Horizontal branch
The horizontal branch (HB) is a stage of
Discovery
Horizontal branch stars were discovered with the first deep photographic
and were notable for being absent from allEvolution
After exhausting their core hydrogen, stars leave the
If the
Stars initially between about 2.3 M☉ and 8 M☉ have larger helium cores that do not become degenerate. Instead their cores reach the Schönberg–Chandrasekhar mass at which they are no longer in hydrostatic or thermal equilibrium. They then contract and heat up, which triggers helium fusion before the core becomes degenerate. These stars also become hotter during core helium fusion, but they have different core masses and hence different luminosities from HB stars. They vary in temperature during core helium fusion and perform a blue loop before moving to the asymptotic giant branch. Stars more massive than about 8 M☉ also ignite their core helium smoothly, and also go on to burn heavier elements as a red supergiant.[5]
Stars remain on the horizontal branch for around 100 million years, becoming slowly more luminous in the same way that main sequence stars increase luminosity as the virial theorem shows. When their core helium is eventually exhausted, they progress to helium shell burning on the asymptotic giant branch (AGB). On the AGB they become cooler and much more luminous.[3]
Horizontal branch morphology
Stars on the horizontal branch all have very similar core masses, following the helium flash. This means that they have very similar luminosities, and on a Hertzsprung–Russell diagram plotted by visual magnitude the branch is horizontal.
The size and temperature of an HB star depends on the mass of the hydrogen envelope remaining around the helium core. Stars with larger hydrogen envelopes are cooler. This creates the spread of stars along the horizontal branch at constant luminosity. The temperature variation effect is much stronger at lower metallicity, so old clusters usually have more pronounced horizontal branches.[6]
Although the horizontal branch is named because it consists largely of stars with approximately the same absolute magnitude across a range of temperatures, lying in a horizontal bar on a color–magnitude diagrams, the branch is far from horizontal at the blue end. The horizontal branch ends in a "blue tail" with hotter stars having lower luminosity, occasionally with a "blue hook" of extremely hot stars. It is also not horizontal when plotted by bolometric luminosity, with hotter horizontal branch stars being less luminous than cooler ones.[7]
The hottest horizontal-branch stars, referred to as extreme horizontal branch, have temperatures of 20,000–30,000 K. This is far beyond what would be expected for a normal core helium burning star. Theories to explain these stars include binary interactions, and "late thermal pulses", where a thermal pulse that asymptotic giant branch (AGB) stars experience regularly, occurs after fusion has ceased and the star has entered the superwind phase.[8] These stars are "born again" with unusual properties. Despite the bizarre-sounding process, this is expected to occur for 10% or more of post-AGB stars, although it is thought that only particularly late thermal pulses create extreme horizontal-branch stars, after the planetary nebular phase and when the central star is already cooling towards a white dwarf.[9]
The RR Lyrae gap
Globular cluster CMDs (Color-Magnitude diagrams) generally show horizontal branches that have a prominent gap in the HB. This gap in the CMD incorrectly suggests that the cluster has no stars in this region of its CMD. The gap occurs at the instability strip, where many pulsating stars are found. These pulsating horizontal-branch stars are known as RR Lyrae variable stars and they are obviously variable in brightness with periods of up to 1.2 days.[10]
It requires an extended observing program to establish the star's true (that is, averaged over a full period)
Different globular clusters often display different HB morphologies, by which is meant that the relative proportions of HB stars existing on the hotter end of the RR Lyr gap, within the gap, and to the cooler end of the gap varies sharply from cluster to cluster. The underlying cause of different HB morphologies is a long-standing problem in
Relationship to the red clump
A related class of stars is the clump giants, those belonging to the so-called
References
- ^
doi:10.1086/106674
- ^
Sandage, A. R. (1953), "The color-magnitude diagram for the globular cluster M 3", doi:10.1086/106822
- ^ ISBN 978-3-540-34143-7
- ^ "Post Main Sequence Stars". Australia Telescope Outreach and Education. Retrieved 2 December 2012.
- Bibcode:2005essp.book.....S.
- ISBN 978-3-642-30304-3.
- ^ doi:10.1086/173803.
- .
- Bibcode:2008ASPC..391....3J.
- American Association of Variable Star Observers. "Types of Variables". Archived from the originalon 17 October 2018. Retrieved 12 March 2011.
- ISBN 978-3-319-14234-0.
- ISBN 978-3-540-34144-4.