Stars cannot get energy out of fusing iron into heavier elements. As a result, when a star does fuse lighter elements to make iron, the iron just sits there in its core.

There is a limit to how much mass you can have before gravity becomes stronger, the normal forces that prevent you from squishing matter together. When the amount of iron gets big enough that gravity becomes this strong, the iron core of the star collapses. This collapse releases the energy from gravity that powers a supernova.

Supernovae happen about once per century per Milky Way-sized galaxy.

Congrats - you have the correct plural for 'supernova.' A supernova happens where there is a change in the core, or center, of a star.

One of the stars, a carbon-oxygen white dwarf, steals matter from its companion star. Eventually, the white dwarf accumulates too much matter. Having too much matter causes the star to explode, resulting in a supernova.

You can check out this link.

From the link: white dwarf: a star near the end of its life that has used most or all of its nuclear fuel and collapsed into a size similar to Earth.