Editing Betelgeuse (section)

==== 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.