Horizontal branch

The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha process) and by hydrogen fusion (via the CNO cycle) in a shell surrounding the core. The onset of core helium fusion at the tip of the red giant branch causes substantial changes in stellar structure, resulting in an overall reduction in luminosity, some contraction of the stellar envelope, and the surface reaching higher temperatures.

M5 colour magnitude diagram
Hertzsprung–Russell diagram for globular cluster M5, with the horizontal branch marked in yellow, RR Lyrae stars in green, and some of the more luminous red giant branch stars in red

Discovery

Horizontal branch stars were discovered with the first deep photographic photometric studies of globular clusters[1][2] and were notable for being absent from all open clusters that had been studied up to that time. The horizontal branch is so named because in low-metallicity star collections like globular clusters, HB stars lie along a roughly horizontal line in a Hertzsprung–Russell diagram.

Evolution

Evolutionary track 1m
The evolutionary track of a sun-like star, showing the horizontal branch and red clump region

After exhausting their core hydrogen, stars leave the main sequence and begin fusion in a hydrogen shell around the helium core and become giants on the red giant branch. In stars with masses up to 2.3 times the mass of the Sun the helium core becomes a region of degenerate matter that does not contribute to the generation of energy. It continues to grow and increase in temperature as the hydrogen fusion in the shell contributes more helium.[3]

If the star has more than about 0.5 solar masses,[4] the core eventually reaches the temperature necessary for the fusion of helium into carbon through the triple-alpha process. The initiation of helium fusion begins across the core region, which will cause an immediate temperature rise and a rapid increase in the rate of fusion. Within a few seconds the core becomes non-degenerate and quickly expands, producing an event called helium flash. Non-degenerate cores initiate fusion more smoothly, without a flash. The output of this event is absorbed by the layers of plasma above, so the effects are not seen from the exterior of the star. The star now changes to a new equilibrium state, and its evolutionary path switches from the red giant branch (RGB) onto the horizontal branch of the Hertzsprung–Russell diagram.[3]

Stars initially between about 2.3 M and 8 M have larger helium cores that do not become degenerate. Instead their cores reach the Schoenberg-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.

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.

The hottest horizontal-branch stars, referred to as extreme horizontal branch, have temperatures of 20,000–30,000K. 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. 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.

The RR Lyrae gap

M3 color magnitude diagram
Hertzsprung–Russell diagram for the globular cluster M3

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.[6] It requires an extended observing program to establish the star's true (that is, averaged over a full period) apparent magnitude and color. Such a program is usually beyond the scope of an investigation of a cluster's color–magnitude diagram. Because of this, while the variable stars are noted in tables of a cluster's stellar content from such an investigation, these variable stars are not included in the graphic presentation of the cluster CMD because data adequate to plot them correctly are unavailable. This omission often results in the RR Lyrae gap seen in many published globular cluster CMDs.

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 stellar astrophysics. Chemical composition is one factor (usually in the sense that more metal-poor clusters have bluer HBs), but other stellar properties like age, rotation and helium content have also been suggested as affecting HB morphology. This has sometimes been called the "Second Parameter Problem" for globular clusters, because there exist pairs of globular clusters which seem to have the same metallicity yet have very different HB morphologies; one such pair is NGC 288 (which has a very blue HB) and NGC 362 (which has a rather red HB). The label "second parameter" acknowledges that some unknown physical effect is responsible for HB morphology differences in clusters that seem otherwise identical.

Relationship to the red clump

A related class of stars is the clump giants, those belonging to the so-called red clump, which are the relatively younger (and hence more massive) and usually more metal-rich population I counterparts to HB stars (which belong to population II). Both HB stars and clump giants are fusing helium to carbon in their cores, but differences in the structure of their outer layers result in the different types of stars having different radii, effective temperatures, and color. Since color index is the horizontal coordinate in a Hertzsprung–Russell diagram, the different types of star appear in different parts of the CMD despite their common energy source. In effect, the red clump represents one extreme of horizontal-branch morphology: all the stars are at the red end of the horizontal branch, and may be difficult to distinguish from stars ascending the red giant branch for the first time.

