--- title: Red giant summary: A star in a late stage of life that has run out of hydrogen in its core and swollen to tens or hundreds of times the Sun's size, with a cool, orange-red surface. Stars from about 0.8 to 8 solar masses pass through this stage; the Sun will, in about seven and a half billion years. science_status: [observed, model, sim] categories: [Stars, Giant stars, Stellar evolution] aliases: [Red giants, Red giant star, Red giant branch, RGB, Red clump, Red-clump star, Asymptotic giant branch, AGB star, K giant, M giant, Orange giant, RGI, OGI, Helium flash, Tip of the red giant branch] infobox: type: star_class name: Red giant image: File:Pi1_Gruis_surface_ESO_eso1741a.jpg caption: "Observation: the surface of the red giant pi1 Gruis, showing a few huge convection cells, imaged with ESO's Very Large Telescope Interferometer. Credit: ESO" code: "RGI (M giants), OGI (K giants), YGI (G giants)" stellar_class_key: "red_giant, orange_giant, yellow_giant" spectral_types_covered: "Mostly late G, K and M" luminosity_class: "III (giants); II (bright giants)" effective_temperature: {value: "about 3,000 to 5,000", unit: K, source: observed} mass: {value: "about 0.8 to 8 (initial)", unit: M_Sun, source: model} radius: {value: "about 10 to a few hundred", unit: R_Sun, source: observed} luminosity: {value: "tens to several thousand", unit: L_Sun, source: observed} main_sequence_lifetime: {value: "red giant branch lasts a few hundred million years for a Sun-like star; core helium burning about 100 million", unit: yr, source: model} perceived_colour: "Orange to orange-red" activity: "Pulsations and huge convection cells; strong mass loss on the asymptotic giant branch" share_of_stars: "None within 10 pc; far more common in catalogues of visible stars because they are bright" planet_occurrence: "Planets known around many giants; close-in planets are engulfed as the star grows" subtypes: "Red giant branch, red clump (core helium burning), asymptotic giant branch, Mira variables. Sim: RGI and OGI subtypes for the giant branch, clump, AGB and Miras" real_examples: "Arcturus (K1.5III), Aldebaran (K5+III), Pollux (K0IIIb), Gacrux (M3.5III), R Doradus, pi1 Gruis" physics_engine: "Red giant and orange giant physics engines and subtype classifiers" science_doc: "The sim's giant star science references" last_verified: "2026-09-27, writer B" sim: entity: stellar_class.red_giant refs: - id: ss08 type: article-journal author: [{family: Schröder, given: K.-P.}, {family: Connon Smith, given: Robert}] title: "Distant future of the Sun and Earth revisited" container-title: Monthly Notices of the Royal Astronomical Society volume: 386 page: 155-163 issued: 2008 DOI: 10.1111/j.1365-2966.2008.13022.x - id: sackmann93 type: article-journal author: [{family: Sackmann, given: I.-Juliana}, {family: Boothroyd, given: Arnold I.}, {family: Kraemer, given: Kathleen E.}] title: "Our Sun. III. Present and Future" container-title: The Astrophysical Journal volume: 418 page: 457 issued: 1993 DOI: 10.1086/173407 - id: bildsten12 type: article-journal author: [{family: Bildsten, given: Lars}, {family: Paxton, given: Bill}, {family: Moore, given: Kevin}, {family: Macias, given: Phillip J.}] title: "Acoustic Signatures of the Helium Core Flash" container-title: The Astrophysical Journal Letters volume: 744 page: L6 issued: 2012 DOI: 10.1088/2041-8205/744/1/L6 - id: girardi16 type: article-journal author: [{family: Girardi, given: Léo}] title: "Red Clump Stars" container-title: Annual Review of Astronomy and Astrophysics volume: 54 page: 95-133 issued: 2016 DOI: 10.1146/annurev-astro-081915-023354 - id: hawkins17 type: article-journal author: [{family: Hawkins, given: Keith}, {family: Leistedt, given: Boris}, {family: Bovy, given: Jo}, {family: Hogg, given: David W.}] title: "Red clump stars and Gaia: calibration of the standard candle using a hierarchical probabilistic model" container-title: Monthly Notices of the Royal Astronomical Society volume: 471 page: 722-729 issued: 2017 DOI: 10.1093/mnras/stx1655 - id: freedman19 type: article-journal author: [{family: Freedman, given: Wendy L.}, {family: Madore, given: Barry F.}, {family: Hatt, given: Dylan}, {literal: "et al."}] title: "The Carnegie-Chicago Hubble Program. VIII. An Independent Determination of the Hubble Constant Based on the Tip of the Red Giant Branch" container-title: The Astrophysical Journal volume: 882 page: 34 issued: 2019 DOI: 10.3847/1538-4357/ab2f73 - id: bedding11 type: article-journal author: [{family: Bedding, given: Timothy R.