Why do globular-cluster colour-magnitude diagrams show a giant branch and a horizontal branch?

The presence of a giant branch and a horizontal branch in the colour-magnitude diagram of a globular cluster is a direct consequence of the evolutionary stages of low-mass stars that formed at approximately the same time.12

• Giant Branch: As stars exhaust the hydrogen in their cores, they evolve away from the main sequence. The core shrinks and heats up while the outer layers expand and cool, causing the star to become more luminous and move toward the red giant domain on the diagram.2
• Horizontal Branch: In stars with masses between roughly half and three times that of the Sun, the core eventually reaches temperatures sufficient to begin helium fusion—a process that often initiates with a "helium flash." Following this, the star becomes smaller and hotter, shifting its position on the diagram to the left to form the horizontal branch.2

These features reflect the different paths stars take as they age, with the "turnoff point" where stars leave the main sequence serving as a primary indicator of the cluster's overall age.13