Supernova remnant IC 443, optical image. This is part of a shell of gas that was ejected by a supernova, the explosive death of a massive star. The gas is glowing as it collides with a dark, cool cloud of molecular gas. This collision excites the hydrogen molecules, causing them to emit red light. It is thought that the supernova explosion responsible for this shell of gas occurred around 8000 years ago. IC 433 lies in the constellation Gemini.