Q&A: Supernova Remnants and Neutron Stars

Q:
I was looking at the NASA site that has an AVI of the time lapse shots taken of the Crab Nebula. I have a crazy question to ask. Those shots are 8 shots taken over a 6 month period, 1/2 light year. The caption states the inner ring is one light year across. That would make the next ring about 2 light years across. I note that there are at least 3 "waves" that pass the distance between the 2 rings in 6 months time. Wouldn't that mean that the waves would be traveling at least 2 C if not faster?

A:
It is often difficult to sort out what is moving in such images. In the paper by J. Hester et al. in The Astrophysical Journal, Volume 577, Issue 1, pp. L49-L52 (September 20 issue) the authors report that the wisps move about 11-15 arcsec/year, which converts to about 0.4 light years/yr, implying a speed of about 40% of the speed of light. This looks consistent with the image at http://chandra.harvard.edu/photo/2002/0052/index.html .