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62-492: 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

124-425: A habitable zone for several billion years at 2 astronomical units (AU) out to around 100 million years at 9 AU out, giving perhaps enough time for life to develop on a suitable world. After the red-giant stage, there would for such a star be a habitable zone between 7 and 22 AU for an additional one billion years. Later studies have refined this scenario, showing how for a 1  M ☉ star

186-404: A type II supernova . The most massive stars can become Wolf–Rayet stars without becoming giants or supergiants at all. Although traditionally it has been suggested the evolution of a star into a red giant will render its planetary system , if present, uninhabitable, some research suggests that, during the evolution of a 1  M ☉ star along the red-giant branch, it could harbor

248-456: A white dwarf . [REDACTED] Media related to Red giants at Wikimedia Commons Long-period variable The descriptive term long-period variable star refers to various groups of cool luminous pulsating variable stars . It is frequently abbreviated to LPV . The General Catalogue of Variable Stars does not define a long-period variable star type, although it does describe Mira variables as long-period variables. The term

310-425: A dynamic and interesting chemistry , much of which is difficult to reproduce in a laboratory environment because of the low densities involved. The nature of the chemical reactions in the envelope changes as the material moves away from the star, expands and cools. Near the star the envelope density is high enough that reactions approach thermodynamic equilibrium. As the material passes beyond about 5 × 10  km

372-569: A few large cells, the features of which cause the variations of brightness so common on both types of stars. Red giants are evolved from main-sequence stars with masses in the range from about 0.3  M ☉ to around 8  M ☉ . When a star initially forms from a collapsing molecular cloud in the interstellar medium , it contains primarily hydrogen and helium, with trace amounts of " metals " (in astrophysics, this refers to all elements heavier than hydrogen and helium). These elements are all uniformly mixed throughout

434-408: A high proportion of observed supernovae. Detecting examples of these supernovae would provide valuable confirmation of models that are highly dependent on assumptions. 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

496-405: A hot chromosphere above the photosphere of red giants, where investigating the heating mechanisms for the chromospheres to form requires 3D simulations of red giants. Another noteworthy feature of red giants is that, unlike Sun-like stars whose photospheres have a large number of small convection cells ( solar granules ), red-giant photospheres, as well as those of red supergiants , have just

558-407: 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 × 10  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 . These envelopes have

620-439: A much larger effect would be Roche lobe overflow causing mass-transfer from the star to the planet when the giant expands out to the orbital distance of the planet. (A similar process in multiple star systems is believed to be the cause of most novas and type Ia supernovas .) Many of the well-known bright stars are red giants, because they are luminous and moderately common. The red-giant branch variable star Gamma Crucis

682-535: A period, although it is an open question whether they are truly non-periodic. LPVs have spectral class F and redwards, but most are spectral class M, S or C . Many of the reddest stars in the sky, such as Y CVn , V Aql , and VX Sgr are LPVs. Most LPVs, including all Mira variables, are thermally-pulsing asymptotic giant branch stars with luminosities several thousand times the sun. Some semiregular and irregular variables are less luminous giant stars, while others are more luminous supergiants including some of

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744-401: A star with the shell structure. The core contracts and heats up due to the lack of fusion, and so the outer layers of the star expand greatly, absorbing most of the extra energy from shell fusion. This process of cooling and expanding is the subgiant stage. When the envelope of the star cools sufficiently it becomes convective , the star stops expanding, its luminosity starts to increase, and

806-458: A star's life is called the horizontal branch in metal-poor stars , so named because these stars lie on a nearly horizontal line in the H–R diagram of many star clusters. Metal-rich helium-fusing stars instead lie on the so-called red clump in the H–R diagram. An analogous process occurs when the core helium is exhausted, and the star collapses once again, causing helium in a shell to begin fusing. At

868-490: A temperature of roughly 1 × 10  K , hot enough to begin fusing helium to carbon via the triple-alpha process . Once the degenerate core reaches this temperature, the entire core will begin helium fusion nearly simultaneously in a so-called helium flash . In more-massive stars, the collapsing core will reach these temperatures before it is dense enough to be degenerate, so helium fusion will begin much more smoothly, and produce no helium flash. The core helium fusing phase of

