The horizontal branch is a feature seen in the Hertzsprung-Russell Diagrams of low-metallicity globular clusters (i.e. clusters of stars whose chemical compositions are more deficient in metallic elements than the Sun). Stars on the horizontal branch are very evolved stars in a short phase of core helium-burning, after the star has evolved through the main sequence, the red giant branch, and the helium flash.

As its name implies, the horizontal branch is a horizontal, linear feature seen in cluster H-R diagrams, meaning that stars on the horizontal branch all have similar luminosities, but can have a range of effective temperatures.The horizontal branch is only seen in low-metallicity clusters. All stars regardless of the metal content can undergo core helium burning, but more metal rich stars are constrained to a very small range of effective temperatures lying adjacent to the red giant branch. In the H-R diagrams of metal-rich clusters, these stars will instead appear as a clump of stars, known as the red clump. The reason for this is that the metal atoms in the photospheres of metal-rich stars are more efficient at absorbing blue photons, in a process known as line blanketing. This results in a cooler photosphere, and hence a lower effective temperature.

Stars on the horizontal branch or red clump have luminosities about 100 times the luminosity of the Sun. Structurally, they will have a convective, helium-burning core containing a little less than half the mass of the star, surrounded by a layer of burning hydrogen, in turn surrounded by a non-burning envelope and photosphere. Aside from the additional hydrogen-burning layer, a horizontal branch star is somewhat like a zero-age main sequence star in that it will continue to evolve until it exhausts its supply of helium. However, this takes much less time than the main sequence lifetime, and may be as short as ten million years as opposed to the billions of years a star may spend on the main sequence. When all the core helium is fully burned into carbon and oxygen, the core again rapidly contracts, and begins a phase where it burns helium in a shell around the core, and moves up the asymptotic giant branch in the H-R diagram. At this point, the star is very near the end of its life, and is well on its way to becoming a white dwarf.

The horizontal branches of some globular clusters have extended blue tails (at the high-temperature end of the branch) which dip to lower luminosities, whose stars are called extreme horizontal branch stars. The nature of these stars is not understood, though the appearance of a blue tail in some clusters and not in others could be due to different cluster ages, or to different helium abundances. The horizontal branch also contains the RR Lyrae variable stars, where the Cepheid instability strip crosses the horizontal branch. Like the Cepheid variables, RR Lyrae stars are also useful as distance indicators.

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