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Core of a red giant isn't always degenerate. "red giant branch" and "asymptotic branch aren't normally capitalized. H->He fusion may or may not be p-p chain. copy-edit.
→‎Characteristics and lifespan: rewrite section with hopefully-better-organized summary of red giant stellar evolution
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Prominent bright red giants in the night sky include [[Aldebaran]] (Alpha Tauri), [[Arcturus]] (Alpha Bootis), and [[Gamma Crucis]] (Gacrux), while the even larger [[Antares]] (Alpha Scorpii) and [[Betelgeuse]] (Alpha Orionis) are [[red supergiant]]s.
Prominent bright red giants in the night sky include [[Aldebaran]] (Alpha Tauri), [[Arcturus]] (Alpha Bootis), and [[Gamma Crucis]] (Gacrux), while the even larger [[Antares]] (Alpha Scorpii) and [[Betelgeuse]] (Alpha Orionis) are [[red supergiant]]s.


==Characteristics and lifespan==
==Characteristics==

[[Image:Sun red giant.svg|thumb|300px|right|The size of the current Sun (now in the [[main sequence]]) compared to its estimated size during its red giant phase.]]
[[Image:Sun red giant.svg|thumb|300px|right|The size of the current Sun (now in the [[main sequence]]) compared to its estimated size during its red giant phase.]]


Red giants are stars with radii tens to hundreds of times larger than that of the [[Sun]] which have exhausted the supply of [[hydrogen]] in their cores and switched to fusing hydrogen in a shell outside the core. Main sequence stars of [[spectral type]]s A through K are believed to become red giants.<ref>[http://www.vendian.org/mncharity/dir3/starcolor/ Color of star ranging blue through orange]</ref>
⚫
Red giants are evolved from [[main sequence]] stars with masses in the range from about 0.5 solar masses to somewhere between 4 and 6 solar masses <ref>{{cite book

⚫
Actually, such stars are not big red spheres with sharp limbs (when one is close to it) as displayed on many images. Due to the very low density such stars may not have a sharp [[photosphere]] but a star body which gradually transfers into a '[[corona]]'.<ref>[http://www.keckobservatory.org/article.php?id=87 Measurements of the frequency of starspots on red giant stars]</ref><ref>[http://solarscience.msfc.nasa.gov/feature1.shtml orange sphere of the sun]</ref>

==Stellar evolution==
{{main|stellar evolution#Mid-sized stars}}

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Red giants are evolved from [[main sequence]] stars with masses in the range from about 0.5 solar masses to somewhere between 4 and 6 solar masses.<ref>{{cite book
| coauthors = Jean Audouze, Guy Israël, et al.
| coauthors = Jean Audouze, Guy Israël, et al.
| title = The Cambridge Atlas of Astronomy
| title = The Cambridge Atlas of Astronomy
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| year = 1988
| year = 1988
| pages = 255
| pages = 255
| isbn = 0-521-36360-8 }}<!--outdated reference? That number seems a bit smaller than is commonly said nowadays--></ref> When a star initially [[star formation|forms]] from a collapsing [[molecular cloud]] in the [[interstellar medium]], it contains primarily hydrogen and helium, with trace amounts of "[[metallicity|metals]]" (elements with [[atomic number]] > 3, i. e. every element except hydrogen and helium). These elements are all equally mixed throughout the star. The star reaches the main sequence when the core reaches a temperature high enough to begin [[stellar nucleosynthesis|fusing hydrogen]] (a few million Kelvin) and establish [[hydrostatic equilibrium]]. Over its main sequence life, the star slowly converts the hydrogen in the core into helium; its main sequence life ends when nearly all the hydrogen in the core has been exhausted. For the [[Sun]], the main sequence lifetime is approximately 10&nbsp;billion years; the lifetime is shorter for more massive stars and longer for less massive stars.<ref name=zeilik />
| isbn = 0-521-36360-8 }}</ref>.


When the star exhausts the hydrogen fuel in its core, nuclear reactions in the core stop, so the core begins to contract due to its gravity. This heats a shell just outside the core, where hydrogen remains, initiating fusion of hydrogen to helium in the shell. The higher temperatures lead to increasing reaction rates, producing enough energy to increase the star's luminosity by a factor of 1,000&ndash;10,000. The outer layers of the star then expand greatly, beginning the red giant phase of the stars life. Due to the expansion of the outer layers of the star, the energy produced in the core of the star is spread over a much larger surface area, resulting in a lower [[effective temperature|surface temperature]] and a shift in the star's visible light output towards the red — hence ''red giant'', even though the color usually is orange. At this time, the star is said to be ascending the red giant branch of the [[Hertzsprung-Russell diagram| Hertzsprung-Russell (H-R) diagram]].
Red giants are stars with radii tens to hundreds of times larger than that of the [[Sun]] which have exhausted the supply of [[hydrogen]] in their cores and switched to fusing hydrogen in a shell outside the core. Since the inert [[helium]] core has no source of energy of its own, it contracts and heats up, and its gravity compresses the hydrogen in the layer immediately above it, thus causing it to fuse faster. This in turn causes the star to become more luminous (from 1,000 to 10,000 times brighter) and expand; the degree of expansion outstrips the increase in luminosity, thus causing the [[effective temperature]] to decrease. In stars massive enough to ignite [[helium fusion]], an analogous process occurs when central helium is exhausted and the star switches to fusing helium in a shell, although with the additional complication that in many cases hydrogen fusion will continue in a shell at lesser depth — this puts stars onto the [[asymptotic giant branch]].<ref>{{cite journal

