A sequence of stars in the H-R diagram of a globular cluster, extending horizontally across the diagram to the left from the red-giant region. These are probably stars undergoing helium burning in their cores, by the triple-alpha reaction.
band of stars running horizontally at M ~ 0.6 in the HR diagrams of old, low-heavy-element stellar clusters: these are low mass stars burning helium in their cores.
In the H-R diagram of a globular cluster, the sequence of stars extending from the red giants toward the blue side of the diagram; includes RR Lyrae stars.
Region of the Hertzsprung–Russell diagram where post–main sequence stars again reach hydrostatic equilibrium. At this point, the star is burning helium in its core, and hydrogen in a shell surrounding the core.
The horizontal branch is a part of the Hertzsprung -Russell (H-R) diagram that represents stars that burn helium in thier cores. These mostly large stars lie along the top of the H-R diagram.
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.