References

  1. ^ Arp, H. C.; Baum, W. A.; Sandage, A. R. (1952), "The HR diagrams for the globular clusters M 92 and M 3", Astronomical Journal, 57: 4–5, Bibcode:1952AJ.....57....4A, doi:10.1086/106674
  2. ^ Sandage, A. R. (1953), "The color-magnitude diagram for the globular cluster M 3", Astronomical Journal, 58: 61–75, Bibcode:1953AJ.....58...61S, doi:10.1086/106822
  3. ^ a b c Karttunen, Hannu; Oja, Heikki (2007), Fundamental astronomy (5th ed.), Springer, p. 249, ISBN 3-540-34143-9
  4. ^ "Post Main Sequence Stars". Australia Telescope Outreach and Education. Retrieved 2 December 2012.
  5. ^ Salaris, Maurizio; Cassisi, Santi (2005). Evolution of Stars and Stellar Populations. Evolution of Stars and Stellar Populations. p. 400. Bibcode:2005essp.book.....S.
  6. ^ American Association of Variable Star Observers. "Types of Variables". Retrieved 12 March 2011.
110 Virginis

110 Virginis is a star in the zodiac constellation Virgo, located 195 light years away from the Sun. It is visible to the naked eye as an orange-hued star with an apparent visual magnitude of 4.40. The star is moving closer to the Earth with a heliocentric radial velocity of −16 km/s.The stellar classification of 110 Virginis is K0.5 IIIb Fe–0.5, indicating that this is an evolved giant star with a mild underabundance of iron in its spectrum. At the age of 4.5 billion years old, it belongs to a sub-category of giants called the red clump, which means it is on the horizontal branch and is generating energy through the helium fusion at its core. Compared to the Sun, it has 167% of the mass but has expanded to 11 times the size. The enlarged photosphere has an effective temperature of 4,664 K and is radiating 76 times the Sun's luminosity.

14 Andromedae

14 Andromedae, abbreviated 14 And, also named Veritate , is a single, orange-hued giant star situated approximately 247 light-years away in the northern constellation of Andromeda. It is dimly visible to the naked eye with an apparent visual magnitude of 5.22. The star is moving closer to the Earth with a heliocentric radial velocity of −60 km/s. In 2008 an extrasolar planet (designated 14 Andromedae b, later named Spe) was discovered to be orbiting the star.This is a red clump giant with a stellar classification of K0 III, indicating it is on the horizontal branch and is generating energy through helium fusion at its core. The star has 1.12 (or 2.2) times the mass of the Sun and has expanded to 10.5 times the Sun's radius. It is radiating 60.3 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,743 K. This is thought it was formerly an A- or F-type main-sequence star earlier in its life, prior to evolving into a giant.

15 Boötis

15 Boötis is a binary star system in the northern constellation of Boötes, located approximately 260 light years away from the Sun. It is visible to the naked eye as a dim, orange-hued star with an apparent visual magnitude of 5.45. The system has a relatively high proper motion, traversing the celestial sphere at the rate of 0.166 arc seconds per annum. It is moving away from the Earth with a heliocentric radial velocity of +16.8 km/s.The magnitude 5.51 primary, designated component A, is an aging K-type giant star with a stellar classification of K1 III. It is a red clump giant, which indicates it is on the horizontal branch and is generating energy through helium fusion at its core. It is around two billion years old with 1.5 times the mass of the Sun and has expanded to 10 times the Sun's radius. The star is radiating 61 times the luminosity of the Sun from its swollen photosphere at an effective temperature of 4,845 K.Its companion, component B, is a magnitude +8.53 star was located at an angular separation of 0.80″ along a position angle of 111° from the primary, as of 2015. This is the same separation it had when the system was discovered in 1936.

1 Aquarii

1 Aquarii is a binary star system in the zodiac constellation of Aquarius, about 257 light years away from the Sun. 1 Aquarii is the Flamsteed designation. It is visible to the naked eye as a faint, orange-hued star with an apparent visual magnitude of 5.151, located a degree north of the celestial equator. The system is moving closer to the Earth with a heliocentric radial velocity of −41 km/s.Systematic observation for determining the orbit of this system began in 2002, some eighty years following the first radial velocity measurements. It is a single-lined spectroscopic binary with an orbital period of 5.385 yr and an eccentricity of 0.368. The visible component is an aging giant star with a stellar classification of K1III. At the age of 1.26 billion years old it is a red clump giant, which indicates it is on the horizontal branch and is generating energy through helium fusion at its core. The star has 1.5 times the mass of the Sun and has expanded to 11 times the Sun's radius. It is radiating 53.7 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,715 K.The mass of the companion appears small, suggesting a red dwarf no higher than class M5. In addition to the spectroscopic companion there are two faint optical companions that have no physical relation to 1 Aqr.

