White dwarf

After our Sun fuses carbon -> oxygen, the outer layers will expand to the radius of mars. After those layers dissipate, the inner core (carbon & oxygen) will compress to the size of earth (extremely dense.) This is a white dwarf, and will burn fo billions of years

When one pair of a binary star becomes a white dwarf, it can steal matter from its pair, fuse carbon and oxygen into iron, and go Supernova.