Cosmic rays are one of the few samples of matter we have from outside our Solar System , for comets and large meteorites that bombard the Earth all originate from within our Solar System. Cosmic rays, therefore, carry great histories. We know from previous research on the origin of the elements that heavier elements are created as a star depletes its nuclear fuel of the lighter elements, hydrogen and helium.
Lighter elements fuse into heavier ones.
When a star explodes, these heavier elements fly into space, often creating even heavier elements, such as uranium. The elements are accelerated to nearly the speed of light by a mechanism not well understood.