COSMIC-RAY ACCELERATOR is believed to arise from a supernova explosion. Astrophysicists hypothesize that atomic nuclei crossing the supernova shock front will pick up energy from the turbulent magnetic fields embedded in the shock. A particle may be deflected in such a way that it crosses the boundary of the shock hundreds or even thousands of times, picking up more energy on each passage, until it escapes as a cosmic ray. Most of the particles travel on paths that result in relatively small accelerations, accounting for the general shape of the cosmic-ray energy spectrum (see image), which falls off at higher energies. The "knee," or bend, in the curve suggests that most of the particles are accelerated by a mechanism incapable of imparting more than about 1015 electron volts. The relative excess of ultrahigh-energy particles indicates an additional source of acceleration whose nature is as yet unknown.
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