The mechanism that ends the collapse of the core and the ascent up the red giant branch depends on the mass of the star. For the Sun and red giants less than 2.57&nbsp;solar masses,{{cn|date=July 2008}} 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 10<sup>8</sup>&nbsp;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 10<sup>8</sup>&nbsp;K before it is dense enough to be degenerate, so helium fusion will begin much more smoothly, with no helium flash. Once the star is fusing helium in its core, it contracts and is no longer considered a red giant.<ref name=zeilik /> 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 instead lie on the so-called [[red clump]] in the H-R diagram.<ref>[http://cfa-www.harvard.edu/~kstanek/RedClump/ Harvard University search for orange-yellow clumps]</ref>

In stars massive enough to ignite [[helium fusion]], an analogous process occurs when central helium is exhausted and the star switches to fusing helium in a shell, although with the additional complication that in many cases hydrogen fusion will continue in a shell at lesser depth. This puts stars onto the [[asymptotic giant branch]], a second red giant phase.<ref>{{cite journal
| last = Sackmann
| last = Sackmann
| first = I.-Juliana
| first = I.-Juliana
| authorlink =
| coauthors = Boothroyd, Arnold I.; Kraemer, Kathleen E.
| coauthors = Boothroyd, Arnold I.; Kraemer, Kathleen E.
| title = Our Sun. III. Present and Future
| title = Our Sun. III. Present and Future
| journal = Astrophysical Journal
| journal = Astrophysical Journal
| volume = 418
| volume = 418
| issue =
| pages = 457
| pages = 457
⚫
| year = 1993 }}</ref><ref>{{cite web
| publisher = The American Astronomical Society
| date = November 1993
| url =
| doi =
| id =
⚫
| accessdate = }}</ref><ref>{{cite web
| last = Pogge
| last = Pogge
| first = Richard W.
| first = Richard W.
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| format =
| format =
| doi =
| doi =
| accessdate = 2006-12-29 }}</ref> More massive stars continue to repeat this cycle, fusing heavier elements in successive phases, each lasting more briefly than the previous.
| accessdate = 2006-12-29 }}</ref> <!-- Should this source [http://www.astro.umd.edu/~miller/teaching/astr606/lecture18.ps] be included? After all, it's a postscript file that majority will not be able to see without installing some software --> The decrease in surface temperature shifts the star's visible light output '''towards''' the red — hence ''red giant'', even though the color usually is orange. Main sequence stars of [[spectral type]]s A through K are believed to become red giants. <ref>[http://www.vendian.org/mncharity/dir3/starcolor/ Color of star ranging blue through orange]</ref>

A solar mass star will never fuse carbon. 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 planetary nebula finally ends the red giant phase of the star's evolution.<ref name=zeilik />

===Stars that do not become red giants===


Very low mass stars are thought to be fully convective<ref>{{cite web
Very low mass stars are thought to be fully convective<ref>{{cite web
Line 72: Line 79:
| format =
| format =
| doi =
| doi =
| accessdate = 2006-12-29 }}</ref> Such stars are commonly referred to as [[red dwarfs]]. The predicted lifespan of these stars is much larger than the current age of the universe, and hence there are no actual observations of these stars aging.
| accessdate = 2006-12-29 }}</ref> Such stars are commonly referred to as [[red dwarfs]]. The predicted lifespan of these stars is much larger than the current age of the universe, and hence there are no actual observations of these stars aging.


Very high mass stars instead develop to [[supergiant]] stars that wander back and forth horizontally over the HR diagram, at the right end constituting [[red supergiant]]s. These usually end their life as type II [[supernova]]e.
Very high mass stars instead develop to [[supergiant]] stars that wander back and forth horizontally over the HR diagram, at the right end constituting [[red supergiant]]s. These usually end their life as type II [[supernova]]e.

