Horizontal branch
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The horizontal branch (HB) is a stage of stellar evolution which immediately follows the red giant branch in stars whose masses are similar to the Sun's. Horizontal branch stars are powered by helium fusion (aka the triple-alpha reaction) in the core and hydrogen fusion in a shell surrounding the core. The horizontal branch is so named because HB stars lie along a roughly horizontal line in a color-magnitude diagram. Many horizontal branch stars vary with a similar period and are known as RR Lyrae stars.