| Term: | Horizontal branch |
| Definition: |
A set of roughly horizontal points in the Hertzsprung-Russell diagram of a typical globular cluster. It displays a stage of stellar evolution which immediately follows the red giant branch (RGB) in stars with an initial mass < 1.2 M_sun. When the star's ascent of the RGB is terminated by the helium flash, it moves down to the Horizontal Branch (HB). The star's effective temperature on the HB is higher than it was on the RGB, but the luminosity is considerably less than at the helium flash. Usually HB stars have two energy sources: in addition to the helium burning in their cores, they experience hydrogen fusion in a surrounding shell. The thickness of the shell determines the color of the HB stars. A thin shell, involving low opacity, makes the star look blue. The HB domain encompasses a very large effective temperature range with several members: extreme HB, blue HB, RR Lyrae, red HB, and red clump stars. The locations depend on many parameters, including stellar mass, metallicity, age, helium abundance, and rotation. |