What causes a supernova?
Medium-mass stars (less than 3 times the mass of our sun) become a red giants and eventually become a supernova. A supernova is the massive explosion of a star accompanied by emission of light and matter so intense that it can outshine an entire galaxy. After a supernove, when all the accessible fuel in a medium-mass star is exhausted, the iron core collapses and proton-electron pairs are converted into neutrons. Such stars are called neutron stars. Neutron stars might spin rapidly giving off light and X-rays or they might emit pulses of energy regularly and be known as pulsars.