Final answer:
After exhausting hydrogen, a star's core initially contains helium. High-mass stars can further fuse heavier elements up to iron, which signals the star's impending end, leading to either a supernova or the formation of a neutron star.
Step-by-step explanation:
Nuclear Fusion in Stars After Hydrogen
When a star depletes its hydrogen fuel, the core typically contains helium, and in the case of high-mass stars, the process progresses through the fusion of heavier elements. Initially, a low-mass star will accumulate helium as a product of hydrogen fusion. Over time, as the core contracts due to gravity, it becomes hot enough to start fusing helium into more complex elements like carbon and oxygen. For high-mass stars, capable of fusing elements beyond carbon, the process continues through silicon and up to iron. Before they exhaust all their fuel, massive stars resemble an onion with layers of different elements fusing in shells around a core. Finally, when the core fuses into iron, this marks the end of the energy-producing nuclear reactions since iron fusion requires more energy than it releases, leading to the star's eventual supernova explosion or collapse into a dense neutron star.