Editing Betelgeuse (section)

== Observation ==
[[File:Orion Head to Toe.jpg|thumb|right|Image showing Betelgeuse (top left) and the dense nebulae of the [[Orion molecular cloud complex]] ([[Rogelio Bernal Andreo]])]]
As a result of its distinctive orange-red color and position within Orion, Betelgeuse is easy to find with the naked eye. It is one of three stars that make up the [[Winter Triangle]] [[asterism (astronomy)|asterism]], and it marks the center of the [[Winter Hexagon]]. It can be seen rising in the east at the beginning of January of each year, just after sunset. Between mid-September and mid-March (best in mid-December), it is visible to virtually every inhabited region of the globe, except in [[Antarctica]] at latitudes south of 82°. In May (moderate northern latitudes) or June (southern latitudes), the red supergiant can be seen briefly on the western horizon after sunset, reappearing again a few months later on the eastern horizon before sunrise. In the intermediate period (June–July, centered around mid June), it is invisible to the naked eye (visible only with a telescope in daylight), except around midday low in the north in Antarctic regions between 70° and 80° south latitude (during midday twilight in [[polar night]], when the Sun is below the horizon).

Betelgeuse is a variable star whose [[apparent magnitude|visual magnitude]] ranges between 0.0 and +1.6&nbsp;.<ref name=vsx/> There are periods during which it surpasses Rigel to become the sixth brightest star, and occasionally it will become even brighter than [[Capella]]. At its faintest, Betelgeuse can fall behind [[Deneb]] and [[Beta Crucis]], themselves both slightly variable, to be the twentieth-brightest star.<ref name=BURNHAM/>

Betelgeuse has a B–V [[color index]] of 1.85 – a figure which points to its pronounced "redness". The photosphere has an extended [[Stellar atmosphere|atmosphere]], which displays strong lines of [[Spectral line|emission]] rather than [[Spectral line|absorption]], a phenomenon that occurs when a star is surrounded by a thick gaseous envelope (rather than ionized). This extended gaseous atmosphere has been observed moving toward and away from Betelgeuse, depending on fluctuations in the photosphere. Betelgeuse is the brightest near-infrared source in the sky with a [[J band (infrared)|J&nbsp;band]] [[Magnitude (astronomy)|magnitude]] of −2.99;<ref name="2MASS-brightest">{{cite press release |date=7 September 2009 |title=Very bright stars in the 2MASS Point Source Catalog (PSC) |publisher=The Two Micron All Sky Survey at IPAC |last1=Cutri |first1=R. |last2=Skrutskie |first2=M. |url=http://www.ipac.caltech.edu/2mass/releases/allsky/doc/sec1_6b.html#satr1|access-date=28 December 2011}}</ref> only about 13% of the star's [[radiant energy]] is emitted as visible light. If human eyes were sensitive to radiation at all wavelengths, Betelgeuse would appear as the brightest star in the night sky.<ref name="BURNHAM" />
[[File:Betelgeuse_(star).jpg|thumb|219x219px|Betelgeuse seen close-up]]
Catalogues list up to nine faint visual companions to Betelgeuse. They are at distances of about one to four arc-minutes and all are fainter than 10th&nbsp;magnitude.<ref name="CCDM">{{cite web |title=CCDM (Catalog of Components of Double & Multiple stars (Dommanget+ 2002) |work=[[VizieR]] |publisher=[[Centre de Données astronomiques de Strasbourg]] |url=http://vizier.u-strasbg.fr/viz-bin/VizieR-S?CCDM%20J05552%2b0724AP |access-date=22 August 2010}}</ref><ref name="wds">{{cite journal |bibcode=2001AJ....122.3466M |title=The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog |journal=The Astronomical Journal |volume=122 |issue=6 |pages=3466 |last1=Mason |first1=Brian D. |last2=Wycoff |first2=Gary L. |last3=Hartkopf |first3=William I. |last4=Douglass |first4=Geoffrey G. |last5=Worley |first5=Charles E. |year=2001 |doi=10.1086/323920 |doi-access=free}}</ref>

=== Distance measurements ===
[[File:USA.NM.VeryLargeArray.02.jpg|thumb|right|[[National Radio Astronomy Observatory|NRAO]]'s [[Very Large Array]] used to derive Betelgeuse's 2008 distance estimate]]

