What are Cosmic Rays?

The term “cosmic rays” is actually misleading as they are not rays at all. In the early 1900s, when cosmic rays were first discovered, they were thought to be made of electromagnetic radiation. However, within a few decades, experimental data showed that cosmic rays were actually made from pieces of atoms: protons, electrons, and atomic nuclei.

Relative abundances of cosmic-ray elements and the ranges covered by different experiments

Where Do They Come From?

Galactic cosmic rays are high-energy particles that have been rapidly accelerated from the magnetic fields of supernovae remnants. Charged particles are accelerated by repeated interactions with the expanding hydromagnetic shock that was produced by the supernova explosion. The maximum energy to which a supernova shockwave can accelerate cosmic rays depends on the strength of the magnetic field and the size of supernova remnant.

OB association model for cosmic-ray origins that worked for cosmic rays through Zirconium (Zr, Z=40) and does not work for the elements above through Barium (Ba, Z=56)

Why are Cosmic Rays Important?

Cosmic rays are one of the best ways to directly study matter that originated outside of our solar system. However, by the time the cosmic rays reach us, the magnetic fields they have passed through have entirely distorted the original flight path and it is impossible to determine where the cosmic ray originated. Therefore, we have to use other means to determine their origins. The elemental composition of galactic cosmic rays is very similar to that of our solar system. However, there are slight differences that give us information about its source. By measuring the quantity of each element in the cosmic rays, we gain information about where it could have originated. Although cosmic rays have been studied for over a century, there are still unanswered questions. Some cosmic rays have been observed at energies much higher than is possible just by a supernova remnant. What caused these ultra-high energy particles is still a mystery yet there is not a lack of theories. Active galactic nuclei (supermassive black holes at the center of nearby galaxies), quasars from loud radio galaxies, and gamma-ray bursts have all been suggested as possible explanations. Questions about cosmic rays tie astrophysics to particle physics in an attempt to further understand the fundamental nature of the universe.