A supernova occurs when a high-mass star reaches the end of its life. When nuclear fusion in the core of the star stops, the star collapses and explodes . The expanding shell of gas forms a supernova remnant. The Crab nebula is a supernova remnant which probably exploded in 1054 AD. Light and X-ray emission from supernova remnants comes from ionized gas. There is a huge amount of radio emission called synchrotron emission. This emission originates from high-velocity electrons oscillating in magnetic fields.