[[Parallax]] is the apparent change of the position of an object, measured in seconds of arc, caused by the change of position of the observer of that object. [[Parallax in astronomy|Parallax is used in astronomy]] to estimate distances to the nearest stars. As the Earth orbits the Sun, every star is seen to shift by a fraction of an arc second, which measure, combined with the baseline provided by the Earth's orbit gives the distance to that star. Since the first successful [[parallax]] measurement by [[Friedrich Bessel]] in 1838, astronomers have been puzzled by Betelgeuse's apparent distance. Knowledge of the star's distance improves the accuracy of other stellar parameters, such as [[Stellar luminosity|luminosity]] that, when combined with an angular diameter, can be used to calculate the physical radius and [[effective temperature]]; luminosity and [[Natural abundance|isotopic abundances]] can also be used to estimate the [[Stellar evolution|stellar age]] and [[Intermediate mass star|mass]].<ref name="HARPER">
{{cite journal
 | last1=Harper |first1=Graham M.
 | last2=Brown  |first2=Alexander
 | last3=Guinan |first3=Edward F.
 | title=A new VLA-Hipparcos distance to Betelgeuse and its implications
 | journal=The Astronomical Journal
 | date=April 2008
 | volume=135 | issue=4 | pages=1430–40
 | bibcode=2008AJ....135.1430H
 | doi=10.1088/0004-6256/135/4/1430 | doi-access=free
 }}</ref>

When the first interferometric studies were performed on the star's diameter in 1920, the assumed parallax was {{val|0.0180|ul="}}. This equated to a distance of {{val|56|ul=pc}} or roughly {{val|180|ul=ly}}, producing not only an inaccurate radius for the star but every other stellar characteristic. Since then, there has been ongoing work to measure the distance of Betelgeuse, with proposed distances as high as {{val|400|u=pc}} or about {{val|1300|u=ly|fmt=commas}}.<ref name="HARPER" />

Before the publication of the [[Hipparcos Catalogue]] (1997), there were two slightly conflicting parallax measurements for Betelgeuse. The first, in 1991, gave a parallax of {{val|9.8|4.7|ul=mas}}, yielding a distance of roughly {{val|102|u=pc}} or {{val|330|u=ly}}.<ref name=YALEPLX>
{{cite journal
 | last1=van&nbsp;Altena |first1=W.F.
 | last2=Lee  |first2=J.T.
 | last3=Hoffleit |first3=D.
 | title=Yale Trigonometric Parallaxes Preliminary
 | journal=Yale University Observatory (1991)
 | date=October 1995
 | bibcode=1995yCat.1174....0V
 | volume=1174
 | page=0
}}
</ref> The second was the [[Hipparcos#Hipparcos Input Catalogue|Hipparcos Input Catalogue]] (1993) with a trigonometric parallax of {{val|5|4|u=mas}}, a distance of {{val|200|u=pc}} or {{val|650|u=ly}}.<ref name="HIC">
{{cite web
 | title=Hipparcos Input Catalogue, Version&nbsp;2 (Turon+ 1993)
 | work=[[VizieR]]
 | year=1993
 | publisher=[[Centre de Données astronomiques de Strasbourg]]
 | url=http://vizier.u-strasbg.fr/viz-bin/VizieR-S?HIC%2027989
 | access-date=20 June 2010
}}
</ref> Given this uncertainty, researchers were adopting a wide range of distance estimates, leading to significant variances in the calculation of the star's attributes.<ref name="HARPER" />

The results from the Hipparcos mission were released in 1997. The measured parallax of Betelgeuse was {{val|7.63|1.64|u=mas}}, which equated to a distance of roughly {{val|131|u=pc}} or {{val|427|u=ly}}, and had a smaller reported error than previous measurements.<ref name="PERRYMAN">
{{cite journal
 | display-authors=6
 | last1=Perryman | first1=M.A.C. | last2=Lindegren | first2=L.
 | last3=Kovalevsky | first3=J. | last4=Hoeg | first4=E.
 | last5=Bastian | first5=U. | last6=Bernacca | first6=P.L.
 | last7=Crézé | first7=M. | last8=Donati | first8=F.
 | last9=Grenon | first9=M.
 | title=The Hipparcos Catalogue
 | journal=[[Astronomy & Astrophysics]]
 | year=1997 | volume=323 | pages=L49–L52
 | bibcode=1997A&A...323L..49P
}}
</ref>
However, later evaluation of the Hipparcos parallax measurements for variable stars like Betelgeuse found that the uncertainty of these measurements had been underestimated.<ref name=EYER>
{{cite conference
 | last1=Eyer |first1=L.
 | last2=Grenon |first2=M.
 | year=2000
 | title=Problems encountered in the Hipparcos variable stars analysis
 | conference=6th Vienna Workshop in Astrophysics
 | book-title=Delta Scuti and Related Stars – Reference Handbook and Proceedings of the 6th Vienna Workshop in Astrophysics
 | series=ASP Conference Series
 | publisher=[[Astronomical Society of the Pacific]]
 | volume=210 | page=482
 | place=Vienna, Austria
 | bibcode=2000ASPC..210..482E
 | isbn=978-1-58381-041-5 | arxiv = astro-ph/0002235
}}
</ref>
In 2007, an improved figure of {{val|6.55|0.83}} was calculated, hence a much tighter [[Margin of error|error factor]] yielding a distance of roughly {{val|152|20|u=pc}} or {{val|500|65|u=ly}}.<ref name=hipparcos>{{cite journal | title=Hipparcos, the new reduction | last1=van&nbsp;Leeuwen | first1=F. |display-authors=etal |date=November 2007 | id=[[VizieR]] | issue=2 | pages=653–664 | volume=474 | doi=10.1051/0004-6361:20078357 | journal=[[Astronomy and Astrophysics]] | bibcode=2007A&A...474..653V |arxiv = 0708.1752| s2cid=18759600 }}</ref>

