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 in the core (via the triple-alpha reaction) and by 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. Many horizontal branch stars pulsate and these are known as RR Lyrae stars.
A related class of stars are those belonging to the so-called red clump, which are the (relatively younger) population I counterparts to HB stars (which belong to population II).
