Physics – 6.2.1 The Sun as a star | e-Consult

As a star ages and exhausts the hydrogen fuel in its core, the rate of nuclear fusion in the core significantly decreases. This is because the fusion process requires a certain temperature and density to occur efficiently. With the hydrogen fuel depleted, the core temperature and density decrease, slowing down the fusion reaction.

The decrease in fusion rate leads to a reduction in the outward pressure supporting the core. This causes the core to contract under gravity. The contraction increases the temperature of the core, but not enough to reignite hydrogen fusion. As the core contracts, the outer layers of the star expand and become cooler, forming a red giant.

The eventual fate of the star depends on its initial mass. For stars with relatively low mass, the core will eventually become hot enough to initiate helium fusion. However, these stars do not have enough mass to fuse helium into heavier elements. They will eventually shed their outer layers, forming a planetary nebula, leaving behind a white dwarf – a dense, hot remnant core supported by electron degeneracy pressure. For more massive stars, the core will continue to fuse heavier elements, eventually leading to a supernova explosion and the formation of either a neutron star or a black hole.