Supernova Remnants
Supernovae are the most energetic and spectacular events in the universe. When a large, hot star reaches the end of it's main sequence lifetime, it implodes and the material that once made up the star is emitted into the interstellar medium. This ejected material sweeps up all of the surrounding gas and dust as it expands, this produces a shock waves that excites and ionizes nearby gas. X-rays emitted by the supernova are also instrumental in the ionization of the gas. We can observe these supernovae remnants as they are strong radio emitters due to their synchrotron radiation.
This massive shock wave ploughs through the interstellar medium and heats the material to millions of Kelvins which means that X-rays are emitted. In general these remnants show very symmetrical shells with variations in brightness around there rims which indicates the patchy nature of the surrounding interstellar medium. Below is an example of a supernova remnant.