The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red giant branch in stars whose masses are similar to the Sun's. The helium core flash that occurs to stars at the top of the red giant branch causes substantial changes in stellar structure, resulting in an overall reduction in luminosity, some contraction of the stellar envelope, and surfaces reaching higher temperatures. Horizontal branch stars are powered by helium fusion in the core (via the triple-alpha reaction) and by hydrogen fusion in a shell surrounding the core.
Horizontal branches were discovered with the first deep photographic photometric studies of globular clusters and were notable for being absent from all open clusters that had been studied up to that time. The horizontal branch is so named because in low-metallicity samples like globular clusters, HB stars lie along a roughly horizontal line in a Hertzsprung–Russell diagram (CMD).
Read more about Horizontal Branch: Evolution, The RR Lyrae "gap" and Horizontal Branch Morphology, Relationship To The Red Clump
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