Red giant#Planets

File:Structure of Stars (artist’s impression).jpg

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 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 ({{Solar luminosity|link=y}}); spectral types of K or M have surface temperatures of {{val|3000|–|4000|ul=K|fmt=commas}} (compared with the Sun's photosphere temperature of nearly {{val|6000|u=K|fmt=commas}}) and radii up to about 200 times the Sun ({{Solar radius|link=y}}). Stars on the horizontal branch are hotter, with only a small range of luminosities around {{solar luminosity|75}}. 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 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.{{Cite journal | last1 = Boothroyd | first1 = A. I. | last2 = Sackmann | first2 = I. -J. | doi = 10.1086/306546 | title = The CNO Isotopes: Deep Circulation in Red Giants and First and Second Dredge-up | journal = The Astrophysical Journal | volume = 510 | pages = 232–250 | year = 1999 | issue = 1 |bibcode = 1999ApJ...510..232B | arxiv = astro-ph/9512121 | s2cid = 561413 }} 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 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'.{{cite journal|bibcode=2007ApJ...659.1592S|arxiv=astro-ph/0608195|title=Structured Red Giant Winds with Magnetized Hot Bubbles and the Corona/Cool Wind Dividing Line|journal=The Astrophysical Journal|volume=659|issue=2|pages=1592–1610|last1=Suzuki|first1=Takeru K.|year=2007|doi=10.1086/512600|s2cid=13957448}} The coolest red giants have complex spectra, with molecular lines, emission features, and sometimes masers, particularly from thermally pulsing AGB stars.{{cite journal|bibcode=2003agbs.conf.....H|title=Asymptotic giant branch stars|journal=Asymptotic Giant Branch Stars|last1=Habing|first1=Harm J.|last2=Olofsson|first2=Hans|year=2003}} Observations have also provided evidence of a hot chromosphere above the photosphere of red giants,{{Cite book|last=Deutsch|first=A. J.|chapter=Chromospheric Activity in Red Giants, and Related Phenomena |date=1970|title=Ultraviolet Stellar Spectra and Related Ground-Based Observations|bibcode=1970IAUS...36..199D|volume=36|pages=199–208|doi=10.1007/978-94-010-3293-3_33|isbn=978-94-010-3295-7}}{{Cite journal|last1=Vlemmings|first1=Wouter|last2=Khouri|first2=Theo|last3=O’Gorman|first3=Eamon|last4=De Beck|first4=Elvire|last5=Humphreys|first5=Elizabeth|last6=Lankhaar|first6=Boy|last7=Maercker|first7=Matthias|last8=Olofsson|first8=Hans|last9=Ramstedt|first9=Sofia|last10=Tafoya|first10=Daniel|last11=Takigawa|first11=Aki|date=December 2017|title=The shock-heated atmosphere of an asymptotic giant branch star resolved by ALMA|journal=Nature Astronomy|language=en|volume=1|issue=12|pages=848–853|doi=10.1038/s41550-017-0288-9|arxiv=1711.01153|bibcode=2017NatAs...1..848V|s2cid=119393687|issn=2397-3366}}{{Cite journal|last1=O’Gorman|first1=E.|last2=Harper|first2=G. M.|last3=Ohnaka|first3=K.|last4=Feeney-Johansson|first4=A.|last5=Wilkeneit-Braun|first5=K.|last6=Brown|first6=A.|last7=Guinan|first7=E. F.|last8=Lim|first8=J.|last9=Richards|first9=A. M. S.|last10=Ryde|first10=N.|last11=Vlemmings|first11=W. H. T.|date=June 2020|title=ALMA and VLA reveal the lukewarm chromospheres of the nearby red supergiants Antares and Betelgeuse|journal=Astronomy & Astrophysics|volume=638|pages=A65|doi=10.1051/0004-6361/202037756|arxiv=2006.08023|bibcode=2020A&A...638A..65O|s2cid=219484950|issn=0004-6361}} where investigating the heating mechanisms for the chromospheres to form requires 3D simulations of red giants.{{Cite journal|last1=Wedemeyer|first1=Sven|last2=Kučinskas|first2=Arūnas|last3=Klevas|first3=Jonas|last4=Ludwig|first4=Hans-Günter|date=2017-10-01|title=Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars - VI. First chromosphere model of a late-type giant|journal=Astronomy & Astrophysics|language=en|volume=606|pages=A26|doi=10.1051/0004-6361/201730405|arxiv=1705.09641|bibcode=2017A&A...606A..26W|s2cid=119510487|issn=0004-6361}}

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 a few large cells, the features of which cause the variations of brightness so common on both types of stars.

