Why does a supernova remnant eventually blend into the interstellar medium?
This nebula is the product of a supernova explosion; in this section, the blast wave has encountered an area of dense interstellar gas, creating turbulence in the wave and causing it to glow. The picture is a composite of three images taken by the Hubble Space Telescope.
A supernova remnant eventually blends into the interstellar medium because of the natural progression of its expansion and pressure equalization.
As a supernova remnant expands, it evolves through several stages. Initially, it sweeps up surrounding material, but as it continues to grow, the energy from the explosion is distributed over an increasingly larger volume, causing the temperature to drop. Eventually, the remnant reaches a final phase where the internal pressure of the expanding gas becomes comparable to the pressure of the surrounding interstellar medium. At this point, the remnant loses its distinct identity and merges with the local interstellar gas.12
While the bulk of the material eventually integrates into the medium, some regions of very hot gas may persist for a time, producing soft X-rays that can still be observed locally.1