GeV. A proton with 1 GeV of energy can go through almost 2 feet (60 centimeters) of iron. The highest-energy cosmic rays reach 100 billion GeV. They can have more than 100 million times the energy of the most energetic particles scientists can produce using devices called particle accelerators.
Scientists divide cosmic rays into two types: (1) primary cosmic rays, or primaries, which originate in outer space; and (2) secondary cosmic rays, or secondaries, which originate in Earth's atmosphere. 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 Earth's surface.
