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.
Bubble (astronomy) - Text by M. Alan Kazlev A roughly spheroidal shell of interstellar gas blown outward from a star by a stellar explosion or strong stellar wind. Large bubbles are caused by supernova explosions and contain gas that is much hotter but also much more diffuse (about one atom per cubic cm) than that of the surrunding interstellar medium. These are also called loops.