The concentration of core-helium-burning stars on the cool part of the horizontal branch. It must be distinguished from the ascending hydrogen-shell-burning red giant branch.
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Red clump stars are a specific type of red giant star in the later stages of stellar evolution. These stars are typically in the horizontal branch phase, which follows the giant phase of a star's life cycle. Red clump stars represent a phase where stars have exhausted the hydrogen in their cores and are now fusing helium into carbon and oxygen. They are somewhat hotter and brighter than ordinary red giants, and their spectra show strong absorption lines indicative of helium fusion.