Editing Betelgeuse (section)

=== Circumstellar dynamics ===
[[File:ESO-Betelgeuse.jpg|thumb|left|Image from [[ESO]]'s [[Very Large Telescope]] showing the stellar disk and an extended [[Stellar atmosphere|atmosphere]] with a previously unknown plume of surrounding gas]]

In the late phase of [[stellar evolution]], massive stars like Betelgeuse exhibit high rates of [[stellar mass loss|mass loss]], possibly as much as {{Solar mass|one}} every {{val|10,000|fmt=commas|u=years}}, resulting in a complex [[circumstellar envelope|circumstellar environment]] that is constantly in flux. In a 2009 paper, stellar mass loss was cited as the "key to understanding the evolution of the universe from the earliest cosmological times to the current epoch, and of planet formation and the formation of life itself".<ref name=RIDGEWAY>
{{cite journal
 | last1=Ridgway       | first1=Stephen        | last2=Aufdenberg | first2=Jason
 | last3=Creech-Eakman | first3=Michelle       | last4=Elias      | first4=Nicholas
 | last5=Howell        | first5=Steve          | last6=Hutter     | first6=Don
 | last7=Karovska      | first7=Margarita      | last8=Ragland    | first8=Sam
 | last9=Wishnow       | first9=Ed
 | display-authors=6
 | year=2009
 | title=Quantifying stellar mass loss with high angular resolution imaging
 | journal=Astronomy & Astrophysics
 | volume=247 | page=247
 | bibcode=2009astro2010S.247R
 |arxiv = 0902.3008
}}
</ref>
However, the physical mechanism is not well understood.<ref name=KERVELLA2011/> When [[Martin Schwarzschild]] first proposed his theory of huge convection cells, he argued it was the likely cause of mass loss in evolved supergiants like Betelgeuse.<ref name=SCHWARZSCHILD1975/> Recent work has corroborated this hypothesis, yet there are still uncertainties about the structure of their convection, the mechanism of their mass loss, the way dust forms in their extended atmosphere, and the conditions which precipitate their dramatic finale as a type II supernova.<ref name=KERVELLA2011/> In 2001, Graham Harper estimated a stellar wind at {{Solar mass|0.03}} every {{val|10,000|fmt=commas|u=years}},<ref name=HARPER2001>
{{cite journal
 | last1=Harper | first1=Graham M.
 | last2=Brown  | first2=Alexander
 | last3=Lim    | first3=Jeremy
 | date=April 2001
 | title=A spatially resolved, semiempirical model for the extended atmosphere of {{nobr|{{mvar|α}} Orionis}} (M2 Iab)
 | journal=The Astrophysical Journal
 | volume=551  | issue=2  | pages=1073–98
 | doi=10.1086/320215
 | bibcode=2001ApJ...551.1073H|s2cid=120271858 | doi-access=free
 }}</ref> but research since 2009 has provided evidence of episodic mass loss making any total figure for Betelgeuse uncertain.<ref name=OHNAKA2009/> Current observations suggest that a star like Betelgeuse may spend a portion of its lifetime as a [[Red supergiant star|red supergiant]], but then cross back across the H–R diagram, pass once again through a brief [[Yellow supergiant star|yellow supergiant]] phase and then explode as a [[blue supergiant]] or [[Wolf–Rayet star]].<ref name=LEVESQUE1>
{{cite conference
 | last=Levesque |first=E.M.
 | date=June 2010
 | title=The physical properties of red supergiants
 | publisher=[[Astronomical Society of the Pacific]]
 | conference = Hot and Cool: Bridging gaps in massive star evolution
 | series = ASP Conference Series
 | volume = 425   | page = 103
 | bibcode = 2010ASPC..425..103L | arxiv = 0911.4720
}}
</ref>
[[File:Betelgeuse Plume eso0927d.jpg|thumb|right|Artist's rendering from [[European Southern Observatory|ESO]] showing Betelgeuse with a gigantic bubble boiling on its surface and a radiant plume of gas being ejected to six photospheric radii or roughly the orbit of Neptune]]

