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| − | A '''giant star''' is a [[star]] with substantially larger [[radius]] and [[luminosity]] than a [[main sequence]] star of the same [[effective temperature|surface temperature]]. It is therefore placed above the main sequence group in the [[Hertzsprung-Russell diagram]]. A giant star is thought to represent an intermediate state in the evolution of a star. After all the [[hydrogen]] available for [[nuclear fusion]] at the core of a main sequence star has become depleted, the star swells up to a much larger size and becomes a giant star. Stars of greater luminosity than giant stars are classified as [[supergiant]]s and [[hypergiant]]s. | + | A '''giant star''' is a [[star]] with substantially larger [[radius]] and [[luminosity]] than a [[main sequence]] star of the same [[effective temperature|surface temperature]]. It is, therefore, placed above the main sequence group in the [[Hertzsprung-Russell diagram]]. A giant star is thought to represent an intermediate state in the evolution of a star. After all the [[hydrogen]] available for [[nuclear fusion]] at the core of a main sequence star has become depleted, the star swells up to a much larger size and becomes a giant star. |
| + | {{toc}} | ||
| + | Stars of greater luminosity than giant stars are classified as [[supergiant]]s and [[hypergiant]]s. | ||
== Characteristics == | == Characteristics == | ||
| − | + | Typically, giant stars have radii between 10 and 100 [[solar radii]] and luminosities between 10 and 1,000 times that of the [[Sun]].<ref name="oxford">Patrick Moore, "Giant star," in ''Astronomy Encyclopedia'' (New York, NY: Oxford University Press, 2002, ISBN 0195218337).</ref> Stars still more luminous than giants are referred to as [[supergiants]] and [[hypergiants]].<ref name="darlingsg">David Darling, [http://www.daviddarling.info/encyclopedia/S/supergiant.html Supergiant,] The Encyclopedia of Astrobiology, Astronomy, and Spaceflight. Retrieved December 14, 2008.</ref><ref>David Darling, [http://www.daviddarling.info/encyclopedia/H/hypergiant.html Hypergiant,] The Encyclopedia of Astrobiology, Astronomy, and Spaceflight. Retrieved December 14, 2008.</ref> A hot, luminous main sequence star may also be referred to as a giant.<ref name="cambridge">Jacqueline Mitton, "Giant star," in ''Cambridge Dictionary of Astronomy'' (Cambridge, UK: Cambridge University Press, 2001, ISBN 0521800455).</ref> Apart from this, because of their large radii and luminosities, giant stars lie above the main sequence (luminosity class '''V''' in the [[Spectral classification#Yerkes spectral classification|Yerkes spectral classification]]) on the [[Hertzsprung-Russell diagram]] and correspond to luminosity classes '''II''' or '''III.'''<ref name="fof">John Daintith and William Gould (eds.), "Giant," in ''The Facts on File Dictionary of Astronomy,'' 5th ed. (New York, NY: Facts On File, Inc., 2006, ISBN 0816059985).</ref> | |
| − | Typically, giant stars have radii between 10 and 100 [[solar radii]] and luminosities between 10 and 1,000 times that of the [[Sun]].<ref name="oxford">Moore, | ||
==Formation== | ==Formation== | ||
| − | + | A star becomes a giant star after all the [[hydrogen]] available for [[nuclear fusion|fusion]] at its core has been depleted and, as a result, it has left the [[main sequence]].<ref name="fof" /> A star whose initial mass is less than approximately 0.4 [[solar mass]]es will not become a giant star. This is because such stars have their interior thoroughly mixed by [[convection]] and therefore continue fusing hydrogen until it is exhausted throughout the star, at which point they become [[white dwarf]]s, composed chiefly of [[helium]]. This exhaustion, however, is predicted to take significantly longer than the lifetime of the [[Universe]] up to now.<ref name="rln">Michael Richmond, [http://spiff.rit.edu/classes/phys230/lectures/planneb/planneb.html Late stages of evolution for low-mass stars.] Retrieved December 14, 2008.</ref> | |
| − | A star becomes a giant star after all the [[hydrogen]] available for [[nuclear fusion|fusion]] at its core has been depleted and, as a result, it has left the [[main sequence]].<ref name="fof" /> A star whose initial mass is less than approximately 0.4 [[solar mass]]es will not become a giant star. This is because such stars have their interior thoroughly mixed by [[convection]] and therefore continue fusing hydrogen until it is exhausted throughout the star, at which point they become [[white dwarf]]s, composed chiefly of [[helium]]. This exhaustion, however, is predicted to take significantly longer than the lifetime of the [[Universe]] up to now.<ref name="rln">Richmond, | ||
