Supernovae in high density circumstellar medium
From MassiveWiki
The expansion of a supernova explosion is influenced by the morphology of the surrounding medium, which in turn was shaped by the stellar wind. An aspherical density distribution will cause part of the supernova to expand faster, creating an asymmetric supernova remnant. If the supernova encounters a circumstellar shell, such as produced by an LBV type outburst, the expansion will slow down significantly.
