The horizontal branch (HB) is a grouping of stars (HB stars, or horizontal branch stars) on the H-R diagram that form a horizontal line on the diagram, leading away from the red-giant branch (RGB). They vary in color (as suggested by the line covering some distance across the diagram), are mostly red (red giants) but can range to blue (blue horizontal branch stars), and are generally small giants (subgiants). HB stars have passed through the RGB (for which they needed at least half a solar mass as main sequence stars) and have begun helium fusion (triple alpha process) in the core, with a surrounding shell of hydrogen fusion. The term zero age HB or ZAHB indicates such a star that has just entered this phase. Their horizontal grouping within the diagram indicates their luminosity is similar despite varying colors, the latter of which reflects temperature. Not all stars within this phase show these varying colors: some (that have a high metallicity) are at a specific temperature, occupying a small region of the diagram known as the red clump at the cooler end of the usual HB line location.
The hottest region of the horizontal branch (20000 K and more) is termed the extreme horizontal branch (EHB), i.e., with extreme horizontal branch stars (EHB stars).
After the horizontal branch (or red clump) stage, a core of carbon (product of the triple-alpha fusion) has accumulated to the point that the helium fusion is confined to a surrounding shell, and the star enters the asymptotic giant branch (AGB) stage.