{{cite journal

| volume = 195

| pages = 137–144

| last1 = Schwarzschild

| first1 = Martin

| title = On the scale of photospheric convection in red giants and supergiants

| journal = Astrophysical Journal

| date = 1975

|bibcode = 1975ApJ...195..137S

| doi = 10.1086/153313

| doi-access = free

}}

{{Main|Stellar evolution#Mid-sized stars}}

File:The life cycle of a Sun-like star (annotated).jpg, from its birth on the left side of the frame to its evolution into a red giant on the right after billions of years]]

Red giants are evolved from main-sequence stars with masses in the range from about {{Solar mass|0.3|link=y}} to around {{Solar mass|8}}. 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 other than hydrogen and helium). These elements are all uniformly mixed throughout 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 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 their fuel disproportionately faster and so have a shorter lifetime than less massive stars.{{cite book | last=Zeilik | first=Michael A. |author2=Gregory, Stephan A. | title=Introductory Astronomy & Astrophysics | edition=4th | date=1998 | publisher=Saunders College Publishing | isbn=0-03-006228-4 | pages=321–322 }}

When the star has mostly exhausted the hydrogen fuel in its core, the core's rate of nuclear reactions declines, and thus so do 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" the star's outer layers and causes them to expand.{{cite web | title=Stars | website=NASA Science Mission Directorate | date=2012-03-16 | url=https://science.nasa.gov/astrophysics/focus-areas/how-do-stars-form-and-evolve#:~:text=Hydrogen%20is%20still%20available%20outside,star%20into%20a%20red%20giant. | access-date=2023-08-29}} 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 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 the star is ascending the red-giant branch of the Hertzsprung–Russell (H–R) diagram.{{cite book|author1=Tiago L. Campante|author2=Nuno C. Santos|author3=Mário J. P. F. G. Monteiro|title=Asteroseismology and Exoplanets: Listening to the Stars and Searching for New Worlds: IVth Azores International Advanced School in Space Sciences|url=https://books.google.com/books?id=keM8DwAAQBAJ&pg=PA99|date=3 November 2017|publisher=Springer|isbn=978-3-319-59315-9|pages=99–}}

File:Seeing into the Heart of Mira A and its Partner.jpg is an old star, already shedding its outer layers into space]]

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 {{Solar mass|2}}{{cite journal|bibcode=1994A&AS..105...29F|title=Evolutionary sequences of stellar models with new radiative opacities. IV. Z=0.004 and Z=0.008|journal=Astronomy and Astrophysics Supplement Series |volume=105|last1=Fagotto|first1=F.|last2=Bressan|first2=A.|last3=Bertelli|first3=G.|last4=Chiosi|first4=C.|year=1994|pages = 29}} 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 a temperature of roughly {{val|1e8|u=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 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.{{cite journal|bibcode=1999ApJ...511..225A|arxiv=astro-ph/9808253|title=The Age-dependent Luminosities of the Red Giant Branch Bump, Asymptotic Giant Branch Bump, and Horizontal Branch Red Clump|journal=The Astrophysical Journal|volume=511|pages=225–234|last1=Alves|first1=David R.|last2=Sarajedini|first2=Ata|year=1999|issue=1|doi=10.1086/306655|s2cid=18834541}}

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 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.{{Cite journal | last1 = Sackmann | first1 = I. -J. | last2 = Boothroyd | first2 = A. I. | last3 = Kraemer | first3 = K. E. | title = Our Sun. III. Present and Future | doi = 10.1086/173407 | journal = The Astrophysical Journal | volume = 418 | pages = 457 | year = 1993 |bibcode = 1993ApJ...418..457S | doi-access = free }} The helium fusion results in the build-up of a carbon–oxygen core. A star below about {{Solar mass|8}} will never start fusion in its degenerate carbon–oxygen core. Instead, at the end of the asymptotic-giant-branch phase 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 the star has about 0.2 to {{Solar mass|0.5}}, red dwarfs earlier than about M5V, 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 a white dwarf.

