Cassiopeia A in many colors Supernovae are the violent explosions of stars occurring at the end of their lives. On average, one supernova goes off every 50 years or so in our Galaxy. The enormous explosion from these stars blasts material out into the surroundings at very high speeds, sweeping up the surrounding gas into a giant bubble. This is known as a supernova remnant. The shell (or bubble) shines at many different wavelengths, and is very bright in the X-ray because the material from the explosion is very hot.