What happens in the core of the star when it runs out of hydrogen fuel?

When a star exhausts its hydrogen fuel in the core, nuclear fusion ceases, leading to a drop in outward pressure. This causes the core to collapse under gravity, increasing temperature and pressure until it becomes hot enough to fuse helium into heavier elements. As the core contracts, the outer layers may expand and cool, often transforming the star into a red giant. Eventually, this process leads to the star's evolution into later stages, such as a supernova or a white dwarf, depending on its mass.