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Horizontal branch

Hertzsprung–Russell diagram for globular cluster M5, with the horizontal branch marked in yellow, RR Lyrae stars in green, and some of the more luminous red-giant branch stars in red
  Asymptotic giant branch (AGB)
  Upper red-giant branch (RGB)
  Horizontal branch (HB)
  RR Lyrae variable (RR)
  End of main sequence, subgiant branch, and lower RGB

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. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha process) and by hydrogen fusion (via the CNO cycle) in a shell surrounding the core. The onset of core helium fusion at the tip 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 the surface reaching higher temperatures.

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