What is the main sequence of the red giant?

The main sequence of a red giant refers to the phase in a star's life cycle prior to its expansion into a red giant. During the main sequence, a star fuses hydrogen into helium in its core, generating energy that counteracts gravitational collapse. Once the hydrogen is depleted, the core contracts and heats up, leading to the outer layers expanding and cooling, which transforms the star into a red giant. Thus, the main sequence is characterized by stable hydrogen burning, while the red giant phase marks the transition to helium burning and further stellar evolution.