Astronomers may be close to solving this mystery. They noticed a large plume of gas extending at least six times its stellar radius indicating that Betelgeuse is not shedding matter evenly in all directions.<ref name="KERVELLA2009" /> The plume's presence implies that the spherical symmetry of the star's photosphere, often observed in the infrared, is ''not'' preserved in its close environment. Asymmetries on the stellar disk had been reported at different wavelengths. However, due to the refined capabilities of the [[List of instruments at the Very Large Telescope|NACO]] adaptive optics on the VLT, these asymmetries have come into focus. The two mechanisms that could cause such asymmetrical mass loss, were large-scale convection cells or polar mass loss, possibly due to rotation.<ref name=KERVELLA2009/> Probing deeper with ESO's AMBER, gas in the supergiant's extended atmosphere has been observed vigorously moving up and down, creating bubbles as large as the supergiant itself, leading his team to conclude that such stellar upheaval is behind the massive plume ejection observed by Kervella.<ref name=OHNAKA2009>
{{cite journal
 | last1=Ohnaka  | first1=K.      | last2=Hofmann  | first2=K.-H.
 | last3=Benisty | first3=M.      | last4=Chelli   | first4=A.
 | last5=Driebe  | first5=T.      | last6=Millour  | first6=F.
 | last7=Petrov  | first7=R.      | last8=Schertl  | first8=D.
 | last9=Stee    | first9=Ph.
 | display-authors=6
 | year=2009
 | title=Spatially resolving the inhomogeneous structure of the dynamical atmosphere of Betelgeuse with VLTI/AMBER
 | journal = [[Astronomy & Astrophysics]]
 | volume= 503 | issue=1 | pages=183–195
 | bibcode=2009A&A...503..183O
 | doi=10.1051/0004-6361/200912247
 | doi-access=free
 | arxiv = 0906.4792 | s2cid=17850433
}}
</ref>

==== Asymmetric shells ====
In addition to the photosphere, six other components of Betelgeuse's atmosphere have now been identified. They are a molecular environment otherwise known as the MOLsphere, a gaseous envelope, a chromosphere, a dust environment and two outer shells (S1 and S2) composed of [[carbon monoxide]] (CO). Some of these elements are known to be asymmetric while others overlap.<ref name=HAUBOIS/>
[[File:ESO Paranal Platform.jpg|thumb|left|Exterior view of ESO's Very Large Telescope ([[Very Large Telescope|VLT]]) in Paranal, Chile]]
At about 0.45 stellar radii (~2–{{val|3|u=AU}}) above the photosphere, there may lie a molecular layer known as the MOLsphere or molecular environment. Studies show it to be composed of water vapor and carbon monoxide with an effective temperature of about {{val|1500|500|u=K|fmt=commas}}.<ref name=HAUBOIS/><ref name=TSUJI>
{{cite journal
 | last=Tsuji | first=T.
 | year=2000
 | title=Water on the early M&nbsp;supergiant stars {{nobr|{{mvar|α}} Orionis}} and {{nobr|{{mvar|μ}} Cephei}}
 | journal=The Astrophysical Journal
 | volume=538 | issue=2 | pages=801–07
 | bibcode=2000ApJ...538..801T
 | doi=10.1086/309185 | doi-access=free
}}
</ref>
Water vapor had been originally detected in the supergiant's spectrum in the 1960s with the two Stratoscope projects but had been ignored for decades. The MOLsphere may also contain [[Silicon monoxide|SiO]] and [[Aluminium oxide|Al<sub>2</sub>O<sub>3</sub>]]—molecules which could explain the formation of dust particles.
[[File:Eso-paranal-16.jpg|thumb|left|Interior view of one of the four 8.2-meter Unit Telescopes at ESO's VLT]]

