Subgiants

Subgiants represent a transitional phase in a star's evolution. When a star like our Sun exhausts the hydrogen fuel in its core, it leaves the main sequence and begins to expand. The core contracts and heats up, while the outer layers cool down and swell outward. The star maintains roughly the same mass, but its diameter grows significantly. A shift in spectral class reflects this drop in surface temperature, turning the star's color from yellow toward orange or red. This brief cosmic phase is the first step toward becoming a full red giant.