Supernova remnants are the dramatic objects produced by the violent explosion of a massive star at the end of its life. This explosion, called a supernova, is one of the most energetic events in the universe, and causes a single star to briefly outshine the entire galaxy in which it is located.
Astronomers believe that supernovas occur in our galaxy roughly once every hundred years on average.
The explosion itself involves the core of the massive star, which is primarily composed of iron by the time it explodes. Once the hydrogen in the core is exhausted, the helium is fused to form carbon, nitrogen, and oxygen, releasing more energy. The structure is like an onion. There are two primary results. It blasts a hole in the ISM that gradually expands until it reaches up to several hundred light years in diameter.