Sired by a Supernova

Cosmic rays born in the shock waves of explosive star death.

Sunday, November 01, 1998

Cosmic rays are high-energy particles that travel throughout the galaxy at velocities near the speed of light. Some hit our atmosphere, creating, among other things, radioactive carbon 14. Astrophysicists have long known that cosmic rays consist of electrons and protons and other particles but haven't had a precise idea about how they were created. Now a new theory holds that cosmic rays are born in the shock waves following the explosive deaths of massive stars.

When such a star explodes as a supernova, says astrophysicist Richard Lingenfelter of the University of California at San Diego, "the whole thing sort of blows its guts out," forming an enormous cloud of material that streams out at thousands of miles per second. The cloud consists of ionized gas, or plasma, and contains the elements hydrogen, helium, and oxygen, along with heavier elements that were generated during the blast itself.

As the cloud expands, the atoms and ions within it begin to cool and condense into grains of graphite, aluminum oxide, various metal oxides, and quartz. Ahead of these growing grains, a shock wave forms at the front of the expanding cloud, which sweeps up whatever material it encounters. Eventually, after a thousand years or so, the plowed-up material begins to slow the shock wave. This allows the grains in the body of the cloud to catch up with and hit particles in the shock wave. These collisions, says Lingenfelter, kick the protons and electrons and other particles up to cosmic-ray speeds.

The theory dovetails nicely with other observations as well. It predicts that the element beryllium should form when cosmic rays break apart carbon and oxygen in the supernova debris cloud. The amount of beryllium in our galaxy seems to agree with the theory's predictions.

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