A supernova is when a huge star explodes. It usually happens when its nuclear fusion cannot hold the core against its own gravity. The core collapses, and explodes.
The biggest supernovae are called hypergiants and smaller ones are called supergiants. They are massive: because of gravity they use up their energy very quickly. Normally they only live for a few million years.
During the explosion, the total energy radiated by supernovae may briefly outshine the entire output of a galaxy. They emit energy equal to that of the whole lifetime of a solar-like star. The explosion blows of its stellar material away from the star, at velocities up to 30,000 km/s or 10% of the speed of light. This drives a shock wave into the surrounding interstellar medium. This sweeps up an expanding shell of gas and dust, which we see as a supernova remnant. After exploding, what is left becomes a black hole or a neutron star.
Most stars are small and do not explode. They become colder and smaller, and they become white dwarf stars.

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