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.