Another cooler region, the asymmetric gaseous envelope, extends for several radii (~10–{{val|40|u=AU}}) from the photosphere. It is enriched in oxygen and especially in [[nitrogen]] relative to carbon. These composition anomalies are likely caused by contamination by [[CNO cycle|CNO]]-processed material from the inside of Betelgeuse.<ref name=HAUBOIS/><ref name=LAMBERT>
{{cite journal
 | last1=Lambert | first1=D.L.      | last2=Brown   | first2=J.A.
 | last3=Hinkle  | first3=K.H.      | last4=Johnson | first4=H.R.
 | year=1984
 | title=Carbon, nitrogen, and oxygen abundances in Betelgeuse
 | journal=Astrophysical Journal
 | volume=284 | pages=223–37
 | bibcode=1984ApJ...284..223L
 | doi=10.1086/162401
}}</ref>

Radio-telescope images taken in 1998 confirm that Betelgeuse has a highly complex atmosphere,<ref name=NRAO/> with a temperature of {{val|3450|850|u=K|fmt=commas}}, similar to that recorded on the star's surface but much lower than surrounding gas in the same region.<ref name=NRAO>
{{cite press release
 |first=Dave |last=Finley
 |date=8 April 1998
 |title=VLA shows "boiling" in atmosphere of Betelgeuse
 |publisher=[[National Radio Astronomy Observatory]]
 |url=http://www.nrao.edu/pr/1998/betel/
 |access-date=7 September 2010
}}
</ref><ref name=LIM>
{{cite journal
 | title=Large Convection Cells as the Source of Betelgeuse's Extended Atmosphere
 | last1=Lim    | first1=Jeremy        | last2=Carilli | first2=Chris L.
 | last3=White  | first3=Stephen M.    | last4=Beasley | first4=Anthony J.
 | last5=Marson | first5=Ralph G.
 | year=1998
 | journal=[[Nature (journal)|Nature]]
 | volume=392  | issue=6676  | pages=575–577
 | bibcode=1998Natur.392..575L
 | doi=10.1038/33352
 |s2cid=4431516
}}
</ref> The VLA images also show this lower-temperature gas progressively cools as it extends outward. Although unexpected, it turns out to be the most abundant constituent of Betelgeuse's atmosphere. "This alters our basic understanding of red-supergiant star atmospheres", explained Jeremy Lim, the team's leader. "Instead of the star's atmosphere expanding uniformly due to gas heated to high temperatures near its surface, it now appears that several giant convection cells propel gas from the star's surface into its atmosphere."<ref name=NRAO/> This is the same region in which Kervella's 2009 finding of a bright plume, possibly containing carbon and nitrogen and extending at least six photospheric radii in the southwest direction of the star, is believed to exist.<ref name=HAUBOIS/>

The [[chromosphere]] was directly imaged by the Faint Object Camera on board the Hubble Space Telescope in ultraviolet wavelengths. The images also revealed a bright area in the southwest quadrant of the disk.<ref name=LOBEL2004/> The average radius of the chromosphere in 1996 was about 2.2&nbsp;times the optical disk (~{{val|10|u=AU}}) and was reported to have a temperature no higher than {{val|5500|u=K|fmt=commas}}.<ref name=HAUBOIS/><ref name=DUPREE>
{{cite journal
 | last1=Dupree    |first1=Andrea K.
 | last2=Gilliland |first2=Ronald L.
 | date=December 1995
 | title=HST direct image of Betelgeuse
 | journal=[[Bulletin of the American Astronomical Society]]
 | volume=27  | page=1328
 | bibcode=1995AAS...187.3201D
}}
</ref>{{efn|
"Such a major single feature is distinctly different from scattered smaller regions of activity typically found on the Sun although the strong ultraviolet flux enhancement is characteristic of stellar magnetic activity. This inhomogeneity may be caused by a large scale convection cell or result from global pulsations and shock structures that heat the chromosphere."<ref name=DUPREE/>
}}
However, in 2004 observations with the STIS, Hubble's high-precision spectrometer, pointed to the existence of warm chromospheric plasma at least one arcsecond away from the star. At a distance of {{val|197|u=pc}}, the size of the chromosphere could be up to {{val|200|u=AU}}.<ref name=LOBEL2004>
{{cite conference
 | last1=Lobel    |first1=A.          | last2=Aufdenberg |first2=J.
 | last3=Dupree   |first3=A.K.        | last4=Kurucz     |first4=R.L.
 | last5=Stefanik |first5=R.P.        | last6=Torres     |first6=G.
 | year=2004
 | title=Spatially resolved STIS spectroscopy of Betelgeuse's outer atmosphere
 | conference=219th Symposium of the IAU
 | page= 641
 | bibcode=2004IAUS..219..641L
 | volume=219
 | doi=10.1017/s0074180900182671
 | s2cid=15868906
 |arxiv = astro-ph/0312076
 | quote=
}}
</ref>{{efn|
"In the article, Lobel ''et al.'' equate 1&nbsp;arcsecond to approximately 40&nbsp;stellar radii, a calculation which in 2004 likely assumed a Hipparcos distance of 131&nbsp;pc (430&nbsp;ly) and a photospheric diameter of 0.0552″ from Weiner ''et al''."<ref name=LOBEL2004/>
}}
The observations have conclusively demonstrated that the warm chromospheric plasma spatially overlaps and co-exists with cool gas in Betelgeuse's gaseous envelope as well as with the dust in its circumstellar dust shells.<ref name="HAUBOIS" /><ref name=LOBEL2004/>