[[Image:Solar-type Red Giant structure.jpg|right|thumb|400px|Internal structure of a Sun-like star and a red giant. ''Image from the [[European Southern Observatory]] (ESO).'']] | [[Image:Solar-type Red Giant structure.jpg|right|thumb|400px|Internal structure of a Sun-like star and a red giant. ''Image from the [[European Southern Observatory]] (ESO).'']] | ||
| − | If a star is more massive than this lower limit, then when it consumes all of the [[hydrogen]] in its core available for [[nuclear fusion|fusion]], the core will begin to contract. Hydrogen now fuses to [[helium]] in a shell around the helium-rich core, and the portion of the star outside the shell expands and cools. During this portion of its [[stellar evolution|evolution]], labeled the [[subgiant]] branch on the [[Hertzsprung-Russell diagram]], the [[luminosity]] of the star remains approximately constant and its [[effective temperature|surface temperature]] decreases. Eventually the star will start to ascend the [[red giant branch]] on the Hertzsprung-Russell diagram. At this point the surface temperature of the star, now typically a [[red giant]], will remain approximately constant as its luminosity and radius increase drastically. The core will continue to contract, raising its temperature.<ref name="evo">Salaris | + | If a star is more massive than this lower limit, then when it consumes all of the [[hydrogen]] in its core available for [[nuclear fusion|fusion]], the core will begin to contract. Hydrogen now fuses to [[helium]] in a shell around the helium-rich core, and the portion of the star outside the shell expands and cools. During this portion of its [[stellar evolution|evolution]], labeled the [[subgiant]] branch on the [[Hertzsprung-Russell diagram]], the [[luminosity]] of the star remains approximately constant and its [[effective temperature|surface temperature]] decreases. Eventually the star will start to ascend the [[red giant branch]] on the Hertzsprung-Russell diagram. At this point the surface temperature of the star, now typically a [[red giant]], will remain approximately constant as its luminosity and radius increase drastically. The core will continue to contract, raising its temperature.<ref name="evo">Maurizio Salaris and Santi Cassisi, ''Evolution of Stars and Stellar Populations'' (Chichester, UK: John Wiley & Sons, Ltd., 2005, ISBN 047009219X).</ref> |
| − | If the star's mass, when on the main sequence, was below approximately 0.5 [[solar mass]]es, it is thought that it will never attain the central temperatures necessary to fuse [[helium]].<ref> | + | If the star's mass, when on the main sequence, was below approximately 0.5 [[solar mass]]es, it is thought that it will never attain the central temperatures necessary to fuse [[helium]].<ref>S.O. Kepler and P.A. Bradley, [http://adsabs.harvard.edu/abs/1995BaltA...4..166K Structure and Evolution of White Dwarfs,] ''Baltic Astronomy'' 4: 166–220. Retrieved December 14, 2008.</ref> It will therefore remain a hydrogen-fusing red giant until it eventually becomes a helium white dwarf.<ref name="evo" /> Otherwise, when the core temperature reaches approximately 10<sup>8</sup> K, helium will begin to fuse to [[carbon]] and [[oxygen]] in the core by the [[triple-alpha process]].<ref name="evo" /> The energy generated by helium fusion causes the core to expand. This causes the pressure in the surrounding hydrogen-burning shell to decrease, which reduces its energy-generation rate. The luminosity of the star decreases, its outer envelope contracts again, and the star leaves the [[red giant branch]].<ref name="psu">Robin Ciardullo, [http://www.astro.psu.edu/users/rbc/a534/lec23.pdf Giants and Post-Giants.] Retrieved December 14, 2008.</ref> |
| − | Its subsequent evolution will depend on its mass. If not very massive, it may be found in the [[horizontal branch]] on the Hertzsprung-Russell diagram, or its position in the diagram may move in loops.<ref name="evo" / | + | Its subsequent evolution will depend on its mass. If not very massive, it may be found in the [[horizontal branch]] on the Hertzsprung-Russell diagram, or its position in the diagram may move in loops.<ref name="evo" /> If the star is not heavier than approximately 8 solar masses, it will eventually exhaust the helium at its core and begin to fuse helium in a shell around the core. It will then increase in luminosity again as, now an [[AGB star]], it ascends the [[asymptotic giant branch]] of the Hertzsprung-Russell diagram. After the star sheds most of its mass, its core will remain as a carbon-oxygen [[white dwarf]].<ref name="evo" /> |
