Iron is the “kiss of death” for a star. Stars are just giant engines fusing elements - at the center you have a dense mass that gravity is pushing together and fusing elements into heavier elements. Once a star fuses it’s way up the chain and reaches iron - a chain reaction is set into place because there’s just not enough energy in a star to fuse iron. Since fusion can no longer occur this is where the engine shuts down and gravity overcomes the star, collapsing in on itself