[[File:Nebula around Betelgeuse.jpg|thumb|This [[infrared]] image from the [[ESO]]'s [[Very Large Telescope|VLT]] shows complex shells of gas and dust around Betelgeuse – the [[:File:Nebula and betelgeuse VLT.jpg|tiny red circle]] in the middle is the size of the photosphere.]]

The first claim of a dust shell surrounding Betelgeuse was put forth in 1977 when it was noted that dust shells around mature stars often emit large amounts of radiation in excess of the photospheric contribution. Using [[Interferometry#Heterodyne detection|heterodyne interferometry]], it was concluded that the red supergiant emits most of its excess radiation from positions beyond 12 stellar radii or roughly the distance of the [[Kuiper belt]] at 50 to 60 AU, which depends on the assumed stellar radius.<ref name="SUTTON1977"/><ref name="HAUBOIS"/> Since then, there have been studies done of this dust envelope at varying wavelengths yielding decidedly different results. Studies from the 1990s have estimated the inner radius of the dust shell anywhere from 0.5 to {{val|1.0|ul=arcseconds}}, or 100 to {{val|200|u=AU}}.<ref name=SKINNER>
{{cite journal
 | last1=Skinner |first1=C.J.        | last2=Dougherty | first2=S.M.
 | last3=Meixner | first3=M.         | last4=Bode      | first4=M.F.
 | last5=Davis   | first5=R.J.       | last6=Drake     | first6=S.A.
 | last7=Arens   | first7=J.F.       | last8=Jernigan  | first8=J.G.
 | display-authors=6
 | year=1997
 | title=Circumstellar environments – V. The asymmetric chromosphere and dust shell of Alpha&nbsp;Orionis
 | journal=Monthly Notices of the Royal Astronomical Society
 | volume=288 | issue=2 | pages=295–306
 | bibcode=1997MNRAS.288..295S
 | doi=10.1093/mnras/288.2.295 | doi-access=free
}}
</ref><ref name=DANCHI>
{{cite journal
 | last1=Danchi    | first1=W.C.       | last2=Bester    | first2=M.
 | last3=Degiacomi | first3=C.G.       | last4=Greenhill | first4=L.J.
 | last5=Townes    | first5=C.H.
 | year=1994
 | title=Characteristics of dust shells around 13&nbsp;late-type stars
 | journal=The Astronomical Journal
 | volume=107 | issue=4 | pages=1469–1513
 | doi=10.1086/116960
 | bibcode=1994AJ....107.1469D
}}
</ref>
These studies point out that the dust environment surrounding Betelgeuse is not static. In 1994, it was reported that Betelgeuse undergoes sporadic decades-long dust production, followed by inactivity. In 1997, significant changes in the dust shell's morphology in one year were noted, suggesting that the shell is asymmetrically illuminated by a stellar radiation field strongly affected by the existence of photospheric hotspots.<ref name="SKINNER" /> The 1984 report of a giant asymmetric dust shell {{val|1|u=pc}} ({{val|206,265|fmt=commas|u=AU}}) has not been corroborated by recent studies, although another published the same year said that three dust shells were found extending four light-years from one side of the decaying star, suggesting that Betelgeuse sheds its outer layers as it moves.<ref name="BAUD">
{{cite journal
 | last1=Baud          | first1=B.        | last2=Waters    | first2=R.
 | last3=de&nbsp;Vries | first3=J.        | last4=van&nbsp;Albada | first4=G.D.
 | last5=Boulanger     | first5=F.        | last6=Wesselius | first6=P.R.
 | last7=Gillet        | first7=F.        | last8=Habing    | first8=H.J.
 | last9=van der Kruit | first9=P.C.
 | display-authors=6
 | date=January 1984
 | title=A giant asymmetric dust shell around Betelgeuse
 | journal=[[Bulletin of the American Astronomical Society]]
 | volume=16 | page=405
 | bibcode=1984BAAS...16..405B
}}
</ref><ref name=DAVID>
{{cite magazine
 | last1=David | first1=L.
 | last2=Dooling |first2=D.
 | year=1984
 | title=The Infrared Universe
 | magazine=Space World
 | issue=2
 | pages=4–7
 | bibcode=1984SpWd....2....4D
}}
</ref>

