Green ring: Wavefront or debris feild? (8-26-04 APOD)

[dkdowell]

In the explanation for the August 26 APOD, the false-color green ring is estimated to be 10 light-years in diameter. Also, it states that light from the explosion first reached Earth just over 300 years ago.

If the green ring represents the wavefront from the explosion, it must be moving very quickly to have traveled 5 light-years in 300 years. Lets say 1.6% of the speed of light. Quite a few furlongs per fortnight.

What is the medium in which the wavefront or shock wave is traveling? Is it an actual compresional wave? If so we can compute the density of the medium. Instead of a wave, is it actual debris?

Somebody set me straight.