Cosmic Rays


	What Are Cosmic Rays?

	Cosmic rays are high energy charged particles, originating in outer space, that travel at nearly the speed of light and strike the Earth from all directions. Most cosmic rays are the nuclei of atoms, ranging from the lightest to the heaviest elements in the periodic table. Cosmic rays also include high energy electrons, positrons, and other subatomic particles. The term "cosmic rays" usually refers to galactic cosmic rays, which originate in sources outside the solar system, distributed throughout our Milky Way galaxy. However, this term has also come to include other classes of energetic particles in space, including nuclei and electrons accelerated in association with energetic events on the Sun (called solar energetic particles), and particles accelerated in interplanetary space.


	Where Do Cosmic Rays Come From? The origin of cosmic rays is the burning question in high energy astrophysics research. Unfortunately it is hard to tell where the cosmic ray has came from. The problem is that cosmic rays carry electric charge and do not travel in straight lines. Their trajectories are bent by the magnetic fields that are known to exist between stars and galaxies.

Cosmic Ray trajectories: The diagram illustrates the trajectories of cosmic rays and gamma rays from their point of origin to the Earth.

1. Electrically charged cosmic rays are bent by interstellar magnetic fields and do not travel in straight lines. When we measure their trajectory at the Earth we cannot tell where they came from.

2. Gamma rays are neutral particles and so travel in straight lines. If we can measure their trajectory when they hit the Earth, then we can see where they came from.




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	Andrea T. Hughes - March 2004