In 2008, measurements using the [[Very Large Array]] (VLA) produced a [[Radio astronomy|radio]] solution of {{val|5.07|1.10|u=mas}}, equaling a distance of {{val|197|45|u=pc}} or {{val|643|146|u=ly}}.<ref name="HARPER" /> As the researcher, Harper, points out: "The revised Hipparcos parallax leads to a larger distance ({{val|152|20|u=pc}}) than the original; however, the [[astrometric]] solution still requires a significant [[cosmic noise]] of 2.4 mas. Given these results it is clear that the Hipparcos data still contain systematic errors of unknown origin." Although the radio data also have systematic errors, the Harper solution combines the datasets in the hope of mitigating such errors.<ref name="HARPER" /> An updated result from further observations with [[Atacama Large Millimeter Array|ALMA]] and [[e-Merlin]] gives a parallax of {{val|4.51|0.8}} mas and a distance of {{val|222|34|48}} pc or {{val|724|111|156}} ly.<ref name=harper2017>{{cite journal|bibcode=2017AJ....154...11H|arxiv=1706.06020|title=An Updated 2017 Astrometric Solution for Betelgeuse|journal=The Astronomical Journal|volume=154|issue=1|pages=11|last1=Harper|first1=G. M.|last2=Brown|first2=A.|last3=Guinan|first3=E. F.|last4=O'Gorman|first4=E.|last5=Richards|first5=A. M. S.|last6=Kervella|first6=P.|last7=Decin|first7=L.|year=2017|doi=10.3847/1538-3881/aa6ff9|s2cid=59125676 |doi-access=free }}</ref>

In 2020, new observational data from the space-based ''Solar Mass Ejection Imager'' aboard the [[Coriolis (satellite)|Coriolis satellite]] and three different modeling techniques produced a refined parallax of {{val|5.95|0.58|0.8|u=mas}}, a radius of {{val|764|116|62|u=Solar radius}}, and a distance of {{val|168.1|27.5|14.4|u=pc}} or {{val|548|90|49|u=ly}}, which would imply Betelgeuse is nearly 25% smaller and 25% closer to Earth than previously thought.<ref name=joyce2020>{{cite journal |doi=10.3847/1538-4357/abb8db |title=Standing on the Shoulders of Giants: New Mass and Distance Estimates for Betelgeuse through Combined Evolutionary, Asteroseismic, and Hydrodynamic Simulations with MESA |year=2020 |last1=Joyce |first1=Meridith |last2=Leung |first2=Shing-Chi |last3=Molnár |first3=László |last4=Ireland |first4=Michael |last5=Kobayashi |first5=Chiaki |last6=Nomoto |first6=Ken'Ichi |journal=The Astrophysical Journal |volume=902 |issue=1 |page=63 |arxiv=2006.09837 |bibcode=2020ApJ...902...63J |s2cid=221507952 |doi-access=free }}</ref>

Another study in 2022 suggests Betelgeuse to be smaller and closer than previously thought based on historical records which revealed Betelgeuse changed in color from yellow to red in the last thousand years. This color change suggests a mass of {{solar mass|14}}, considerably less than previous estimates, and the best-fit [[stellar evolution|evolutionary track]] gives an estimate as low as 125 parsecs (410 light-years), consistent with the ''Hipparcos'' data.<ref name="Neuhauser2022" />