}, {family: Mosser, given: Benoit}, {family: Huber, given: Daniel}, {literal: "et al."}] title: "Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars" container-title: Nature volume: 471 page: 608-611 issued: 2011 DOI: 10.1038/nature09935 - id: herwig05 type: article-journal author: [{family: Herwig, given: Falk}] title: "Evolution of Asymptotic Giant Branch Stars" container-title: Annual Review of Astronomy and Astrophysics volume: 43 page: 435-479 issued: 2005 DOI: 10.1146/annurev.astro.43.072103.150600 - id: kalirai08 type: article-journal author: [{family: Kalirai, given: Jasonjot S.}, {family: Hansen, given: Brad M. S.}, {family: Kelson, given: Daniel D.}, {literal: "et al."}] title: "The Initial-Final Mass Relation: Direct Constraints at the Low-Mass End" container-title: The Astrophysical Journal volume: 676 page: 594-609 issued: 2008 DOI: 10.1086/527028 - id: paladini18 type: article-journal author: [{family: Paladini, given: C.}, {family: Baron, given: F.}, {family: Jorissen, given: A.}, {literal: "et al."}] title: "Large granulation cells on the surface of the giant star pi1 Gruis" container-title: Nature volume: 553 page: 310-312 issued: 2018 DOI: 10.1038/nature25001 - id: vlemmings24 type: article-journal author: [{family: Vlemmings, given: Wouter}, {family: Khouri, given: Theo}, {family: Bojnordi Arbab, given: Behzad}, {literal: "et al."}] title: "One month convection timescale on the surface of a giant evolved star" container-title: Nature volume: 633 page: 323-326 issued: 2024 DOI: 10.1038/s41586-024-07836-9 - id: de23 type: article-journal author: [{family: De, given: Kishalay}, {family: MacLeod, given: Morgan}, {family: Karambelkar, given: Viraj}, {literal: "et al."}] title: "An infrared transient from a star engulfing a planet" container-title: Nature volume: 617 page: 55-60 issued: 2023 DOI: 10.1038/s41586-023-05842-x - id: hon23 type: article-journal author: [{family: Hon, given: Marc}, {family: Huber, given: Daniel}, {family: Rui, given: Nicholas Z.}, {literal: "et al."}] title: "A close-in giant planet escapes engulfment by its star" container-title: Nature volume: 618 page: 917-920 issued: 2023 DOI: 10.1038/s41586-023-06029-0 - id: rap11 type: article-journal author: [{family: Ramírez, given: I.}, {family: Allende Prieto, given: C.}] title: "Fundamental Parameters and Chemical Composition of Arcturus" container-title: The Astrophysical Journal volume: 743 page: 135 issued: 2011 DOI: 10.1088/0004-637X/743/2/135 - id: reffert06 type: article-journal author: [{family: Reffert, given: Sabine}, {family: Quirrenbach, given: Andreas}, {family: Mitchell, given: David S.}, {literal: "et al."}] title: "Precise Radial Velocities of Giant Stars. II. Pollux and Its Planetary Companion" container-title: The Astrophysical Journal volume: 652 page: 661-665 issued: 2006 DOI: 10.1086/507516 - id: hatzes06 type: article-journal author: [{family: Hatzes, given: A. P.}, {family: Cochran, given: W. D.}, {family: Endl, given: M.}, {literal: "et al."}] title: "Confirmation of the planet hypothesis for the long-period radial velocity variations of beta Geminorum" container-title: Astronomy & Astrophysics volume: 457 page: 335-341 issued: 2006 DOI: 10.1051/0004-6361:20065445 - id: hatzes15 type: article-journal author: [{family: Hatzes, given: A. P.}, {family: Cochran, given: W. D.}, {family: Endl, given: M.}, {literal: "et al."}] title: "Long-lived, long-period radial velocity variations in Aldebaran: A planetary companion and stellar activity" container-title: Astronomy & Astrophysics volume: 580 page: A31 issued: 2015 DOI: 10.1051/0004-6361/201425519 - id: reichert19 type: article-journal author: [{family: Reichert, given: Katja}, {family: Reffert, given: Sabine}, {family: Stock, given: Stephan}, {literal: "et al."}] title: "Precise radial velocities of giant stars. XII. Evidence against the proposed planet Aldebaran b" container-title: Astronomy & Astrophysics volume: 625 page: A22 issued: 2019 DOI: 10.1051/0004-6361/201834028 - id: reyle21 type: article-journal author: [{family: Reylé, given: C.}, {family: Jardine, given: K.}, {family: Fouqué, given: P.}, {literal: "et al."