What describes a large star that has expanded and turned crimson?

A large star that has expanded and turned crimson is aptly described as a red giant. This phase occurs when a star exhausts the hydrogen fuel in its core, causing the core to contract and heat up. As a result, the outer layers of the star expand and cool, giving it a reddish hue due to the decreased temperature of its surface compared to when it was in its main sequence phase.

Red giants are significant in stellar evolution, signaling that the star is nearing the end of its life cycle. They are often characterized by their size—they can be hundreds of times larger than the Sun—and their appearance, which is distinctly red due to the cooler surface temperature (typically between 3,000 to 5,000 degrees Kelvin) compared to hotter stars. This transformation illustrates the dynamic processes stars undergo as they age and change their position in the Hertzsprung-Russell diagram, an important tool in understanding stellar classification and evolution.