Astronomers have suspected for more than a decade that supernova shock waves can act like giant particle accelerators. The basic idea is this: As the remnant of a dead star hurtles through space at up to 30 million kilometers per hour, it creates a shock wave as it interacts with the so-called interstellar medium (ISM). Protons in the shock wave get trapped by the magnetic field of the ISM, which bounces the protons back toward the remnant. But the remnant has its own magnetic field, which repels the protons.
Each bounce adds more energy, and eventually the magnetic tennis match accelerates the protons to nearly the speed of light. Knocked free of the remnant and out into deep space, some of the protons finally hit Earth's atmosphere. The particles are so energetic that astronauts have reported seeing flashes of light--caused by single protons striking their retinas--even when their eyes are closed.
That last part is fascinating! I've never heard about that before. I wonder if that can do damage to one's eyesight. How do you have ample shielding to prevent that? I suppose it doesn't occur too frequently for it to be a concern. I also wonder if these protons are also the one detected by the Auger observatory?