Editing Horizontal branch (section)

{{Short description|Stage of stellar evolution}}
[[File:M5 colour magnitude diagram.png|thumb|right|upright=1.4|[[Hertzsprung–Russell diagram]] for [[globular cluster]] [[Messier 5|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

{{leftlegend|#0A88FF|[[Asymptotic giant branch]] (AGB)|outline=gray}} {{leftlegend|#FF420E|Upper [[red-giant branch]] (RGB)|outline=gray}} {{leftlegend|#FFD010|Horizontal branch (HB)|outline=gray}} {{leftlegend|#479406|[[RR Lyrae variable]] (RR)|outline=gray}} {{leftlegend|#7E0021|End of [[main sequence]], [[subgiant|subgiant branch]], and lower RGB|outline=gray}}]]
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 [[Star#Structure|stellar structure]], resulting in an overall reduction in [[Luminosity#In astronomy|luminosity]], some contraction of the stellar envelope, and the surface reaching higher temperatures.