Very-low-mass stars are fully convective{{cite journal |bibcode=2009A&A...496..787R |arxiv=0901.1659 |title=On the magnetic topology of partially and fully convective stars |journal=Astronomy and Astrophysics |volume=496 |issue=3 |pages=787 |last1=Reiners |first1=Ansgar |last2=Basri |first2=Gibor |year=2009 |doi=10.1051/0004-6361:200811450 |s2cid=15159121 }}{{cite web |last=Brainerd |first=Jerome James |date=2005-02-16 |title=Main-Sequence Stars |url=http://www.astrophysicsspectator.com/topics/stars/MainSequence.html |archive-url=https://web.archive.org/web/20061206065847/http://www.astrophysicsspectator.com/topics/stars/MainSequence.html |archive-date=2006-12-06 |access-date=2006-12-29 |work=Stars |publisher=The Astrophysics Spectator}} and may continue to fuse hydrogen into helium for up to a trillion years{{cite web

| last=Richmond | first=Michael

| title=Late stages of evolution for low-mass stars

| url=http://spiff.rit.edu/classes/phys230/lectures/planneb/planneb.html

| access-date=2006-12-29 }} 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 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 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.{{Cite journal | last1 = Laughlin | first1 = G. | last2 = Bodenheimer | first2 = P. | last3 = Adams | first3 = F. C. | title = The End of the Main Sequence | doi = 10.1086/304125 | journal = The Astrophysical Journal | volume = 482 | pages = 420–432 | year = 1997 | issue = 1 |bibcode = 1997ApJ...482..420L | doi-access = free }}

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 a type II supernova. The most massive stars can become Wolf–Rayet stars without becoming giants or supergiants at all.{{Cite journal |last=Crowther |first=P. A. |date=2007 |title=Physical Properties of Wolf-Rayet Stars |journal=Annual Review of Astronomy and Astrophysics |volume=45 |issue= 1|pages=177–219 |doi=10.1146/annurev.astro.45.051806.110615 |bibcode=2007ARA&A..45..177C|arxiv = astro-ph/0610356 |s2cid=1076292 }}{{cite journal|version=v1|display-authors=4|author1=Georges Meynet|author2=Cyril Georgy|author3=Raphael Hirschi|author4=Andre Maeder|author5=Phil Massey|author6=Norbert Przybilla|author7=Fernanda Nieva|title=Red Supergiants, Luminous Blue Variables and Wolf-Rayet stars: The single massive star perspective |pages=266–278 |volume=80 |issue=39 |journal=Société Royale des Sciences de Liège, Bulletin (Proceedings of the 39th Liège Astrophysical Colloquium) |location=Liège |date=12–16 July 2010 |display-editors=4 |editor=G. Rauw |editor2=M. De Becker |editor3=Y. Nazé |editor4=J.-M. Vreux |editor5=P. Williams|arxiv=1101.5873|bibcode = 2011BSRSL..80..266M }}

{{Update|section|date=April 2015|reason=May be outdated}}

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 {{Solar mass|1}} star along the red-giant branch, it could harbor a habitable zone for several billion years at 2 astronomical units (AU) out to around 100 million years at {{Val|9|u=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 {{Val|7|and|22|u=AU}} for an additional one billion years.{{cite journal

| author=Lopez, Bruno

| author2=Schneider, Jean

| author3=Danchi, William C.

| title=Can Life Develop in the Expanded Habitable Zones around Red Giant Stars?

| journal=The Astrophysical Journal

| date=2005

| volume=627

| issue=2

| pages=974–985

| bibcode=2005ApJ...627..974L

| doi=10.1086/430416|arxiv = astro-ph/0503520

| s2cid=17075384

}} Later studies have refined this scenario, showing how for a {{Solar mass|1}} star 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 {{Solar mass|0.5}} star in equivalent orbits to those of Jupiter and Saturn would be in 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.{{cite journal

| author=Ramirez, Ramses M.

