Cosmic rays are extremely high-energy, high-velocity particles which bombard Earth on a regular basis. Most of those which reach Earth actually originate outside of the solar system, and new research published in 2013 has finally proven that the main source of cosmic rays are supernovae, or exploding stars. A minority of cosmic rays come from solar flares from Earth's own Sun and energy processes caused by particles entering the solar system from the interstellar medium.
Scientists like Theodor Wulf and Victor Hess first discovered cosmic rays in the early 1900s by measuring background radiation at various altitudes using hot-air balloons. Initially, they could only speculate about the cause, but the term "cosmic ray" was attached because the ionization levels they were detecting seemed to increase with altitude. That seemed to suggest that the cause of the radiation existed beyond the Earth, and that the atmosphere was working as a sort of filter, keeping detectable levels of ionization on the ground comparatively low. This initial speculation naturally led astronomers to speculate that distant stars might be the culprit, although in the absence of hard evidence, that actually left the field quite open for guesses. Over the years, various astronomers have identified supernovae (exploding stars), magnetic variable stars, active galactic nuclei or quasars.
Over time, increasingly sensitive equipment allowed the beginning of piecing together the puzzle. Cosmic rays must ultimately be emitted by some original source, at which time they are made up of protons - the positively charged particles that make up part of an atomic nucleus. When they plunge into the atmosphere of a planet, like Earth, they collide with the molecules in the atmosphere, causing them to burst apart in a shower of subatomic particles. Some may be created by solar flares from the Sun, but the overwhelming number of high-energy cosmic rays have definitely been identified as originating outside of the solar system, and even outside of the Milky Way Galaxy.
There are limits to the research which can be performed on cosmic rays using telescopes and other detection devices on the surface of the Earth, but in 2008, the U.S. government launched the Fermi Gamma-Ray Space Telescope, which, despite the name, is also responsible for some of the latest advances in cosmic ray research. A team of nearly 200 researchers, led by Stefan Funk of Stanford University, has been studying cosmic rays indirectly, by examining the gamma rays, or gamma radiation, which can also be emitted as protons and are accelerated to the extraordinarily high speeds at which they become cosmic rays. Then, Funk's team turned the telescope toward supernovae to measure whether cosmic rays were being created there.
The new research, announced this year and published in the influential journal Science under the somewhat modest title "Detection of the Characteristic Pion-Decay Signature in Supernova Remnants," confirms that when large stars explode as supernovae, they accelerate protons to create cosmic rays. However, Funk's team hasn't yet turned to evaluating all of the other hypothetical sources of cosmic rays, so they can't say for certain yet whether supernovae are the only main cause of cosmic rays or whether there are also a few others.