How does the galaxy’s magnetic field affect an old supernova remnant?
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.
In the later stages of a supernova remnant's evolution, the galaxy’s magnetic field plays a significant role in determining the motions of the weakly expanding gas.
As a supernova remnant expands, it progresses through several stages. Initially, it expands violently, sweeping up interstellar material. Over time, the expansion slows as the remnant accumulates a mass of interstellar material comparable to or greater than its own. Eventually, the remnant reaches a final phase where its internal pressure becomes comparable to the pressure of the surrounding interstellar medium. At this point, the remnant loses its distinct identity, and the galactic magnetic field becomes a key factor in governing the movement of the remaining gas as it merges with the local environment.1