}] title: "The 10 parsec sample in the Gaia era" container-title: Astronomy & Astrophysics volume: 650 page: A201 issued: 2021 DOI: 10.1051/0004-6361/202140985 images_wanted: - file: File:Pi1_Gruis_surface_ESO_eso1741a.jpg subject: "Interferometric image of the red giant pi1 Gruis: a mottled disc with a few very large bright and dark convection cells" source: eso page_url: https://www.eso.org/public/images/eso1741a/ credit: "ESO" licence: "CC BY 4.0 (ESO usage terms)" - file: File:R_Doradus_surface_ALMA_eso2412a.jpg subject: "ALMA images of the surface of the giant star R Doradus showing convective bubbles changing over weeks" source: eso page_url: https://www.eso.org/public/images/eso2412a/ credit: "ALMA (ESO/NAOJ/NRAO)/W. Vlemmings et al." licence: "CC BY 4.0 (ESO usage terms)" - file: File:Red_giant_evolution_HR_diagram.svg subject: "Diagram: a 1 solar-mass track on the Hertzsprung-Russell diagram from the main sequence through the subgiant branch, up the red giant branch to the helium flash, down to the red clump, up the asymptotic giant branch, and across to a white dwarf, with ages marked" source: other note: "shot list: to be drawn as an SVG; stage values from Schröder and Smith (2008)" --- A **red giant** is a star near the end of its life that has used up the hydrogen in its core and swollen to tens or hundreds of times its former size. Its outer layers, spread over so large a surface, cool to about 3,000 to 5,000 K, which gives the star an orange-red colour and a spectral type of late G, K or M with luminosity class III. Every star born with between roughly 0.8 and 8 times the Sun's mass passes through the red giant stages, and the Sun will begin to in about five and a half billion years, reaching the peak of the red giant branch about seven and a half billion years from now :cite[ss08]. Red giants are rare near the Sun (there are none within 33 light years :cite[reyle21]), but they are so bright that they are among the most familiar stars in the night sky: Arcturus, Aldebaran and Pollux are all red giants. ## How a star becomes a red giant A main-sequence star fuses hydrogen into helium in its core. When the core's hydrogen runs out, fusion continues in a shell around an inert helium core. The core contracts and heats, the shell burns faster, and the star's envelope expands and cools: the star leaves the main sequence, crosses the subgiant branch and climbs the **red giant branch** of the Hertzsprung-Russell diagram, growing brighter and larger as its core grows. In stars below about two solar masses the helium core becomes so dense that it is supported by electron degeneracy pressure, the resistance of electrons to being packed together. When the core reaches about 0.48 solar masses its temperature reaches the point at which helium fuses to carbon, and because a degenerate core cannot expand to cool itself, helium ignites in a runaway **helium flash**. The flash begins off-centre and proceeds in a series of subflashes that lift the degeneracy over about two million years :cite[bildsten12]. Heavier stars ignite helium gently. The brightest point a star reaches on the red giant branch, the tip, is almost the same for all old, low-mass stars, because it is set by the core mass at ignition. That makes the **tip of the red giant branch** a standard candle: calibrated at an absolute I-band magnitude of -4.05, it is used to measure distances to nearby galaxies and from them the expansion rate of the universe :cite[freedman19]. ## The red clump After the flash, a star settles down to burn helium in its core and hydrogen in a shell, at a much smaller size and a nearly fixed luminosity. Stars in this phase gather in a compact **red clump** on the Hertzsprung-Russell diagram, the sharpest feature of the diagram of nearby stars and a widely used tool for measuring distances and dust :cite[girardi16]. Gaia calibrates the clump at an absolute magnitude of -1.61 in the near-infrared K band and +0.44 in Gaia's own G band, with a scatter of about 0.17 magnitudes :cite[hawkins17]. A red clump star and a star still climbing the red giant branch can look identical from outside. Starquakes tell them apart. Oscillations measured by the Kepler spacecraft include gravity modes that probe the core: the spacing between their periods is about 50 seconds in stars burning hydrogen in a shell and about 100 to 300 seconds in stars burning helium in the core :cite[bedding11]. ## The asymptotic giant branch and the end When core helium runs out, the star has a carbon-oxygen core surrounded by helium- and hydrogen-burning shells, and it swells again, up the **asymptotic giant