Subgiant Branch

The Subgiant Branch is a phase in a star’s life cycle that occurs after it has exhausted the hydrogen fuel in its core. During this phase, the star begins to expand and cool slightly, moving away from the main sequence on a star cluster’s color-magnitude diagram. It’s a transitional stage where the star is preparing to become a giant, with its outer layers expanding more significantly. This phase provides insight into stellar evolution, showing how stars change as they run out of hydrogen in their cores before entering the giant stage.