Encyclopedia of Astrobiology

Living Edition
| Editors: Muriel Gargaud, William M. Irvine, Ricardo Amils, Henderson James Cleaves, Daniele Pinti, José Cernicharo Quintanilla, Michel Viso

Horizontal Branch

  • Nikos Prantzos
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-27833-4_735-2


The horizontal branch (HB) is the region of the Hertzsprung-Russell diagram occupied by evolved stars – after the red-giant stage – of low mass and low metallicity (abundance of elements heavier than He less than 0.1 that of the Sun), during their central He-burning (He-fusion) phase. The HB is observed in globular clusters, whereas in open clusters (having approximately solar metallicity), stars are confined to the bottom of the red-giant branch during core He burning. Metallicity is the key factor explaining that difference (low-metallicity stars have lower opacities and are hotter), but other factors (composition, age, rotation, envelope mass) appear to affect the morphology of the HB.

See Also


Bioorganic Chemistry Open Cluster Globular Cluster Horizontal Branch Solar Metallicity 
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© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  1. 1.Institut d’Astrophysique de ParisParisFrance