Q&A: Supernova Remnants and Neutron Stars

Q:
I was reading from the ABC Science site that the diameter of the Cas A gas cloud is 10 light years and the event occurred 300 years ago. That would mean that this gas cloud is moving at a rate of close to a tenth of the speed of light. Is that true?

A:
The diameter of the Cas A remnant is about 10 light years, so the radius is about 5 light years. Since the remnant is about 300 years old, its expansion velocity is about 5/300 (=1/60) of the speed of light!