| − | For main-sequence stars with masses great enough to eventually fuse [[carbon]] (approximately 8 [[solar mass]]es)<ref name="evo" /> | + | For main-sequence stars with masses great enough to eventually fuse [[carbon]] (approximately 8 [[solar mass]]es)<ref name="evo" /> this picture must be modified in many ways. These stars do not increase greatly in luminosity after leaving the main sequence, but they will become redder. They may become [[red supergiant]]s, or [[Stellar mass loss|mass loss]] may cause them to become [[blue supergiant]]s.<ref>T.W. Hartquist, J.E. Dyson, and D. P. Ruffle, ''Blowing Bubbles in the Cosmos: Astronomical Winds, Jets, and Explosions'' (New York, NY: Oxford University Press, 2004, ISBN 0195130545).</ref><ref name="darlingsg" /> Eventually, they will become [[white dwarf]]s composed of [[oxygen]] and [[neon]], or will undergo a [[core-collapse supernova]] to form [[neutron star]]s, or [[black hole]]s.<ref name="evo" /> |
==Examples== | ==Examples== | ||
| − | |||
Well-known giant stars of various colors include the following: | Well-known giant stars of various colors include the following: | ||
| − | * '''[[Alcyone (star)|Alcyone]]''' (η Tauri), a blue-white (B-type) giant,<ref>[http://simbad.u-strasbg.fr/simbad/sim-id?Ident=name%20alcyone Alcyone] | + | * '''[[Alcyone (star)|Alcyone]]''' (η Tauri), a blue-white (B-type) giant,<ref>Simbad, [http://simbad.u-strasbg.fr/simbad/sim-id?Ident=name%20alcyone Alcyone.] Retrieved December 14, 2008.</ref> the brightest star in the [[Pleiades (star cluster)|Pleiades]].<ref>Jim Kaler, [http://www.astro.uiuc.edu/~kaler/sow/alcyone.html Alcyone,] STARS. Retrieved December 14, 2008.</ref> |
| − | * '''[[Thuban]]''' (α Draconis), a white (A-type) giant.<ref>[http://simbad.u-strasbg.fr/simbad/sim-id?Ident=name%20thuban Thuban] | + | * '''[[Thuban]]''' (α Draconis), a white (A-type) giant.<ref>SIMBAD, [http://simbad.u-strasbg.fr/simbad/sim-id?Ident=name%20thuban Thuban.] Retrieved December 14, 2008.</ref> |
| − | * '''[[Sigma Octantis|σ Octantis]]''' | + | * '''[[Sigma Octantis|σ Octantis]],''' a yellow-white (F-type) giant.<ref>SIMBAD, [http://simbad.u-strasbg.fr/simbad/sim-id?Ident=sigma%20octantis Sigma Octantis.] Retrieved December 14, 2008.</ref> |
| − | * '''[[Capella (star)|α Aurigae Aa]]''' | + | * '''[[Capella (star)|α Aurigae Aa]],''' a yellow (G-type) giant, one of the stars making up Capella.<ref>SIMBAD, [http://simbad.u-strasbg.fr/simbad/sim-id?Ident=alf%20Aur%20A α Aurigae Aa.] Retrieved December 14, 2008.</ref> |
| − | * '''[[Pollux (star)|Pollux]]''' (β Geminorum), an orange (K-type) giant.<ref>[http://simbad.u-strasbg.fr/simbad/sim-id?Ident=name%20pollux Pollux] | + | * '''[[Pollux (star)|Pollux]]''' (β Geminorum), an orange (K-type) giant.<ref>SIMBAD, [http://simbad.u-strasbg.fr/simbad/sim-id?Ident=name%20pollux Pollux.] Retrieved December 14, 2008.</ref> |
| − | * '''[[Mira]]''' (ο Ceti), a red (M-type) giant.<ref>[http://simbad.u-strasbg.fr/simbad/sim-id?Ident=name%20mira Mira] | + | * '''[[Mira]]''' (ο Ceti), a red (M-type) giant.<ref>SIMBAD, [http://simbad.u-strasbg.fr/simbad/sim-id?Ident=name%20mira Mira.] Retrieved December 14, 2008.</ref> |
== See also == | == See also == | ||
| − | |||
* [[Hertzsprung-Russell diagram]] | * [[Hertzsprung-Russell diagram]] | ||
* [[Hypergiant]] | * [[Hypergiant]] | ||
| Line 45: | Line 43: | ||
==References== | ==References== | ||
| − | + | * Daintith, John, and William Gould (eds.). ''The Facts on File Dictionary of Astronomy,'' 5th ed. New York, NY: Facts On File, Inc., 2006. ISBN 0816059985. | |
| − | * Daintith, John, and William Gould eds. | + | * Hartquist, T.W., J.E. Dyson, and D. P. Ruffle. ''Blowing Bubbles in the Cosmos: Astronomical Winds, Jets, and Explosions''. New York, NY: Oxford University Press, 2004. ISBN 0195130545. |
| − | * Hartquist, T.W., J.E. Dyson, and D. P. Ruffle | + | * Mitton, Jacqueline. ''Cambridge Dictionary of Astronomy''. Cambridge, UK: Cambridge University Press, 2001. ISBN 0521800455. |
| − | * Mitton, Jacqueline | + | * Moore, Patrick (ed.). ''Astronomy Encyclopedia''. New York, NY: Oxford University Press, 2002. ISBN 0195218337. |
| − | * Moore, Patrick ed. | + | * Salaris, Maurizio, and Santi Cassisi. ''Evolution of Stars and Stellar Populations''. Chichester, UK: John Wiley & Sons, Ltd., 2005. ISBN 047009219X. |
| − | * Salaris, Maurizio, and Santi Cassisi | ||
== External links == | == External links == | ||
| + | All links retrieved June 21, 2017. | ||
| + | * [http://genesismission.jpl.nasa.gov/gm2/mission/pdf/Giantstars.pdf Giant Stars]. Genesis Fact Sheet. | ||
| + | * [http://www.giant-stars.de/ Interactive] giant star comparison. | ||
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[[Category:Physical sciences]] | [[Category:Physical sciences]] | ||
Latest revision as of 07:44, 24 January 2023
