Iron

Iron is all around us. By mass, it’s the most common element in our entire planet, making up most of the core and a good bit of the crust. It even gives our blood its red color. And every bit of it was created inside a star.
All stars “fuse” together lightweight elements to make heavier ones. Right now, the Sun is fusing hydrogen, the lightest element of all, to make helium.
Stars like the Sun eventually make a few other elements, such as carbon and oxygen. But most of the elements beyond those are made inside much more massive stars, where temperatures are hot enough to fuse a succession of heavier elements. These reactions produce energy, which makes the star shine and keeps it from collapsing under its own gravitational pull.
At the end of this chain of reactions is nickel-56, a radioactive element that decays to make iron. At that point, it takes too much energy to make the iron fuse to create heavier elements, so fusion stops. Without the fusion reactions, though, the star’s core