Glossary term: Cosmic Ray

Description: Cosmic rays are energetic charged particles (e.g. protons, nuclei of heavy elements, and electrons) that move through the cosmos.

Cosmic rays can enter Earth's atmosphere. Primary cosmic rays could come from the Sun, Solar System, our Milky Way, or distant galaxies. They are composed of protons (about 90%), helium nuclei (about 9%), heavier atomic nuclei and electrons (about 1%), and a very small amount of antimatter. Light coming from the cosmos is not cosmic rays.

If an energetic primary cosmic ray enters the Earth's atmosphere it can interact with atmospheric particles and produce a large number of secondary charged particles, called secondary cosmic rays. The highest energy cosmic rays are nuclear particles that have kinetic energy equivalent to a tennis ball moving at about 150 kilometers per hour. However, such highest energy particles are rare and most have lower energies.

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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".