The solar neighborhood out to 2000 light years or so has several 'bubbles' called Loop I, Loop II and Loop III each of which forms its own roughly spherical volume in space, several hundred light years across. Astronomers think these are formed by ancient supernova explosions which generate expanding shells of gas. We can see these other bubbles because the plasma trapped inside emits X-rays. Our own Local Bubble is about 200 to 300 light years across, and is a low-density region populated by very tenuous clouds (see above sketch). We are entering one of these called the Local Fluff. The properties inside the other Bubbles may be similar, but the density of the gas may be higher if they are younger that the Local Bubble ( about 100,000 years ). It is all very complicated. Our Sun will eventually pass through many of these bubbles in the next few millions of years because supernova are rather common, and it is believed the interstellar medium contains many of these old remnants of supernovae. Entering one of these, we may find the cosmic ray flux increase for thousands of years before subsiding again.
Copyright 1997 Dr. Sten Odenwald
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