20 Boötis

20 Boötis is a single star in the northern constellation of Boötes, located 183 light years away from the Sun. It is visible to the naked eye as a faint, orange-hued star with an apparent visual magnitude of 4.84. The star has a relatively high proper motion, traversing the celestial sphere at the rate of 0.154 arc seconds per annum. It is moving closer to the Earth with a heliocentric radial velocity of −8 km/s.This is an aging K-type giant star with a stellar classification of K3 III. It is a red clump giant, which indicates it is on the horizontal branch and is generating energy through helium fusion at its core. The star is around five billion years old with 1.1 times the mass of the Sun and has expanded to 12 times the Sun's radius. It is radiating 52 times the luminosity of the Sun from its swollen photosphere at an effective temperature of 4,472 K.

30 Aquarii

30 Aquarii is a single star located about 301 light years away from the Sun in the zodiac constellation of Aquarius. 30 Aquarii is its Flamsteed designation. It is visible to the naked eye as a dim, orange-hued star with an apparent visual magnitude of 5.56. The star is moving further from the Earth with a heliocentric radial velocity of 40 km/s.This object is an aging G-type giant star with a stellar classification of G8 III, although Houk and Swift (1999) found a class of K1 IV. It is a red clump giant, which indicates it is on the horizontal branch and is generating energy through helium fusion at its core. The star is nearly two billion years old with a leisurely rotation rate, showing a projected rotational velocity of 1.6 km/s. It has double the mass of the Sun and has expanded to ten times the Sun's radius. The star is radiating 55 times the luminosity of the Sun from its swollen photosphere at an effective temperature of 4,944 K.

32 Boötis

32 Boötis is a single star in the northern constellation of Boötes, located 360 light years away from the Sun. It is visible to the naked eye as a faint, yellow-hued star with an apparent visual magnitude of 5.55. This object is moving closer to the Earth with a heliocentric radial velocity of −23 km/s. It has a relatively high proper motion, traversing the celestial sphere at the rate of 0.195 arc seconds per annum.This is an aging giant star with a stellar classification of G8 III. It is most likely on the horizontal branch and is a candidate red clump giant. The star is an estimated 1.46 billion years old with 2.15 times the mass of the Sun. With the hydrogen at its core exhausted, it has expanded to 12 times the Sun's radius. 32 Boötis is radiating 79 times the luminosity of the Sun from its swollen photosphere at an effective temperature of 4958 K.

49 Andromedae

49 Andromedae (abbreviated 49 And) is a star in the constellation Andromeda. 49 Andromedae is the Flamsteed designation though it also bears the Bayer designation A Andromedae. It is visible to the naked eye under good viewing conditions with an apparent visual magnitude of 5.269. The distance to 49 Andromedae, as determined from its annual parallax shift of 10.4 mas, is around 314 light years. It is moving closer to the Sun with a heliocentric radial velocity of −11.5 km/s.With an estimated age of 1.75 Gyr years, this is an aging red clump giant star with a stellar classification of K0 III, indicating it is generating energy by helium fusion at its core. The spectrum displays "slightly strong" absorption lines of cyanogen (CN). It has 2.07 times the mass of the Sun and has expanded to 11 times the Sun's radius. The star is radiating 71 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,879 K.

52 Cygni

52 Cygni is a giant star in the northern constellation of Cygnus with an apparent magnitude of 4.22. Based on its Hipparcos parallax, it is about 291 light-years (89 pc) away.

