Cassiopeia A: Supernova remnant with no pulsar
This image combines all three types of light we observed.
The round, wispy shape in visible light told us this was probably a supernova, a ball of super-hot gas expanding away from an exploded star.
The fact that X-rays were coming from the same place as the visible light told us that there was no other X-ray source.
The infrared light formed an empty circle, meaning any heat was being generated by gas around the edges of the explosion, and that the gas in the central region, if there is any, isn’t generating heat.
So, this is just a shell of hot gas.