If the star is heavier than 0.4 but less than 2.57 solar masses, the addition of [[helium]] to the core by shell hydrogen fusing will cause a [[helium flash]] — a rapid burst of helium fusing in the core, after which the star will commence a brief period of helium fusing before beginning ''another'' ascent of the red giant branch. Stars more massive than 2.5 solar masses, but less than about 4 to 6, enter the helium fusing phase of their lives much more smoothly. 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 [[Hertzsprung-Russell diagram]] of many star clusters. Metal-rich helium-fusing stars do not lie on a horizontal branch, but instead lie in a clump (the [[red clump]]) in the Hertzsprung-Russell diagram.<ref>[http://cfa-www.harvard.edu/~kstanek/RedClump/ Harvard University search for orange-yellow clumps]</ref>

⚫
Actually, such stars are not big red spheres with sharp limbs (when one is close to it) as displayed on many images. Due to the very low density such stars may not have a sharp [[photosphere]] but a star body which gradually transfers into a '[[corona]]'.<ref>[http://www.keckobservatory.org/article.php?id=87 Measurements of the frequency of starspots on red giant stars]</ref><ref>[http://solarscience.msfc.nasa.gov/feature1.shtml orange sphere of the sun]</ref>


==The Sun as a red giant==
==The Sun as a red giant==

Revision as of 15:07, 23 July 2008

A red giant is a luminous giant star of low or intermediate mass (roughly 0.5–10 solar masses) that is in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and 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 the so-called red giant branch stars (RGB stars) whose shells are still fusing hydrogen into helium, while the core is inactive helium. Another case of red giants are the asymptotic giant branch stars (AGB) that produces carbon from helium by the triple-alpha process.[1] To the AGB stars belong the carbon stars of type C-N and late C-R.

Prominent bright red giants in the night sky include Aldebaran (Alpha Tauri), Arcturus (Alpha Bootis), and Gamma Crucis (Gacrux), while the even larger Antares (Alpha Scorpii) and Betelgeuse (Alpha Orionis) are red supergiants.

Characteristics

The size of the current Sun (now in the main sequence) compared to its estimated size during its red giant phase.

Red giants are stars with radii tens to hundreds of times larger than that of the Sun which have exhausted the supply of hydrogen in their cores and switched to fusing hydrogen in a shell outside the core. Main sequence stars of spectral types A through K are believed to become red giants.[2]

Actually, such stars are not big red spheres with sharp limbs (when one is close to it) as displayed on many images. Due to the very low density such stars may not have a sharp photosphere but a star body which gradually transfers into a 'corona'.[3][4]

Stellar evolution

Main article: stellar evolution § Mid-sized stars

Red giants are evolved from main sequence stars with masses in the range from about 0.5 solar masses to somewhere between 4 and 6 solar masses.[5] 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" (elements with atomic number > 3, i. e. every element except hydrogen and helium). These elements are all equally mixed throughout the star. The star reaches the main sequence when the core reaches a temperature high enough to begin fusing hydrogen (a few million Kelvin) and establish hydrostatic equilibrium. Over its main sequence life, the star slowly converts the hydrogen in the core into helium; its main sequence life ends when nearly all the hydrogen in the core has been exhausted. For the Sun, the main sequence lifetime is approximately 10 billion years; the lifetime is shorter for more massive stars and longer for less massive stars.[1]

When the star exhausts the hydrogen fuel in its core, nuclear reactions in the core stop, so the core begins to contract due to its gravity. This heats a shell just outside the core, where hydrogen remains, initiating fusion of hydrogen to helium in the shell. The higher temperatures lead to increasing reaction rates, producing enough energy to increase the star's luminosity by a factor of 1,000–10,000. The outer layers of the star then expand greatly, beginning the red giant phase of the stars life. Due to the expansion of the outer layers of the star, the energy produced in the core of the star is spread over a much larger surface area, resulting in a lower surface temperature and a shift in the star's visible light output towards the red — hence red giant, even though the color usually is orange. At this time, the star is said to be ascending the red giant branch of the Hertzsprung-Russell (H-R) diagram.

The mechanism that ends the collapse of the core and the ascent up the red giant branch depends on the mass of the star. For the Sun and red giants less than 2.57 solar masses,[citation needed] 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 108 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 108 K before it is dense enough to be degenerate, so helium fusion will begin much more smoothly, with no helium flash. Once the star is fusing helium in its core, it contracts and is no longer considered a red giant.[1] 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 instead lie on the so-called red clump in the H-R diagram.[6]

In stars massive enough to ignite helium fusion, an analogous process occurs when central helium is exhausted and the star switches to fusing helium in a shell, although with the additional complication that in many cases hydrogen fusion will continue in a shell at lesser depth. This puts stars onto the asymptotic giant branch, a second red giant phase.[7][8] More massive stars continue to repeat this cycle, fusing heavier elements in successive phases, each lasting more briefly than the previous.

