Supernova remnant IC 443. Optical image of IC 443 (red, centre right), a supernova remnant in the constellation Gemini. North is at top. Two bright stars (orange) are Eta Geminorum (right) and Mu Geminorum (left). IC 443 is part of a shell of gas that was ejected by a supernova, the explosive death of a massive star. The hydrogen gas in the remnant 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.