Speculation into the cause of the rings includes beamed jets emanating from a dense star left over from the supernova, and a superposition of two stellar winds ionized by the supernova explosion. By looking at external galaxies, astronomers can guess that these events, known as a supernovae , should occur about once every 30 years in a typical spiral galaxy like our MilkyWay. In fact, in 1572, the revered Danish astronomer, Tycho Brahe, witnessed one of the last to be seen. The remnant of this explosion is still visible today as the shockwave it generated continues to expand into the gas and dust between the stars. The nebula is known as Tycho's Supernova Remnant .
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