The [[European Space Agency]]'s current [[Gaia mission]] is unable to produce good parallax results for stars like Betelgeuse which are brighter than the approximately V=6 saturation limit of the mission's instruments.<ref name="ESA1">{{cite web|title=Science Performance|publisher=[[European Space Agency]]|url=http://www.rssd.esa.int/index.php?page=Science_Performance&project=GAIA|date=19 February 2013|access-date=1 March 2013}}</ref><ref name=prusti>{{cite journal |first1 = T. |last1 = Prusti |collaboration = GAIA Collaboration | date = 2016 | title = The ''Gaia'' mission | type = forthcoming article | journal = [[Astronomy and Astrophysics]] | doi = 10.1051/0004-6361/201629272 | url = http://www.aanda.org/articles/aa/pdf/forth/aa29272-16.pdf | access-date = 21 September 2016 | bibcode=2016A&A...595A...1G | volume=595 | pages=A1 |arxiv = 1609.04153 | hdl = 2445/127856 | s2cid = 9271090 }}</ref> Because of this limitation, there was no data on Betelgeuse in [[Gaia Data Release 2]], from 2018<ref>{{cite web |url=https://gea.esac.esa.int/archive/ |title=Welcome to the Gaia Archive |website=[[European Space Agency]] |access-date=2020-09-03 }}</ref> or Data Release 3 from 2022.<ref>{{cite web | url=https://earthsky.org/astronomy-essentials/how-far-is-betelgeuse | title=EarthSky &#124; How far is Betelgeuse, the famous doomed star? | date=8 January 2023 }}</ref>

=== Variability ===
[[File:Light curve of Betelgeuse.png|thumb|upright=1.15|[[AAVSO]] [[Photometric system|V-band]] [[light curve]] of Betelgeuse (Alpha Orionis) from Dec 1988 to Aug 2002]]
[[File:Betelgeuse.jpg|thumb|[[Orion (constellation)|Orion]], with Betelgeuse at its usual [[apparent magnitude|magnitude]] (left) and during the unusually deep minimum in early 2020 (right)]]

Betelgeuse is classified as a [[semiregular variable star]], indicating that some periodicity is noticeable in the brightness changes, but amplitudes may vary, cycles may have different lengths, and there may be standstills or periods of irregularity. It is placed in subgroup SRc; these are pulsating red supergiants with amplitudes around one magnitude and periods from tens to hundreds of days.<ref name=gcvs/>

Betelgeuse typically shows only small brightness changes near to magnitude +0.5, although at its extremes it can become as bright as magnitude 0.0 or as faint as magnitude +1.6. Betelgeuse is listed in the [[General Catalogue of Variable Stars]] with a possible period of 2,335 days.<ref name=gcvs/> More detailed analyses have shown a main period near 400 days, a short period of 185 days,<ref name=joyce2020/> and a longer secondary period around 2,100 days.<ref name=montarges2016>{{cite journal|bibcode=2016A&A...588A.130M|arxiv=1602.05108|title=The close circumstellar environment of Betelgeuse. IV. VLTI/PIONIER interferometric monitoring of the photosphere|journal=Astronomy & Astrophysics|volume=588|pages=A130|last1=Montargès|first1=M.|last2=Kervella|first2=P.|last3=Perrin|first3=G.|last4=Chiavassa|first4=A.|last5=Le Bouquin|first5=J.-B.|last6=Aurière|first6=M.|last7=López Ariste|first7=A.|last8=Mathias|first8=P.|last9=Ridgway|first9=S. T.|last10=Lacour|first10=S.|last11=Haubois|first11=X.|last12=Berger|first12=J.-P.|year=2016|doi=10.1051/0004-6361/201527028|s2cid=53404211}}</ref><ref name=kiss>{{cite journal|bibcode=2006MNRAS.372.1721K|arxiv=astro-ph/0608438|title=Variability in red supergiant stars: Pulsations, long secondary periods and convection noise|journal=Monthly Notices of the Royal Astronomical Society|volume=372|issue=4|pages=1721–1734|last1=Kiss|first1=L. L.|last2=Szabó|first2=Gy. M.|last3=Bedding|first3=T. R.|year=2006|doi=10.1111/j.1365-2966.2006.10973.x|doi-access=free |s2cid=5203133}}</ref> The lowest reliably-recorded [[UBV photometric system|V-band]] magnitude of +1.614 was reported in February 2020.