930-474: 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

992-405: A very large envelope of material of composition similar to main-sequence stars (except in the case of carbon 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

1054-403: A white dwarf. Very-low-mass stars are fully convective and may continue to fuse hydrogen into helium for up to a trillion years until only a small fraction of the entire star is hydrogen. Luminosity and temperature steadily increase during this time, just as for more-massive main-sequence stars, but the length of time involved means that the temperature eventually increases by about 50% and

1116-426: Is 36 light-years away, and Gacrux is the nearest M-class giant at 88 light-years' distance. A red giant will usually produce a planetary nebula and become a white dwarf at the end of its life. A red giant is a star that has exhausted the supply of hydrogen in its core and has begun thermonuclear fusion of hydrogen in a shell surrounding the core. They have radii tens to hundreds of times larger than that of

1178-468: 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

1240-485: Is the nearest M-class giant star at 88 light-years. The K1.5 red-giant branch star Arcturus is 36 light-years away. The Sun will exit the main sequence in approximately 5 billion years and start to turn into a red giant. As a red giant, the Sun will grow so large (over 200 times its present-day radius : ~ 215   R ☉ ; ~ 1  AU ) that it will engulf Mercury , Venus , and likely Earth. It will lose 38% of its mass growing, then will die into

1302-505: 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 oxygen–neon white dwarfs. Since these stars are much more common than higher-mass supergiants, they could form

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1364-483: 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 such as Sakurai's Object and FG Sagittae are being observed as they rapidly evolve through this phase. Mapping

1426-450: The Sun's photosphere temperature of nearly 6,000 K ) and radii up to about 200 times the Sun ( R ☉ ). Stars on the horizontal branch are hotter, with only a small range of luminosities around 75  L ☉ . Asymptotic-giant-branch stars range from similar luminosities as the brighter stars of the red-giant branch, up to several times more luminous at the end of

1488-485: The largest known stars such as VY CMa . Between a quarter and a half of long period variables show very slow variations with an amplitude up to one magnitude at visual wavelengths, and a period around ten times the primary pulsation period. These are called long secondary periods. The causes of the long secondary periods are unknown. Binary interactions, dust formation, rotation, or non-radial oscillations have all been proposed as causes, but all have problems explaining

1550-414: The main sequence and will not have become giants yet) and more massive stars are expected to have more massive planets. However, the masses of the planets that have been found around giant stars do not correlate with the masses of the stars; therefore, the planets could be growing in mass during the stars' red giant phase. The growth in planet mass could be partly due to accretion from stellar wind, although

1612-427: The radiation and thermal pressure the core generates, which are what support the star against gravitational contraction . The star further contracts, increasing the pressures and thus temperatures inside the star (as described by the ideal gas law ). Eventually a "shell" layer around the core reaches temperatures sufficient to fuse hydrogen and thus generate its own radiation and thermal pressure, which "re-inflates"

1674-513: The HR diagram. Eventually, once the temperature in the core has reached approximately 3 × 10  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 a blue loop for stars more massive than about 2.3  M ☉ . After

1736-440: The Sun . However, their outer envelope is lower in temperature, giving them a yellowish-orange hue. Despite the lower energy density of their envelope, red giants are many times more luminous than the Sun because of their great size. Red-giant-branch stars have luminosities up to nearly three thousand times that of the Sun ( L ☉ ); spectral types of K or M have surface temperatures of 3,000–4,000  K (compared with

1798-405: The Sun. After some billions more years, they start to become less luminous and cooler even though hydrogen shell burning continues. These become cool helium white dwarfs. Very-high-mass stars develop into supergiants that follow an evolutionary track that takes them back and forth horizontally over the H–R diagram, at the right end constituting red supergiants . These usually end their life as

1860-577: The asymptotic-giant branch and convects carbon to the surface in sufficiently massive stars. The stellar limb of a red giant is not sharply defined, contrary to their depiction in many illustrations. Rather, due to the very low mass density of the envelope, such stars lack a well-defined photosphere , and the body of the star gradually transitions into a ' corona '. The coolest red giants have complex spectra, with molecular lines , emission features, and sometimes masers , particularly from thermally pulsing AGB stars. Observations have also provided evidence of

