The Gum Nebula (enhanced image); the brighter central region is the Vela Supernova remnant
Extensive emission nebula in Vela and Puppis, 1100 light years in diameter and 450 ly from Sol at the closest point. The Gum Nebula is so large and diffuse it is hard to distinguish without enhanced sensors.
This nebula formed more than a million years ago after a supernova event, and contains two smaller and younger supernova remnants, including the Vela Supernova Remnant.
Supernova Remnant - Text by M. Alan Kazlev The expanding and cooling shell of gas and dust that is visible for thousands of years after a supernova. After a few tens of thousands of years supernova remnants mix with the interstellar medium and dissipate.