Someone else can probably explain this properly, but I believe the fusion reaction inside the Sun ends up leaving it as all Iron. Something about the fusion reaction past a certain point can't produce elements higher than Iron on the period table.

Edit: https://www.ck12.org/flexi/chemistry/nuclear-fusion-in-chemistry/why-do-nuclear-fusion-reactions-stop-once-nickel-and-iron-are-formed-in-the-core-of-stars/

> When very massive stars leave the main sequence, they first become red supergiants and then end their life cycles in with a bang. Unlike a red giant, when all the helium in a red supergiant is gone, fusion continues. Lighter atoms fuse into heavier atoms up to iron atoms. Creating elements heavier than iron through fusion uses more energy than it produces so stars do not ordinarily form any heavier elements.