red giant branch

The red giant branch is a phase in a star's life when it has exhausted the hydrogen fuel in its core. As a result, the core contracts and heats up, causing the outer layers to expand and cool down. The star becomes larger, cooler on the surface, and appears redder. This stage occurs after the star has left the main sequence (its initial, stable phase of burning hydrogen in the core). The red giant branch is a crucial step in stellar evolution, leading eventually to the star shedding its outer layers and becoming a white dwarf.