The answer is Helium to carbon & oxygen. Depending on the mass of the star, the types of elements formed at its core depend on its gravitational force that can fuse lighter elements to heavier elements. All stars can fuse hydrogen to helium at their core. Stars at least 8 times the sun’s solar mass can further fuse helium to carbon and oxygen. Massive stars of over 8 solar masses have immense gravitation forces at the core they can further fuse the carbon and oxygen to silicon and iron.