Q&A: Supernova Remnants and Neutron Stars

Q:
I saw the photo of the remnants of Cassiopeia A which exploded some 300 years ago (from our frame of reference). How can the gases still be at 50 million degrees? Wouldn't the density of the gas have decreased to a point that the temperature dropped significantly?

A:
It's the low density of the gas in supernova remnants that results in its slow cooling. Cooling processes like collisions act faster with high density gas, and slower with low density gas.