branch** (AGB). AGB stars pulsate, suffer periodic thermal pulses as the helium shell ignites in flashes, make heavy elements by slow neutron capture, and shed their envelopes in dense, dusty winds :cite[herwig05]. The exposed core briefly lights up the ejected gas as a planetary nebula and then cools as a [[White dwarf|white dwarf]]. Star clusters show how much mass is lost: a star born with 1.6 solar masses leaves a white dwarf of about 0.54 :cite[kalirai08]. The surfaces of these stars are dominated by a few enormous convection cells. Interferometric images of the giant pi1 Gruis show cells about a quarter of the star's diameter across :cite[paladini18], and radio images of R Doradus from the ALMA array show surface structures about 0.7 AU across that change in about a month :cite[vlemmings24]. ::figure{src="File:R_Doradus_surface_ALMA_eso2412a.jpg" size=wide alt="A sequence of images of a star's disc with bright and dark patches that shift from frame to frame" caption="Observation: the surface of the giant star R Doradus imaged by ALMA over several weeks, its convective bubbles rising and sinking. Credit: ALMA (ESO/NAOJ/NRAO)/W. Vlemmings et al."} ## The Sun as a red giant The Sun will leave the main sequence about 5.4 billion years from now. In the model of Schröder and Smith (2008) it climbs the red giant branch to a peak of 2,730 times its present luminosity and 256 times its present radius, 1.2 AU, while losing a third of its mass in its wind; after the helium flash it shrinks to about 11 solar radii, and on the asymptotic giant branch it grows again to between 150 and 180 solar radii :cite[ss08]. As the Sun loses mass its planets drift outward, but in this model not far enough to save Earth, which is engulfed at the red giant tip. An earlier model, with less mass loss, stops the Sun at 170 solar radii, engulfing Mercury but sparing Venus and Earth :cite[sackmann93]. The difference shows how sensitive the outcome is to the red giant's wind, which is still the least certain part of the physics. ## Planets around red giants Planets are common around giants, and astronomers have now watched one being swallowed. In 2020 the Zwicky Transient Facility caught an outburst, bright in the infrared, from a Sun-like star swallowing a planet of up to about ten Jupiter masses :cite[de23]. Others survive: 8 Ursae Minoris b orbits a helium-burning giant at 0.5 AU, inside the 0.7 AU radius the star should have reached earlier as a red giant, which suggests that the star's history was not that of a single star, perhaps a merger :cite[hon23]. Pollux, the nearest giant to the Sun, has a planet of at least 2.9 Jupiter masses on a 590-day orbit :cite[reffert06] :cite[hatzes06]. Planet claims around giants need care, because the stars' own pulsations and spots can mimic a planet: a proposed planet around Aldebaran :cite[hatzes15] is contradicted by later data, in which its 620-day signal is absent before about 2006 :cite[reichert19]. ## Notable examples | Star | Type | Distance | Notes | |---|---|---|---| | Pollux | K0IIIb | 10.4 pc | Nearest giant; planet on a 590-day orbit :cite[reffert06] | | Arcturus | K1.5III | 11.3 pc | 4,286 K, 25.4 solar radii, 1.08 solar masses, about 7 billion years old; a metal-poor star of the thick disc :cite[rap11] | | Aldebaran | K5+III | 20.4 pc | Proposed planet disputed :cite[hatzes15] :cite[reichert19] | | Gacrux | M3.5III | 27.2 pc | The red star at the top of the Southern Cross | | pi1 Gruis | S-type AGB star | | Giant convection cells imaged :cite[paladini18] | | R Doradus | M-type AGB star | | Surface convection filmed over weeks :cite[vlemmings24] | :::callout{type=sim title="In Pax Abyssi"} The sim names giants by the colour of their spectral letter, so K giants such as Arcturus and Aldebaran are "orange giants" (code OGI) and M giants such as Gacrux are "red giants" (RGI), with G giants as "yellow giants". Their subtypes follow the stages on this page: the lower and upper red giant branch, the red clump, the asymptotic giant branch, and pulsating Mira variables. The game's catalogue, which favours stars bright enough to see, holds about 24,600 K giants and 2,900 M giants, most of the evolved stars in the sky. At the end of their lives the sim turns stars below eight solar masses into white dwarfs. ::: ## See also - [[Stellar classification]] - [[G-type main-sequence star]] - [[K-type main-sequence star]] - [[Supergiant]] - [[White dwarf]] - [[Sol]]