The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (0.6–10 solar masses) late in their lives.
Observationally, an asymptotic-giant-branch star will appear as a bright red giant with a luminosity ranging up to thousands of times greater than the Sun. Its interior structure is characterized by a central and largely inert core of carbon and oxygen, a shell where helium is undergoing fusion to form carbon (known as helium burning), another shell where hydrogen is undergoing fusion forming helium (known as hydrogen burning), and a very large envelope of material of composition similar to main-sequence stars.
When a star exhausts the supply of hydrogen by nuclear fusion processes in its core, the core contracts and its temperature increases, causing the outer layers of the star to expand and cool. The star becomes a red giant, following a track towards the upper-right hand corner of the HR diagram. Eventually, once the temperature in the core has reached approximately 3×108 K, helium burning (fusion of helium nuclei) begins. The onset of helium burning in the core halts the star's cooling and increase in luminosity, and the star instead moves down and leftwards in the HR diagram. This is the horizontal branch (for population II stars) or red clump (for population I stars), or a blue loop for stars more massive than about 2 M☉.
After the completion of helium burning in the core, the star again moves to the right and upwards on the diagram, cooling and expanding as its luminosity increases. Its path is almost aligned with its previous red-giant track, hence the name asymptotic giant branch, although the star will become more luminous on the AGB than it did at the tip of the red giant branch. Stars at this stage of stellar evolution are known as AGB stars.
The AGB phase is divided into two parts, the early AGB (E-AGB) and the thermally pulsing AGB (TP-AGB). During the E-AGB phase, the main source of energy is helium fusion in a shell around a core consisting mostly of carbon and oxygen. During this phase, the star swells up to giant proportions to become a red giant again. The star's radius may become as large as one astronomical unit (~215 R☉).
After the helium shell runs out of fuel, the TP-AGB starts. Now the star derives its energy from fusion of hydrogen in a thin shell, which restricts the inner helium shell to a very thin layer and prevents it fusing stably. However, over periods of 10,000 to 100,000 years, helium from the hydrogen shell burning builds up and eventually the helium shell ignites explosively, a process known as a helium shell flash. The luminosity of the shell flash peaks at thousands of times the total luminosity of the star, but decreases exponentially over just a few years. The shell flash causes the star to expand and cool which shuts off the hydrogen shell burning and causes strong convection in the zone between the two shells. When the helium shell burning nears the base of the hydrogen shell, the increased temperature reignites hydrogen fusion and the cycle begins again. The large but brief increase in luminosity from the helium shell flash produces an increase in the visible brightness of the star of a few tenths of a magnitude for several hundred years, a change unrelated to the brightness variations on periods of tens to hundreds of days that are common in this type of star.
During the thermal pulses, which last only a few hundred years, material from the core region may be mixed into the outer layers, changing the surface composition, a process referred to as dredge-up. Because of this dredge-up, AGB stars may show S-process elements in their spectra and strong dredge-ups can lead to the formation of carbon stars. All dredge-ups following thermal pulses are referred to as third dredge-ups, after the first dredge-up, which occurs on the red-giant branch, and the second dredge up, which occurs during the E-AGB. In some cases there may not be a second dredge-up but dredge-ups following thermal pulses will still be called a third dredge-up. Thermal pulses increase rapidly in strength after the first few, so third dredge-ups are generally the deepest and most likely to circulate core material to the surface.
AGB stars are typically long-period variables, and suffer mass loss in the form of a stellar wind. Thermal pulses produce periods of even higher mass loss and may result in detached shells of circumstellar material. A star may lose 50 to 70% of its mass during the AGB phase.
The extensive mass loss of AGB stars means that they are surrounded by an extended circumstellar envelope (CSE). Given a mean AGB lifetime of one Myr and an outer velocity of 10 km/s, its maximum radius can be estimated to be roughly 3×1014 km (30 light years). This is a maximum value since the wind material will start to mix with the interstellar medium at very large radii, and it also assumes that there is no velocity difference between the star and the interstellar gas. Dynamically, most of the interesting action is quite close to the star, where the wind is launched and the mass loss rate is determined. However, the outer layers of the CSE show chemically interesting processes, and due to size and lower optical depth, are easier to observe.
