Final answer:
The exhaustion of a star's core hydrogen leads to core contraction and outer layer expansion, causing it to become a luminous red giant while increasing in size. A subsequent phase of helium fusion may briefly reduce its luminosity and size before becoming a red giant again.
Step-by-step explanation:
When a star exhausts its core hydrogen supply, several significant changes occur in its structure and behavior. Initially, the core contracts while the outer layers expand due to the newly produced energy from hydrogen fusion occurring in the shell around the helium core. This process increases the luminosity of the star, making it brighter as the expansion makes the star larger. Consequently, the star, such as our Sun, experiences this growth and becomes a red giant, which is characterized by a cooler surface temperature but increased brightness. As the core contracts, heat is generated, aiding in the helium fusion process when the core reaches sufficiently high temperatures. This helium fusion leads to the star briefly shrinking and becoming less luminous but is followed by a reignition of the outer layers, which can cause the star to become a red giant again. Ultimately, the core's collapse and the expansion of the star's exterior give it a 'split personality,' with divergent processes happening simultaneously.