1922-413: The circumstellar magnetic fields of thermal-pulsating (TP-) AGB stars has recently been reported using the so-called Goldreich-Kylafis effect . 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

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1984-449: 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

2046-399: The core region may be mixed into the outer layers, changing the surface composition, in 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

2108-414: The core regions remain, they evolve further into short-lived protoplanetary nebula . The final fate of the AGB envelopes are represented by planetary nebulae (PNe). Physical samples, known as presolar grains, of mineral grains from AGB stars are available for laboratory analysis in the form of individual refractory presolar grains . These formed in the circumstellar dust envelopes and were transported to

2170-426: 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. These changes are 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

2232-431: The density falls to the point where kinetics , rather than thermodynamics, becomes the dominant feature. Some energetically favorable reactions can no longer take place in the gas, because the reaction mechanism requires a third body to remove the energy released when a chemical bond is formed. In this region many of the reactions that do take place involve radicals such as OH (in oxygen rich envelopes) or CN (in

2294-407: The early Solar System by stellar wind . A majority of presolar silicon carbide grains have their origin in 1–3 M ☉ carbon stars in the late thermally-pulsing AGB phase of their stellar evolution. 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

2356-420: The envelopes surrounding carbon stars). In the outermost region of the envelope, beyond about 5 × 10  km , the density drops to the point where the dust no longer completely shields the envelope from interstellar UV radiation and the gas becomes partially ionized. These ions then participate in reactions with neutral atoms and molecules. Finally as the envelope merges with the interstellar medium, most of

2418-466: The first condensates are oxides or carbides, since the least abundant of these two elements will likely remain in the gas phase as CO x . 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

2480-599: The form of a stellar wind . For M-type AGB stars, the stellar winds are most efficiently driven by micron-sized grains. 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 mass-loss rates typically range between 10 and 10 M ⊙ year, and can even reach as high as 10 M ⊙ year; while wind velocities are typically between 5 and 30 km/s. The extensive mass loss of AGB stars means that they are surrounded by an extended circumstellar envelope (CSE). Given

2542-440: The habitable zone for 5.8 billion years and 2.1 billion years, respectively; for stars more massive than the Sun, the times are considerably shorter. As of 2023, several hundred giant planets have been discovered around giant stars. However, these giant planets are more massive than the giant planets found around solar-type stars. This could be because giant stars are more massive than the Sun (less massive stars will still be on

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2604-408: The habitable zone lasts from 100 million years for a planet with an orbit similar to that of Mars to 210 million years for one that orbits at Saturn 's distance to the Sun, the maximum time (370 million years) corresponding for planets orbiting at the distance of Jupiter . However, planets orbiting a 0.5  M ☉ star in equivalent orbits to those of Jupiter and Saturn would be in

2666-482: 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 power of

2728-439: The hydrogen in the core into helium; its main-sequence life ends when nearly all the hydrogen in the core has been fused. For the Sun, the main-sequence lifetime is approximately 10 billion years. More massive stars burn disproportionately faster and so have a shorter lifetime than less massive stars. When the star has mostly exhausted the hydrogen fuel in its core, the core's rate of nuclear reactions declines, and thus so do

2790-447: The long period variables as only AGB and possibly red giant tip stars. The recently classified OSARGs are by far the most numerous of these stars, comprising a high proportion of red giants. Long period variables are pulsating cool giant , or supergiant , variable stars with periods from around a hundred days, or just a few days for OSARGs, to more than a thousand days. In some cases, the variations are too poorly defined to identify

2852-464: The luminosity by around 10 times. Eventually the level of helium increases to the point where the star ceases to be fully convective and the remaining hydrogen locked in the core is consumed in only a few billion more years. Depending on mass, the temperature and luminosity continue to increase for a time during hydrogen shell burning, the star can become hotter than the Sun and tens of times more luminous than when it formed although still not as luminous as

2914-563: 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 , H 2 O , OH , HCN , and SiS . SiO, H 2 O, 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

2976-452: The molecules are destroyed by UV radiation. 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