The temperature of the CSE is determined by heating and cooling properties of the gas and dust, but drops with radial distance from the photosphere of the stars which are 2,000–3,000 K. Chemical peculiarities of an AGB CSE outwards include:
The dichotomy between oxygen-rich and carbon-rich stars has an initial role in determining whether the first condensates are oxides or carbides, since the least abundant of these two elements will likely remain in the gas phase as COx.
In the dust formation zone, refractory elements and compounds (Fe, Si, MgO, etc.) are removed from the gas phase and end up in dust grains. The newly formed dust will immediately assist in surface catalyzed reactions. The stellar winds from AGB stars are sites of cosmic dust formation, and are believed to be the main production sites of dust in the universe.
The stellar winds of AGB stars (Mira variables and OH/IR stars) are also often the site of maser emission. The molecules that account for this are SiO, H2O, OH, HCN, and SiS. SiO, H2O, and OH masers are typically found in oxygen-rich M-type AGB stars such as R Cassiopeiae and U Orionis, while HCN and SiS masers are generally found in carbon stars such as IRC +10216. S-type stars with masers are uncommon.
After these stars have lost nearly all of their envelopes, and only the core regions remain, they evolve further into short-lived preplanetary nebulae. The final fate of the AGB envelopes are represented by planetary nebulae (PNe).
As many as a quarter of all post-AGB stars undergo what is dubbed a "born-again" episode. The carbon–oxygen core is now surrounded by helium with an outer shell of hydrogen. If the helium is re-ignited a thermal pulse occurs and the star quickly returns to the AGB, becoming a helium-burning, hydrogen-deficient stellar object. If the star still has a hydrogen-burning shell when this thermal pulse occurs, it is termed a "late thermal pulse". Otherwise it is called a "very late thermal pulse".
The outer atmosphere of the born-again star develops a stellar wind and the star once more follows an evolutionary track across the Hertzsprung–Russell diagram. However, this phase is very brief, lasting only about 200 years before the star again heads toward the white dwarf stage. Observationally, this late thermal pulse phase appears almost identical to a Wolf–Rayet star in the midst of its own planetary nebula.
Stars close to the upper mass limit to still qualify as AGB stars show some peculiar properties and have been dubbed super-AGB stars. They have masses above 7 M☉ and up to 9 or 10 M☉ (or more). They represent a transition to the more massive supergiant stars that undergo full fusion of elements heavier than helium. During the triple-alpha process, some elements heavier than carbon are also produced: mostly oxygen, but also some magnesium, neon, and even heavier elements. Super-AGB stars develop partially degenerate carbon–oxygen cores that are large enough to ignite carbon in a flash analogous to the earlier helium flash. The second dredge-up is very strong in this mass range and that keeps the core size below the level required for burning of neon as occurs in higher-mass supergiants. The size of the thermal pulses and third dredge-ups are reduced compared to lower-mass stars, while the frequency of the thermal pulses increases dramatically. Some super-AGB stars may explode as an electron capture supernova, but most will end as an oxygen–neon white dwarf. Since these stars are much more common than higher-mass supergiants, they could form a high proportion of observed supernovae. Detecting examples of these supernovae would provide valuable confirmation of models that are highly dependent on assumptions.