52 Cygni is a probable horizontal branch (red clump) star, fusing helium in its core, although there is a 25% chance that it is still on the red giant branch (RGB) and fusing hydrogen in a shell around an insert core. As a clump giant it would be 2.27 gyr old, but only 910 myr if it is an RGB star. It shines with a bolometric luminosity of about 90 L☉ at an effective temperature of 4,677 K. It has a radius of about 14 R☉.At an angular separation of 6.0″ from 52 Cygni is a faint magnitude 9.5 companion.

60 Aquarii

60 Aquarii is a star located 375 light years away from the Sun in the equatorial constellation of Aquarius. 60 Aquarii is its Flamsteed designation. It is visible to the naked eye as a dim, yellow-hued star with an apparent visual magnitude of 5.89. The star is moving closer to the Earth with a heliocentric radial velocity of –8 km/s.This is an aging giant star with a stellar classification of G6 III, most likely on the horizontal branch. It is 437 million years old with 2.77 times the mass of the Sun. Having exhausted the hydrogen at its core, this star has evolved away from the main sequence and expanded to 11 times the Sun's radius. It is radiating 65 times the luminosity of the Sun from its swollen photosphere at an effective temperature of 4,820 K.A magnitude 11.54 companion star is located at an angular separation of 100.90″ along a position angle of 299°, as of 2013.

68 Aquarii

68 Aquarii is a single star located 270 light years away from the Sun in the zodiac constellation of Aquarius. 68 Aquarii is its Flamsteed designation, though it also bears the Bayer designation of g2 Aquarii. It is visible to the naked eye as a dim, yellow-hued star with an apparent visual magnitude of 5.24. The object is moving further from the Earth with a heliocentric radial velocity of +24.5 km/s.This star is 3.79 billion years old with a stellar classification of G8 III, indicating the is a giant star that has exhausted the hydrogen at its core and expanded off the main sequence. It is a red clump giant, which means it is on the horizontal branch and is generating energy through helium fusion at its core. It has 1.39 times the mass of the Sun and 10 times the Sun's radius. The star is radiating 59 times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 5,036 K.

69 Aquilae

69 Aquilae, abbreviated 69 Aql, is a star in the equatorial constellation of Aquila. 69 Aquilae is its Flamsteed designation. It is visible to the naked eye with an apparent visual magnitude of 4.91. Based upon an annual parallax shift of 16.2 mas, it is located 201 light years away. The star is moving closer to the Earth with a heliocentric radial velocity of −22.5 km/s.The stellar classification of 69 Aquilae is K1/2 III, which means this is an evolved giant star. It belongs to a sub-category called the red clump, indicating that it is on the horizontal branch and is generating energy through helium fusion at its core. The star is about 3.4 billion years old with 1.54 times the mass of the Sun and has expanded to 11 times the Sun's radius. It is radiating 45.7 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,529 K.

71 Cygni

71 Cygni is a star in the northern constellation of Cygnus, located 212 light years from the Sun. 71 Cygni is the Flamsteed designation; it has the Bayer designation g Cygni. It is visible to the naked eye as a dim, orange-hued star with an apparent visual magnitude of 5.22. The star is moving closer to the Earth with a heliocentric radial velocity of −21.5 km/s.At the age of one billion years, this is an evolved giant star with a stellar classification of K0− III, which means it has used up its core hydrogen and expanded. It is a red clump giant, indicating that it is on the horizontal branch of the Hertzsprung–Russell diagram and is generating energy by helium fusion at its center. The star has double the mass of the Sun and eight times the Sun's radius. It is radiating 45 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,983 K.

7 Persei

7 Persei is a star in the constellation Perseus, located 774 light years away from the Sun. While the star bears the Bayer designation Chi Persei, it is not to be confused with the entire cluster NGC 884, commonly referred to as Chi Persei. It is faintly visible to the naked eye as a dim, yellow-hued star with an apparent visual magnitude of 5.99. This object is moving closer to the Earth with a heliocentric radial velocity of −12.5 km/s.This is an evolved giant star with a stellar classification of G7 III, most likely (93% chance) on the horizontal branch. At the age of 191 million years, it has 3.84 times the mass of the Sun but has expanded to 24 times the Sun's radius. The star is radiating 316 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,974 K.

