SN1987A before and after image from Anglo-Australian Observatory. It’s in the LMC, 160,000 light-years distant.
When fusion process no longer produces energy to support the star, the core of the star collapses. With nothing to stop it, the atoms are crushed together, and the infalling material bounces off the superdense core, causing the explosion.
A supernova produces 1040 erg/s (a million times more than the sun). The supernova disperses the elements it has created. In addition, the energy of the explosion creates elements heavier than iron.