| author2=Kaltenegger, Lisa

| title=Habitable Zones of Post-Main Sequence Stars

| journal=The Astrophysical Journal

| date=2016

| volume=823

| issue=1

| pages=6

| bibcode=2016ApJ...823....6R

| doi=10.3847/0004-637X/823/1/6

|arxiv = 1605.04924 | s2cid=119225201

| doi-access=free

}}

As of 2023, several hundred giant planets have been discovered around giant stars.{{Cite web |title=Planetary Systems |url=https://exoplanetarchive.ipac.caltech.edu/cgi-bin/TblView/nph-tblView?app=ExoTbls&config=PS |access-date=2023-08-10 |website=exoplanetarchive.ipac.caltech.edu}} 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 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 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.{{cite journal|bibcode=2014A&A...566A.113J|arxiv=1406.0884|title=The properties of planets around giant stars|journal=Astronomy & Astrophysics|volume=566|pages=A113|last1=Jones|first1=M. I.|last2=Jenkins|first2=J. S.|last3=Bluhm|first3=P.|last4=Rojo|first4=P.|last5=Melo|first5=C. H. F.|year=2014|doi=10.1051/0004-6361/201323345|s2cid=118396750}} (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 is the nearest M-class giant star at 88 light-years.{{cite journal | display-authors=1 |last1=Ireland | first1=M. J. | last2=Tuthill | first2=P. G. | last3=Bedding | first3=T. R. | last4=Robertson | first4=J. G. | last5=Jacob | first5=A. P. | title=Multiwavelength diameters of nearby Miras and semiregular variables | journal=Monthly Notices of the Royal Astronomical Society | volume=350 | issue=1 | pages=365–374 |date=May 2004 | doi=10.1111/j.1365-2966.2004.07651.x |doi-access=free | bibcode=2004MNRAS.350..365I |arxiv = astro-ph/0402326 |s2cid=15830460 }} The K1.5 red-giant branch star Arcturus is 36 light-years away.{{cite journal|bibcode=2012A&A...548A..55A|arxiv=1210.1160|title=Carbon and oxygen isotopic ratios in Arcturus and Aldebaran. Constraining the parameters for non-convective mixing on the red giant branch|journal=Astronomy & Astrophysics|volume=548|pages=A55|last1=Abia|first1=C.|last2=Palmerini|first2=S.|last3=Busso|first3=M.|last4=Cristallo|first4=S.|year=2012|doi=10.1051/0004-6361/201220148|s2cid=56386673}}

• Aldebaran (α Tauri)
• Arcturus (α Bootis)
• μ Leonis{{Cite journal |last1=Howes |first1=Louise M. |last2=Lindegren |first2=Lennart |last3=Feltzing |first3=Sofia |last4=Church |first4=Ross P. |last5=Bensby |first5=Thomas |date=February 2019 |title=Estimating stellar ages and metallicities from parallaxes and broadband photometry: successes and shortcomings |url=https://www.aanda.org/10.1051/0004-6361/201833280 |journal=Astronomy & Astrophysics |volume=622 |pages=A27 |doi=10.1051/0004-6361/201833280 |issn=0004-6361|arxiv=1804.08321 |bibcode=2019A&A...622A..27H }}
• Gacrux (γ Crucis)

• Pollux (β Geminorum)
• Capella Aa (α Aurigae)
• α Cassiopeiae (Schedar)
• δ Andromedae{{cite journal|bibcode=2000ApJ...539..732A|arxiv=astro-ph/0003329|title=K-Band Calibration of the Red Clump Luminosity|journal=The Astrophysical Journal|volume=539|issue=2|pages=732–741|last1=Alves|first1=David R.|year=2000|doi=10.1086/309278|s2cid=16673121}}

• ρ Persei (Gorgonea Tertia)
• Mira (ο Ceti)
• χ Cygni
• α Herculis (Rasalgethi)

{{Main|End of the Sun}}

The Sun will exit the main sequence in approximately 5 billion years and start to turn into a red giant.{{cite web |author1=Nola Taylor Redd |title=Red Giant Stars: Facts, Definition & the Future of the Sun |url=http://www.space.com/22471-red-giant-stars.html |website=space.com |access-date=20 February 2016}}{{Cite journal |last1=Schröder |first1=K.-P. |last2=Connon Smith |first2=R. |doi=10.1111/j.1365-2966.2008.13022.x |title=Distant future of the Sun and Earth revisited |journal=Monthly Notices of the Royal Astronomical Society |volume=386 |issue=1 |pages=155–163 |year=2008 |doi-access=free |arxiv=0801.4031 |bibcode=2008MNRAS.386..155S|s2cid=10073988 }} As a red giant, the Sun will grow so large (over 200 times its present-day radius: {{approx|215|tilde=y}}{{nbsp}}{{Solar radius}}; {{Approx|{{Val|1|ul=AU}}|tilde=y}}) that it will engulf Mercury, Venus, and likely Earth. It will lose 38% of its mass growing, then will die into a white dwarf.{{cite news |last1=Siegel |first1=Ethan |title=Ask Ethan: Will The Earth Eventually Be Swallowed By The Sun? |url=https://www.forbes.com/sites/startswithabang/2020/02/08/ask-ethan-will-the-earth-eventually-be-swallowed-by-the-sun/ |access-date=12 March 2021 |work=Forbes |date=8 February 2020 |language=en}}

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