Horizontal branch

The horizontal branch is a stage in the evolution of low-mass stars, when they burn helium in their cores after a helium flash. Its stars span a range of temperatures but have broadly similar luminosities, forming a roughly horizontal band on the Hertzsprung–Russell diagram.

Helium flash: A rapid ignition of helium fusion in the degenerate core of a low-mass red giant. It ends the red-giant ascent and initiates stable core helium burning.
Stellar evolution: The sequence of structural and chemical changes a star undergoes over its lifetime. The horizontal branch is one short-lived phase in this broader sequence.
RR Lyrae variable: A pulsating horizontal-branch star whose period and luminosity make it useful as a distance indicator. These variables are common among the hotter, instability-strip members of the branch.
Second parameter problem: The problem of explaining horizontal-branch differences not accounted for by metallicity alone. It asks why clusters with similar metallicity can have different branch morphologies.
Triple-alpha process: A nuclear reaction sequence that fuses three helium-4 nuclei into carbon-12. It supplies most of the energy from helium burning in the core.
Low-mass star: A star with a mass low enough to develop a degenerate helium core before helium ignition. Such stars undergo helium flashes and populate the horizontal branch.
Globular cluster: A dense, gravitationally bound group of old stars orbiting a galaxy. Its color–magnitude diagram often displays a well-populated horizontal branch.
Red giant branch mass loss: The loss of stellar envelope material during a star’s ascent of the red giant branch. Its amount helps set the envelope mass and temperature at horizontal-branch arrival.
Hertzsprung–Russell diagram: A graph that compares stars’ luminosities with their surface temperatures or spectral types. The horizontal branch is identified by its position on this diagram.
Electron degeneracy pressure: Pressure produced by the quantum-mechanical exclusion principle among electrons, largely independent of temperature. It supports the helium core before ignition, allowing the flash to occur.

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