A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature. It is, therefore, placed above the main sequence group in the Hertzsprung-Russell diagram. A giant star is thought to represent an intermediate state in the evolution of a star. After all the hydrogen available for nuclear fusion at the core of a main sequence star has become depleted, the star swells up to a much larger size and becomes a giant star.
Stars of greater luminosity than giant stars are classified as supergiants and hypergiants.
Characteristics
Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun.[1] Stars still more luminous than giants are referred to as supergiants and hypergiants.[2][3] A hot, luminous main sequence star may also be referred to as a giant.[4] Apart from this, because of their large radii and luminosities, giant stars lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsprung-Russell diagram and correspond to luminosity classes II or III.[5]
Formation
A star becomes a giant star after all the hydrogen available for fusion at its core has been depleted and, as a result, it has left the main sequence.[5] A star whose initial mass is less than approximately 0.4 solar masses will not become a giant star. This is because such stars have their interior thoroughly mixed by convection and therefore continue fusing hydrogen until it is exhausted throughout the star, at which point they become white dwarfs, composed chiefly of helium. This exhaustion, however, is predicted to take significantly longer than the lifetime of the Universe up to now.[6]
If a star is more massive than this lower limit, then when it consumes all of the hydrogen in its core available for fusion, the core will begin to contract. Hydrogen now fuses to helium in a shell around the helium-rich core, and the portion of the star outside the shell expands and cools. During this portion of its evolution, labeled the subgiant branch on the Hertzsprung-Russell diagram, the luminosity of the star remains approximately constant and its surface temperature decreases. Eventually the star will start to ascend the red giant branch on the Hertzsprung-Russell diagram. At this point the surface temperature of the star, now typically a red giant, will remain approximately constant as its luminosity and radius increase drastically. The core will continue to contract, raising its temperature.[7]
If the star's mass, when on the main sequence, was below approximately 0.5 solar masses, it is thought that it will never attain the central temperatures necessary to fuse helium.[8] It will therefore remain a hydrogen-fusing red giant until it eventually becomes a helium white dwarf.[7] Otherwise, when the core temperature reaches approximately 108 K, helium will begin to fuse to carbon and oxygen in the core by the triple-alpha process.[7] The energy generated by helium fusion causes the core to expand. This causes the pressure in the surrounding hydrogen-burning shell to decrease, which reduces its energy-generation rate. The luminosity of the star decreases, its outer envelope contracts again, and the star leaves the red giant branch.[9]
Its subsequent evolution will depend on its mass. If not very massive, it may be found in the horizontal branch on the Hertzsprung-Russell diagram, or its position in the diagram may move in loops.[7] If the star is not heavier than approximately 8 solar masses, it will eventually exhaust the helium at its core and begin to fuse helium in a shell around the core. It will then increase in luminosity again as, now an AGB star, it ascends the asymptotic giant branch of the Hertzsprung-Russell diagram. After the star sheds most of its mass, its core will remain as a carbon-oxygen white dwarf.[7]
For main-sequence stars with masses great enough to eventually fuse carbon (approximately 8 solar masses)[7] this picture must be modified in many ways. These stars do not increase greatly in luminosity after leaving the main sequence, but they will become redder. They may become red supergiants, or mass loss may cause them to become blue supergiants.[10][2] Eventually, they will become white dwarfs composed of oxygen and neon, or will undergo a core-collapse supernova to form neutron stars, or black holes.[7]
Examples
Well-known giant stars of various colors include the following:
- Alcyone (η Tauri), a blue-white (B-type) giant,[11] the brightest star in the Pleiades.[12]
- Thuban (α Draconis), a white (A-type) giant.[13]
- σ Octantis, a yellow-white (F-type) giant.[14]
- α Aurigae Aa, a yellow (G-type) giant, one of the stars making up Capella.[15]
- Pollux (β Geminorum), an orange (K-type) giant.[16]
- Mira (ο Ceti), a red (M-type) giant.[17]
See also
- Hertzsprung-Russell diagram
- Hypergiant
- Main sequence
- Nuclear fusion
- Red giant
- Star
- Supergiant
Notes
- ↑ Patrick Moore, "Giant star," in Astronomy Encyclopedia (New York, NY: Oxford University Press, 2002, ISBN 0195218337).
