The horizontal branch is a stage in the evolution of stars that occurs after the red giant phase, where stars fuse helium into carbon and oxygen in their cores while maintaining a stable outer layer. This stage is significant as it marks a point where stars achieve a balance between gravitational forces and thermal pressure, leading to a characteristic position in the Hertzsprung-Russell diagram. During this phase, stars exhibit a specific set of temperature and luminosity traits that distinguish them from other evolutionary stages.
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