When a star becomes a red giant, what physical change occurs?

When a star becomes a red giant, it undergoes a significant physical change characterized by expansion and cooling. As a star exhausts the hydrogen fuel in its core, nuclear fusion slows down, and gravity causes the core to contract. This contraction increases the core's temperature, allowing helium fusion to initiate. However, the outer layers of the star are pushed outward and expand dramatically.

As the star expands, the outer layers cool down, which leads to the characteristic red color of a red giant. The balance between the intense gravitational forces trying to collapse the star and the outward pressure generated by nuclear reactions in the core is disrupted, resulting in this unique phase of a star's lifecycle.

This process distinguishes red giants from other stellar stages, such as main sequence stars, and highlights how the balance between fusion and gravitational forces affects a star's size, temperature, and brightness during this transformative phase.