- ↑ 2.0 2.1 David Darling, Supergiant, The Encyclopedia of Astrobiology, Astronomy, and Spaceflight. Retrieved December 14, 2008.
- ↑ David Darling, Hypergiant, The Encyclopedia of Astrobiology, Astronomy, and Spaceflight. Retrieved December 14, 2008.
- ↑ Jacqueline Mitton, "Giant star," in Cambridge Dictionary of Astronomy (Cambridge, UK: Cambridge University Press, 2001, ISBN 0521800455).
- ↑ 5.0 5.1 John Daintith and William Gould (eds.), "Giant," in The Facts on File Dictionary of Astronomy, 5th ed. (New York, NY: Facts On File, Inc., 2006, ISBN 0816059985).
- ↑ Michael Richmond, Late stages of evolution for low-mass stars. Retrieved December 14, 2008.
- ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 Maurizio Salaris and Santi Cassisi, Evolution of Stars and Stellar Populations (Chichester, UK: John Wiley & Sons, Ltd., 2005, ISBN 047009219X).
- ↑ S.O. Kepler and P.A. Bradley, Structure and Evolution of White Dwarfs, Baltic Astronomy 4: 166–220. Retrieved December 14, 2008.
- ↑ Robin Ciardullo, Giants and Post-Giants. Retrieved December 14, 2008.
- ↑ T.W. Hartquist, J.E. Dyson, and D. P. Ruffle, Blowing Bubbles in the Cosmos: Astronomical Winds, Jets, and Explosions (New York, NY: Oxford University Press, 2004, ISBN 0195130545).
- ↑ Simbad, Alcyone. Retrieved December 14, 2008.
- ↑ Jim Kaler, Alcyone, STARS. Retrieved December 14, 2008.
- ↑ SIMBAD, Thuban. Retrieved December 14, 2008.
- ↑ SIMBAD, Sigma Octantis. Retrieved December 14, 2008.
- ↑ SIMBAD, α Aurigae Aa. Retrieved December 14, 2008.
- ↑ SIMBAD, Pollux. Retrieved December 14, 2008.
- ↑ SIMBAD, Mira. Retrieved December 14, 2008.
ReferencesISBN links support NWE through referral fees
- Daintith, John, and William Gould (eds.). The Facts on File Dictionary of Astronomy, 5th ed. New York, NY: Facts On File, Inc., 2006. ISBN 0816059985.
- Hartquist, T.W., J.E. Dyson, and D. P. Ruffle. Blowing Bubbles in the Cosmos: Astronomical Winds, Jets, and Explosions. New York, NY: Oxford University Press, 2004. ISBN 0195130545.
- Mitton, Jacqueline. Cambridge Dictionary of Astronomy. Cambridge, UK: Cambridge University Press, 2001. ISBN 0521800455.
- Moore, Patrick (ed.). Astronomy Encyclopedia. New York, NY: Oxford University Press, 2002. ISBN 0195218337.
- Salaris, Maurizio, and Santi Cassisi. Evolution of Stars and Stellar Populations. Chichester, UK: John Wiley & Sons, Ltd., 2005. ISBN 047009219X.
External links
All links retrieved June 21, 2017.
- Giant Stars. Genesis Fact Sheet.
- Interactive giant star comparison.
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