Gamma rays are the very highest energy region of the electromagnetic spectrum. Just as visible light is emitted from atoms when electrons change their orbits, so gamma rays are emitted when the atomic nucleus itself changes, such as in radioactive decay. Supernovae are a main source of gamma radiation, when unstable radioactive elements created in the violence of the explosion later decay. The supernova deaths of the biggest super-giant stars create such extremes of temperature and pressure that they can release a sudden flash of gamma radiation, called a gamma-ray burst.