24 Capricorni is a single star in the southern constellation of Capricornus. This object is visible to the naked eye as a faint, red-hued star with an apparent visual magnitude of +4.49. It is approximately 460 light years from the Sun, based on parallax. The star is moving further from the Earth with a heliocentric radial velocity of +32 km/s.This is an aging red giant, currently on the asymptotic giant branch, with a stellar classification of M1− III; a star that has exhausted the supply of hydrogen at its core and expanded to 54 times the Sun's radius. It is radiating 611 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 3,903 K.27 Cancri
27 Cancri is a single star in the zodiac constellation of Cancer, located around 990 light years away from the Sun. It is visible to the naked eye as a faint, red-hued star with a typical apparent visual magnitude of around +5.56. The star is moving closer to the Earth with a heliocentric radial velocity of −8.3 km/s. It is a member of the Arcturus stream, a group of stars with high proper motion and metal-poor properties thought to be the remnants of a small galaxy consumed by the Milky Way.This is an aging red giant with a stellar classification of M3 IIIa, currently on the asymptotic giant branch. It is classified as a semiregular variable star of type SRb and its brightness varies from magnitude +5.41 to +5.75 with a period of 40 days. The star is radiating around 2,455 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 3,574 K.42 Herculis
42 Herculis is a single star located around 450 light years away from the Sun in the northern constellation of Hercules. It is visible to the naked eye as a faint, red-hued star with an apparent visual magnitude of 4.86. The star is moving closer to the Earth with a heliocentric radial velocity of −56 km/s.This is an aging red giant star on the asymptotic giant branch with a stellar classification of M2.5III. It has been catalogued as a suspected variable star, although a 1992 photometric survey found the brightness to be constant. Having exhausted the supply of hydrogen at its core, the star has expanded to 64 times the Sun's radius. It is radiating 734 times the luminosity of the Sun from its swollen photosphere at an effective temperature of 3761 K.There is an unknown source of X-ray and far ultraviolet emission originating from a location offset by more than one arcsecond from the star. This may indicate there is an undetected main sequence companion.54 Eridani
54 Eridani is a suspected astrometric binary star system located around 400 light years from the Sun in the equatorial constellation of Eridanus. It is visible to the naked eye as a faint, reddish hued star with a baseline apparent visual magnitude of 4.32. The object is moving closer to the Earth with a heliocentric radial velocity of −33 km/s.The visible component is an aging red giant star, currently on the asymptotic giant branch, with a stellar classification of M3/4 III. It is a semiregular variable star of subtype SRb, ranging in magnitude from 4.28 down to 4.36. The star has pulsation periods of 18.8 and 45.5 days, each with an amplitude of 0.019 in magnitude. With the hydrogen at its core exhausted, the star has expanded to around 69 times the Sun's radius and it is radiating 1,021 times the luminosity of the Sun from its swollen photosphere at an effective temperature of 3,915 K.BE Camelopardalis
BE Camelopardalis (BE Cam) is a star in the constellation Camelopardalis.
BE Camelopardalis is a red M-type bright giant with a mean apparent magnitude of +4.39. It is approximately 965 light years from Earth. It is classified as an irregular variable star and its brightness varies from magnitude +4.35 to +4.48.Chi Pegasi
Chi Pegasi (χ Peg) is a class M2+III (red giant) star in the constellation Pegasus. Its apparent magnitude is 4.80 and it is approximately 368 light years away based on parallax.HD 44131
HD 44131 is a class M1III (red giant) star in the constellation Orion. Its apparent magnitude is 4.91 and it is approximately 465 light years away based on parallax.HD 80230
HD 80230, also known as g Carinae (g Car), is a star in the constellation Carina. g Carinae is a M-type red giant, currently on the asymptotic giant branch, with an apparent magnitude of +4.34. It is approximately 536 light years from Earth.Red giant
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses (M☉)) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K (4,700 °C; 8,500 °F) or lower. The appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars.
