Supernova remnant IC 443, optical image. IC 443 is the round shell of gas at upper left. It is thought that it was cast off in a supernova explosion around 8000 years ago. A supernova is the death of a massive star, in which the outer layers are cast off into space after its core collapses then rebounds violently. The ejected gas glows as it collides with surrounding cool dark gas (not seen). It hits this at such high speed that the hydrogen becomes ionised, causing it to glow red. At lower right is the unrelated emission nebula IC 444, which glows as its hydrogen is ionised by stellar radiation. The stars Eta (left) and Mu (right) Geminorum are also seen.