Although the exact size of the two outer [[carbon monoxide|CO]] shells remains elusive, preliminary estimates suggest that one shell extends from about 1.5 to 4.0&nbsp;[[arcseconds]] and the other expands as far as 7.0&nbsp;arcseconds.<ref name=HARPER2009>
{{cite journal
 | last1=Harper |first1=Graham M.  | last2=Carpenter  | first2=Kenneth G.
 | last3=Ryde | first3=Nils  | last4=Smith | first4=Nathan
 | last5=Brown | first5=Joanna  | last6=Brown  | first6=Alexander
 | last7=Hinkle | first7=Kenneth H. | last8=Stempels | first8=Eric
 | display-authors=6
  | year=2009
| title=UV, IR, and mm studies of CO surrounding the red supergiant {{nobr|{{mvar|α}} Orionis}} (M2 Iab)
 | journal=AIP Conference Proceedings
 | volume= 1094
 | pages=868–871
 | doi=10.1063/1.3099254
 | bibcode=2009AIPC.1094..868H
}}
</ref>
Assuming the Jovian orbit of {{val|5.5|u=AU}} as the star radius, the inner shell would extend roughly 50 to 150 stellar radii (~300 to {{val|800|u=AU}}) with the outer one as far as 250 stellar radii (~{{val|1,400|fmt=commas|u=AU}}). The Sun's [[heliopause (astronomy)|heliopause]] is estimated at 100 AU, so the size of this outer shell would be almost fourteen times the size of the Solar System.