The most common red giants are stars on the red-giant branch (RGB) that are still fusing hydrogen into helium in a shell surrounding an inert helium core. Other red giants are the red-clump stars in the cool half of the horizontal branch, fusing helium into carbon in their cores via the triple-alpha process; and the asymptotic-giant-branch (AGB) stars with a helium burning shell outside a degenerate carbon–oxygen core, and a hydrogen burning shell just beyond that.TT Cygni
TT Cygni is a carbon star. It is 561 parsecs (1,830 ly) away in Cygnus. It has an apparent magnitude of 7.44. It is called a carbon star because it has a high ratio of carbon to oxygen in its surface layers. The carbon was produced by helium fusion, dredged up from inside the star. A shell of carbon monoxide, about half a light year across, was emitted 6,000 years before the star was as it appears from Earth now.T Centauri
T Centauri is a variable star located in the far southern constellation Centaurus. It varies between magnitudes 5.56 and 8.44 over 181.4 days. Pulsating between spectral classes K0:e and M4II:e, it has been classed as a semiregular variable, though Sebastian Otero of the American Association of Variable Star Observers has noted its curve more aligned with RV Tauri variable stars and has classified it as one.T Ceti
T Ceti is a semiregular variable star located in the equatorial constellation of Cetus. It varies between magnitudes 5.0 and 6.9 over 159.3 days. The stellar parallax shift measured by Hipparcos is 3.7 mas, which yields a distance estimate of roughly 900 light years. It is moving further from the Earth with a heliocentric radial velocity of +29 km/s.This is an MS-type star on the asymptotic giant branch with a spectral type of M5-6Se. It is often classified simply as an M-type star, for example with the spectral type of M5.5e − M8.8e. (The 'e' notation indicates the presence of emission lines in the spectrum.) It is a long period Mira variable with changing cycle lengths, showing a variation in its spectral features over the course of each cycle. Pulsation periods of 388, 398, and 382 days have been reported, as well as variations in the amplitude, which may indicate dual pulsation cycles that are interfering with each other. The star is losing mass at the rate of 8.2×10−8 M☉ y−1, and it is surrounded by a circumstellar dust shell consisting of crystallized, mostly iron-rich silicates.T Ceti has an estimated three times the mass of the Sun and has expanded to 275 times the Sun's radius. It is radiating 8,128 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 3,396 K.U Hydrae
U Hydrae (U Hya) is a carbon star in the constellation Hydra. It is also a variable star, with its brightness ranging from 4.7 to 5.2 over a 450-day period, with some irregularity.Upsilon Aurigae
Upsilon Aurigae, Latinized from υ Aurigae, is the Bayer designation for a star in the northern constellation of Auriga. It has an apparent visual magnitude of 4.74, which means it is bright enough to be seen with the naked eye. Based upon parallax measurements made during the Hipparcos mission, this star is approximately 520 light-years (160 parsecs) distant from the Earth.
This is an evolved red giant star with a stellar classification of M0 III. It is a suspected variable star and is currently on the asymptotic giant branch, which means it is generating energy at its core through the fusion of helium. The measured angular diameter of this star, after correction for limb darkening, is 4.24 ± 0.05 mas. At the estimated distance of Upsilon Aurigae, this yields a physical size of about 73 times the radius of the Sun.V744 Centauri
V744 Centauri, is a semi-regular variable pulsating star in the constellation Centaurus. Located 3 degrees north north east of Epsilon Centauri, It ranges from apparent magnitude 5.1 to 6.7 over 90 days. It is unusual in that it is a red star with a high proper motion (greater than 50 milliarcseconds a year).V Hydrae
V Hydrae (V Hya) is a carbon star in the constellation Hydra.W43A
W43A or IRAS 18450-0148 is a late-type star with an envelope of OH/IR type with a magnetically collimated jet (a protoplanetary nebula). The star is in the early stages of becoming a planetary nebula, a process that will take several thousand years.W Cygni
W Cygni is a semi-regular variable star in the constellation Cygnus, located 570 light-years from Earth. It lies less than half a degree southeast of ρ Cygni.Y Centauri
Y Centauri or Y Cen (HD 127233, HIP 70969) is a semiregular variable star in the constellation of Centaurus.
The variability in the star was discovered by Williamina Fleming in 1895 and published in the Third Catalogue of Variable Stars. The photographic magnitude range was given as 7.7 - 8.8, but the variability was described as "somewhat doubtful". It was later given the designation HV 52 in the Harvard Catalogue of Variable Stars. The General Catalogue of Variable Stars lists it as a possible semiregular variable star with a period of 180 days and a photographic magnitude range of 8.9 - 10.0. A study of Hipparcos satellite photometry found a small amplitude range of 0.2 magnitudes at a visual magnitude of 8.53.The distance of the star is poorly known. The revised Hipparcos annual parallax of 3.50 mas gives a distance of 900 light years. A study taking into account the variability of the star found a parallax of 5.57 mas, corresponding to a distance of 585 light years. It is an asymptotic giant branch star 330 times as luminous as the sun. Its spectral type varies between M4 and M7 as it pulsates.The star has been observed to produce 22 GHz water maser emission, although later searches did not find any maser emission.
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