If you look for a definition of a “cosmic ray”, you will find something like this:

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.”

So a “cosmic ray” is a bit of matter, like the nucleus of a hydrogen atom, accelerated to something near the speed of light. There are bits of matter accelerated by explosions on the sun as well, but these cosmic rays usually are not as strong as galactic cosmic rays. The article points out: “Most galactic cosmic rays have energies between 100 MeV (corresponding to a velocity for protons of 43% of the speed of light) and 10 GeV (corresponding to 99.6% of the speed of light)… The highest energy cosmic rays measured to date have had more than 10^20 eV, equivalent to the kinetic energy of a baseball traveling at approximately 100 mph!”

You can imagine what it would feel like to get hit by a baseball going 100 MPH. It would hurt! Now imagine all of that energy concentrated into a particle the size of a proton. On earth, that energy normally gets absorbed when the cosmic ray hits an atom in the atmosphere. What you get is an event that looks like an event we would see in a particle accelerator – millions or billions of particles called “charged mesons” spray out, which then collide with other atoms in the atmosphere. This process is here the carbon-14 in the atmosphere comes from (see How Carbon-14 Dating Works), and may also be the seed that gets lightning bolts started (see How Lightning Works).

We are talking about cosmic rays for a reason, which is found in this article:

New Theory on Rays’ Sprint Across Space

As the article points out: “Since these ultrahigh-energy cosmic rays, as they are known, were first glimpsed in 1963, physicists and astronomers have scratched their heads wondering where they came from and what gargantuan process could produce such energies — wondering, even, if they were real.”

Where do they come from? Now, using a new form or telescope, we have an answer:

Using a new array of cosmic ray detectors known as the Pierre Auger Observatory, which is spread over an area the size of Rhode Island near Malargüe, in the pampas of Argentina, the scientists traced some of the highest-energy cosmic rays back to the vicinities of nearby galaxies bubbling with black hole fireworks, so-called active galaxies.

Since cosmic rays (especially the super-high-energy ones) are not super common, not many of them hit the earth. But if you build a big enough collector, you can detect maybe 30 a year. Then you look at the trajectory of the particle when it arrives and simply follow the trajectory backwards to the point of origin. They seem to be originating in the black holes found in the center of galaxies.

Why should we care? For one thing, it’s nice to have the knowledge that comes from a new form of telescope. Who knows where that knowledge wll take us? For another, cosmic rays have an effect on astronauts, who work outside the protective bubble provided by the atmosphere, and maybe with more knowledge we can come up with a way to protect them. See for example this article: Astronauts’ Children Unlikely to Inherit Cosmic Ray-Induced Genetic Defects.

See also: How Atom Smashers Work