==== Supersonic bow shock ====
Betelgeuse is travelling through the interstellar medium at a speed of {{val|30|u=km/second}} (i.e. ~{{val|6.3|u=AU/year}}) creating a [[Bow shocks in astrophysics|bow shock]].<ref name=MOHAMED1/><ref name=LAMERS1999>
{{cite book
 | last1=Lamers |first1=Henny J.G.L.M.
 | last2=Cassinelli |first2=Joseph P.
 | name-list-style=amp
 | date=June 1999
 | title=Introduction to Stellar Winds
 | publisher=Cambridge University Press
 | location=Cambridge, UK
 | isbn=978-0-521-59565-0
 | bibcode=1999isw..book.....L
}}
</ref>
The shock is not created by the star, but by its powerful [[stellar wind]] as it ejects vast amounts of gas into the interstellar medium at a speed of {{val|17|u=km/s}}, heating the material surrounding the star, thereby making it visible in infrared light.<ref name=ESA3>
{{cite press release
 |title=Akari Infrared Space Telescope: Latest science highlights
 |publisher=[[European Space Agency]]
 |url=http://www.esa.int/Our_Activities/Space_Science/Akari_infrared_space_telescope_latest_science_highlights
 |date=19 November 2008
 |access-date=25 June 2012
 |archive-url=https://web.archive.org/web/20110217144724/http://www.esa.int/esaSC/SEMCJT4DHNF_index_1.html
 |archive-date= 17 February 2011}}</ref> Because Betelgeuse is so bright, it was only in 1997 that the bow shock was first imaged. The [[comet]]ary structure is estimated to be at least one parsec wide, assuming a distance of 643&nbsp;light-years.<ref name=NORIEGA1>
{{cite journal
 | last1=Noriega-Crespo | first1=Alberto   | last2=van Buren | first2=Dave
 | last3=Cao        | first3=Yu            | last4=Dgani     | first4=Ruth
 | year=1997
 | title=A parsec-size bow shock around Betelgeuse
 | journal=[[Astronomical Journal]]
 | volume=114  | pages=837–40
 | bibcode=1997AJ....114..837N
 | doi=10.1086/118517  | doi-access=free
}}
</ref>{{efn|
"Noriega in 1997 estimated the size to be 0.8&nbsp;parsecs, having assumed the earlier distance estimate of 400 ly. With a current distance estimate of 643&nbsp;ly, the bow shock would measure ~1.28 parsecs or over 4&nbsp;ly."<ref name=NORIEGA1/>
}}

[[Fluid dynamics|Hydrodynamic]] simulations of the bow shock made in 2012 indicate that it is very young—less than 30,000&nbsp;years old—suggesting two possibilities: That Betelgeuse moved into a region of the interstellar medium with different properties only recently or that Betelgeuse has undergone a significant transformation producing a changed stellar wind.<ref name=ASTROBITES1>
{{cite web
 |last=Newton |first=Elizabeth
 |date=26 April 2012
 |title=This star lives in exciting times, or, how did Betelgeuse make that funny shape?
 |website=Astrobites (astrobites.com)
 |url=http://astrobites.com/2012/04/26/this-star-lives-in-exciting-times-or-how-did-betelgeuse-make-that-funny-shape/
 |access-date=25 June 2012  |url-status=dead
 |archive-url=https://web.archive.org/web/20120430035007/http://astrobites.com/2012/04/26/this-star-lives-in-exciting-times-or-how-did-betelgeuse-make-that-funny-shape/
 |archive-date=30 April 2012
}}
</ref>
A 2012 paper, proposed that this phenomenon was caused by Betelgeuse transitioning from a [[blue supergiant]] (BSG) to a red supergiant (RSG). There is evidence that in the late evolutionary stage of a star like Betelgeuse, such stars "may undergo rapid transitions from red to blue and vice versa on the Hertzsprung–Russell diagram, with accompanying rapid changes to their stellar winds and bow shocks."<ref name="MOHAMED1">{{cite journal
 | last1=Mohamed |first1=S.
 | last2=Mackey |first2=J.
 | last3=Langer |first3=N.
 | year=2012
 | title=3D simulations of Betelgeuse's bow shock
 | journal=[[Astronomy & Astrophysics]]
 | volume= 541  | page=A1
 | bibcode=2012A&A...541A...1M
 | doi=10.1051/0004-6361/201118002
 | arxiv=1109.1555  | s2cid=118435586
}}
</ref><ref name=MACKEY1>
{{cite journal
 | last1=MacKey  | first1=Jonathan     | last2=Mohamed | first2=Shazrene
 | last3=Neilson | first3=Hilding R.   | last4=Langer  | first4=Norbert
 | last5=Meyer   | first5=Dominique M.-A.
 | year=2012
 | title=Double bow shocks around young, runaway red supergiants: Application to Betelgeuse
 | journal=The Astrophysical Journal
 | volume=751  | issue=1  | page=L10
 | bibcode=2012ApJ...751L..10M
 | doi=10.1088/2041-8205/751/1/L10
 | arxiv = 1204.3925  | s2cid=118433862
}}</ref> Moreover, if future research bears out this hypothesis, Betelgeuse may prove to have traveled close to 200,000 AU as a red supergiant scattering as much as {{val|3|u=M_solar}} along its trajectory.