A solar mass star will never fuse carbon. 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 planetary nebula finally ends the red giant phase of the star's evolution.[1]

Stars that do not become red giants

Very low mass stars are thought to be fully convective[9] and thus may not accumulate an inert core of helium, and thus may exhaust all of their fuel without ever becoming red giants. [10] Such stars are commonly referred to as red dwarfs. The predicted lifespan of these stars is much larger than the current age of the universe, and hence there are no actual observations of these stars aging.

Very high mass stars instead develop to supergiant stars that wander back and forth horizontally over the HR diagram, at the right end constituting red supergiants. These usually end their life as type II supernovae.

The Sun as a red giant

The Sun is expected to become a red giant approximately 5 billion years from now.[11] It is calculated that the Sun will become sufficiently large to engulf the current orbits of the solar system's inner planets, including Earth and potentially Mars [12][13][14] and its radius will expand a minimum of 200 times,[15] The Sun will lose a significant fraction of its mass in the process of becoming a red giant, there is a chance that Mars and all the outer planets will escape as their resulting orbits will widen. [16] Mercury and most likely Venus will have been swallowed by sun's outer layer at this time. Earth's fate is less clear. Venus and Earth could technically achieve a widening of its orbit and could potentially maintain a sufficiently high angular velocity to keep it from becoming engulfed. In order to do so, its orbit would increase to 1.3 AU (190,000,000 km) and 1.7 AU (250,000,000 km). [17] However the results of studies announced in 2008 show that due to tidal interaction between sun and Earth, Earth would actually fall back into a lower orbit, and get engulfed and incorporated inside the sun before sun reaches its largest size, despite the sun losing about 38% of its mass. [18] Before this happens, Earth's biosphere will have long been destroyed by the Sun's steady increase in brightness as its hydrogen supply dwindles and its core contracts, even before the transition to a Red Giant. After just over 1 billion years, the extra solar energy input will cause Earth's oceans to evaporate and the hydrogen from the water to be lost permanently to space, with total loss of water by 3 billion years.[19] Earth's atmosphere and lithosphere will become like that of Venus. Over another billion years, most of the atmosphere will get lost in space as well;[20], ultimately leaving Earth as a desiccated, dead planet with a surface of molten rock.[21]

See also

Look up red giant in Wiktionary, the free dictionary.

References

  1. ^ a b c d Zeilik, Michael A. (1998). Introductory Astronomy & Astrophysics (4th ed. ed.). Saunders College Publishing. pp. pp. 321–322. ISBN 0030062284. {{cite book}}: |edition= has extra text (help); |pages= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  2. ^ Color of star ranging blue through orange
  3. ^ Measurements of the frequency of starspots on red giant stars
  4. ^ orange sphere of the sun
  5. ^ The Cambridge Atlas of Astronomy (2 ed.). Cambridge University Press. 1988. p. 255. ISBN 0-521-36360-8. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  6. ^ Harvard University search for orange-yellow clumps
  7. ^ Sackmann, I.-Juliana (1993). "Our Sun. III. Present and Future". Astrophysical Journal. 418: 457. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  8. ^ Pogge, Richard W. (2006-01-21). "Lecture 16: The Evolution of Low-Mass Stars". Astronomy 162: Introduction to Stars, Galaxies, & the Universe. Retrieved 2006-12-29. {{cite web}}: Check date values in: |date= (help); Cite has empty unknown parameter: |coauthors= (help)
  9. ^ "Main-Sequence Stars". Stars. The Astrophysics Spectator. 2005-02-16. Retrieved 2006-12-29. {{cite web}}: Check date values in: |date= (help); Cite has empty unknown parameter: |coauthors= (help)
  10. ^ Richmond, Michael. "Late stages of evolution for low-mass stars". Retrieved 2006-12-29. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
  11. ^ Our Sun. III. Present and Future,by Sackmann, I.-J., Boothroyd, A. I., & Kraemer, K. E., Astrophysical Journal v.418, p.457
  12. ^ "Red Giants". HyperPhysics (hosted by the Department of Physics and Astronomy of Georgia State University). Retrieved 2006-12-29.
  13. ^ Strobel, Nick (2004-06-02). "Stages 5-7". Lives and Deaths of Stars. Retrieved 2006-12-29. {{cite web}}: Check date values in: |date= (help)
  14. ^ "The fading: red giants and white dwarfs". Retrieved 2006-12-29.
  15. ^ Clues of death of our solar system
  16. ^ The Once & Future Sun
  17. ^ Sun's fate 1994
  18. ^ Earth may still vanish before the Sun expands further
  19. ^ Sun is a powerhouse - Death in our solar system
  20. ^ Pogge, Richard W. (1997-06-13). "The Once and Future Sun". New Vistas in Astronomy. Retrieved 2007-01-23. {{cite web}}: Check date values in: |date= (help)
  21. ^ MSN News Earth has little hope to survive over sun's fate update 2/26/2008
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