Editing Horizontal branch (section)

==Relationship to the red clump==
A related class of stars is the ''clump giants'', those belonging to the so-called [[red clump]], which are the relatively [[Star#Age|younger]] (and hence [[Star#Mass|more massive]]) and usually more [[Star#Chemical composition|metal-rich]] [[population I]] counterparts to HB stars (which belong to [[population II]]). Both HB stars and clump giants are fusing [[helium]] to [[carbon]] in their cores, but differences in the [[Star#Structure|structure]] of their outer layers result in the different types of stars having different radii, [[Effective temperature#Star|effective temperatures]], and [[Star#Classification|color]]. Since [[Star#Classification|color index]] is the horizontal coordinate in a [[Hertzsprung–Russell diagram]], the different types of star appear in different parts of the CMD despite their common [[energy]] source. In effect, the red clump represents one extreme of horizontal-branch morphology: all the stars are at the red end of the horizontal branch, and may be difficult to distinguish from stars ascending the red-giant branch for the first time.<ref name="KarttunenKröger2007">{{cite book|author1=Hannu Karttunen|author2=Pekka Kröger|author3=Heikki Oja|author4=Markku Poutanen|author5=Karl Johan Donner|title=Fundamental Astronomy|url=https://books.google.com/books?id=DjeVdb0sLEAC&pg=PA249|date=9 August 2007|publisher=Springer Science & Business Media|isbn=978-3-540-34144-4|pages=249–}}</ref>