Andromeda I

Andromeda I is a dwarf spheroidal galaxy(dSph) about 2.40 million light-years away in the constellation Andromeda. Andromeda I is part of the local group of galaxies and a satellite galaxy of the Andromeda Galaxy (M31). It is roughly 3.5 degrees south and slightly east of M31. As of 2005, it is the closest known dSph companion to M31 at an estimated projected distance of ~40 kpc or ~150,000 light-years.

Andromeda I was discovered by Sidney van den Bergh in 1970 with the Mount Palomar Observatory 48-inch telescope. Further study of Andromeda I was done by the WFPC2 camera of the Hubble Space Telescope. This found that the horizontal branch stars, like other dwarf spheroidal galaxies were predominantly red. From this, and the abundance of blue horizontal branch stars, along with 99 RR Lyrae stars detected in 2005, lead to the conclusion there was an extended epoch of star formation. The estimated age is approximately 10 Gyr. The Hubble telescope also found a globular cluster in Andromeda I, being the least luminous galaxy where such a cluster was found.

Giant star

A giant star is a star with substantially larger radius and luminosity than a main-sequence (or dwarf) star of the same surface temperature. They lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III. The terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type by Ejnar Hertzsprung about 1905.Giant stars have radii up to a few hundred times the Sun and luminosities between 10 and a few thousand times that of the Sun. Stars still more luminous than giants are referred to as supergiants and hypergiants.

A hot, luminous main-sequence star may also be referred to as a giant, but any main-sequence star is properly called a dwarf no matter how large and luminous it is.

Red clump

The red clump is a clustering of red giants in the Hertzsprung–Russell diagram at around 5,000 K and absolute magnitude (MV) +0.5, slightly hotter than most red-giant-branch stars of the same luminosity. It is visible as a more dense region of the red giant branch or a bulge towards hotter temperatures. It is most distinct in many, but not all, galactic open clusters, but it is also noticeable in many intermediate-age globular clusters and in nearby field stars (e.g. the Hipparcos stars).

The red clump giants are cool horizontal branch stars, stars originally similar to the Sun which have undergone a helium flash and are now fusing helium in their cores.

Stellar evolution

Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the universe. The table shows the lifetimes of stars as a function of their masses. All stars are born from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star.

Nuclear fusion powers a star for most of its life. Initially the energy is generated by the fusion of hydrogen atoms at the core of the main-sequence star. Later, as the preponderance of atoms at the core becomes helium, stars like the Sun begin to fuse hydrogen along a spherical shell surrounding the core. This process causes the star to gradually grow in size, passing through the subgiant stage until it reaches the red giant phase. Stars with at least half the mass of the Sun can also begin to generate energy through the fusion of helium at their core, whereas more-massive stars can fuse heavier elements along a series of concentric shells. Once a star like the Sun has exhausted its nuclear fuel, its core collapses into a dense white dwarf and the outer layers are expelled as a planetary nebula. Stars with around ten or more times the mass of the Sun can explode in a supernova as their inert iron cores collapse into an extremely dense neutron star or black hole. Although the universe is not old enough for any of the smallest red dwarfs to have reached the end of their lives, stellar models suggest they will slowly become brighter and hotter before running out of hydrogen fuel and becoming low-mass white dwarfs.Stellar evolution is not studied by observing the life of a single star, as most stellar changes occur too slowly to be detected, even over many centuries. Instead, astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, and by simulating stellar structure using computer models.

Tau Cancri

Tau Cancri (τ Cancri) is a solitary, yellow-hued star in the zodiac constellation of Cancer. With an apparent visual magnitude of +5.42, it is faintly visible to the naked eye. Based upon an annual parallax shift of 11.92 mas as seen from Earth, it is located around 274 light years from the Sun.

With an age of about 620 million years and a stellar classification of G8 III, this is a red clump giant star, which indicates that it has evolved onto the horizontal branch and is generating energy through helium fusion at its core. It is a microvariable, showing a luminosity variation of 0.04 in magnitude. Tau Cancri has an estimated 2.4 times the mass of the Sun and 7.8 times the Sun's radius. The star radiates 40 times the solar luminosity from its photosphere at an effective temperature of 5,153 K.

Formation
Evolution
Luminosity class
Spectral
classification
Remnants
Hypothetical
stars
Nucleosynthesis
Structure
Properties
Star systems
Earth-centric
observations
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