3038-429: 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

3100-476: The radius large and the surface temperature around 5,000  K [K] (4,700 °C; 8,500 °F) or lower. The appearance of the red giant is from yellow-white to reddish-orange, including the spectral types K and M, sometimes G, but also class S stars and most carbon stars . Red giants vary in the way by which they generate energy: Many of the well-known bright stars are red giants because they are luminous and moderately common. The K0 RGB star Arcturus

3162-457: 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

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3224-403: The same time, hydrogen may begin fusion in a shell just outside the burning helium shell. This puts the star onto the asymptotic giant branch , a second red-giant phase. The helium fusion results in the build-up of a carbon–oxygen core. A star below about 8  M ☉ will never start fusion in its degenerate carbon–oxygen core. Instead, at the end of the asymptotic-giant-branch phase

3286-416: The shell flash peaks at thousands of times the observed 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

3348-446: The star has about 0.2 to 0.5  M ☉ , it is massive enough to become a red giant but does not have enough mass to initiate the fusion of helium. These "intermediate" stars cool somewhat and increase their luminosity but never achieve the tip of the red-giant branch and helium core flash. When the ascent of the red-giant branch ends they puff off their outer layers much like a post-asymptotic-giant-branch star and then become

3410-452: The star is ascending the red-giant branch of the Hertzsprung–Russell (H–R) diagram . The evolutionary path the star takes as it moves along the red-giant branch depends on the mass of the star. For the Sun and stars of less than about 2  M ☉ the core will become dense enough that electron degeneracy pressure will prevent it from collapsing further. Once the core is degenerate , it will continue to heat until it reaches

3472-509: The star will eject its outer layers, forming a planetary nebula with the core of the star exposed, ultimately becoming a white dwarf . The ejection of the outer mass and the creation of a planetary nebula finally ends the red-giant phase of the star's evolution. The red-giant phase typically lasts only around a billion years in total for a solar mass star, almost all of which is spent on the red-giant branch. The horizontal-branch and asymptotic-giant-branch phases proceed tens of times faster. If

3534-422: The star's outer layers and causes them to expand. The hydrogen-burning shell results in a situation that has been described as the mirror principle : when the core within the shell contracts, the layers of the star outside the shell must expand. The detailed physical processes that cause this are complex. Still, the behavior is necessary to satisfy simultaneous conservation of gravitational and thermal energy in

3596-402: The star. The star "enters" the main sequence when its core reaches a temperature (several million kelvins ) high enough to begin fusing hydrogen-1 (the predominant isotope), and establishes hydrostatic equilibrium . (In astrophysics, stellar fusion is often referred to as "burning", with hydrogen fusion sometimes termed " hydrogen burning ".) Over its main sequence life, the star slowly fuses

3658-532: The term more restrictively to refer just to Mira and semiregular variables, or solely to Miras. The AAVSO LPV Section covers "Miras, Semiregulars, RV Tau and all your favorite red giants". The AAVSO LPV Section covers the Mira, SR, and L stars, but also RV Tauri variables , another type of large cool slowly varying star. This includes SRc and Lc stars which are respectively semi-regular and irregular cool supergiants. Recent researches have increasingly focused on

3720-448: The thermal pulsing phase. Among the asymptotic-giant-branch stars belong the carbon stars of type C-N and late C-R, produced when carbon and other elements are convected to the surface in what is called a dredge-up . The first dredge-up occurs during hydrogen shell burning on the red-giant branch, but does not produce a large carbon abundance at the surface. The second, and sometimes third, dredge-up occurs during helium shell burning on

3782-417: Was first used in the 19th century, before more precise classifications of variable stars, to refer to a group that were known to vary on timescales typically hundreds of days. By the middle of the 20th century, long period variables were known to be cool giant stars. The relationship of Mira variables, semiregular variables , and other pulsating stars was being investigated and the term long period variable

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3844-680: Was generally restricted to the coolest pulsating stars, almost all Mira variables. Semiregular variables were considered intermediate between LPVs and Cepheids . After the publication of the General Catalogue of Variable Stars, both Mira variables and semiregular variables, particularly those of type SRa, were often considered as long period variables. At its broadest, LPVs include Mira, semiregular, slow irregular variables, and OGLE small amplitude red giants (OSARGs), including both giant and supergiant stars. The OSARGs are generally not treated as LPVs, and many authors continue to use

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