Oldest Known Supernova Remnant

The oldest known supernova remnant is GSH 138-01-94. It’s dynamical age (i.e. ratio between radius and expansion velocity) is about 16 million years. This age estimate is larger by one to three orders of magnitude larger than the typical supernova remnant. The reason for the unusual longevity of this particular supernova remnant is that it occurred in the outer part of the galaxy, where the density is lower than in the bulk of the galaxy, so the cooling is accordingly slower.