Radial pulsations of red supergiants are well-modelled and show that periods of a few hundred days are typically due to [[fundamental mode|fundamental]] and first [[overtone]] pulsation.<ref name=guo>{{cite journal|bibcode=2002ApJ...565..559G|title=Evolution and Pulsation of Red Supergiants at Different Metallicities|journal=The Astrophysical Journal|volume=565|issue=1|pages=559–570|last1=Guo|first1=J. H.|last2=Li|first2=Y.|year=2002|doi=10.1086/324295|doi-access=free}}</ref> [[Absorption line|Lines]] in the [[stellar spectrum|spectrum]] of Betelgeuse show [[doppler shift]]s indicating [[radial velocity]] changes corresponding, very roughly, to the brightness changes. This demonstrates the nature of the pulsations in size, although corresponding temperature and spectral variations are not clearly seen.<ref name=goldberg>{{cite journal|bibcode=1984PASP...96..366G|title=The variability of alpha Orionis|journal=Astronomical Society of the Pacific|volume=96|pages=366|last1=Goldberg|first1=L.|year=1984|doi=10.1086/131347|doi-access=free}}</ref> Variations in the diameter of Betelgeuse have also been measured directly.<ref name=RAVI1/> [[Overtone|First overtone]] pulsations of 185 days have been observed, and the ratio of the fundamental to overtone periods gives valuable information about the internal structure of the star and its age.<ref name=joyce2020/>

The source of the long secondary periods is unknown, but they cannot be explained by [[radial pulsations]].<ref name=kiss/> Interferometric observations of Betelgeuse have shown hotspots that are thought to be created by massive convection cells, a significant fraction of the diameter of the star and each emitting 5–10% of the total light of the star.<ref name=HAUBOIS/><ref name=montarges2016/> One theory to explain long secondary periods is that they are caused by the evolution of such cells combined with the rotation of the star.<ref name=kiss/> Other theories include close binary interactions, [[Chromosphere|chromospheric]] magnetic activity influencing mass loss, or non-radial pulsations such as [[G-mode pulsation|g-mode]]s.<ref name=wood>{{cite journal|bibcode=2004ApJ...604..800W|title=Long Secondary Periods in Pulsating Asymptotic Giant Branch Stars: An Investigation of their Origin|journal=The Astrophysical Journal|volume=604|issue=2|pages=800|last1=Wood|first1=P. R.|last2=Olivier|first2=E. A.|last3=Kawaler|first3=S. D.|year=2004|doi=10.1086/382123|doi-access=free}}</ref>

In addition to the discrete dominant periods, small-amplitude [[stochastic]] variations are seen. It is proposed that this is due to [[Granule (solar physics)|granulation]], similar to the same effect on the sun but on a much larger scale.<ref name=kiss/>

=== Diameter ===
{{See also|List of largest stars}}
[[File:Arcturus to Betelgeuse comparison.jpg|thumb|Size comparison between [[Arcturus]], [[Rigel]], [[S Doradus]], [[Antares]], and Betelgeuse]]
[[File:Mukyv354.png|thumb|upright=1.15|Size comparison of Betelgeuse, [[Mu Cephei]], [[KY Cygni]], and [[V354 Cephei]], according to [[Emily Levesque]]<ref>{{Cite journal |last1=Levesque |first1=Emily M. |last2=Massey |first2=Philip |last3=Olsen |first3=K. A. G. |last4=Plez |first4=Bertrand |last5=Josselin |first5=Eric |last6=Maeder |first6=Andre |last7=Meynet |first7=Georges |date=August 2005  |title=The Effective Temperature Scale of Galactic Red Supergiants: Cool, But Not As Cool As We Thought |journal=The Astrophysical Journal |volume=628 |issue=2 |pages=973–985 |doi=10.1086/430901 |issn=0004-637X|arxiv=astro-ph/0504337 |bibcode=2005ApJ...628..973L }}</ref>]]

On 13 December 1920, Betelgeuse became the first star outside the Solar System to have the angular size of its photosphere measured.<ref name="MICHELSON" /> Although interferometry was still in its infancy, the experiment proved a success. The researchers, using a uniform disk model, determined that Betelgeuse had a diameter of {{val|0.047|u="}}, although the stellar disk was likely 17% larger due to the [[limb darkening]], resulting in an estimate for its angular diameter of about 0.055".<ref name=MICHELSON/><ref name=TOWNES1>{{cite journal
 | title= A Systematic Change with Time in the Size of Betelgeuse
 | last1=Townes |first1=C.H.   | last2=Wishnow |first2=E.H.
 | last3=Hale   |first3=D.D.S. | last4=Walp    |first4=B.
 | journal=The Astrophysical Journal Letters
 | volume= 697
 | issue= 2
 | pages= L127–28
 | year=2009
 | bibcode=2009ApJ...697L.127T
 | doi=10.1088/0004-637X/697/2/L127| doi-access=free
 }}</ref> Since then, other studies have produced angular diameters that range from 0.042 to {{val|0.069|u="}}.<ref name=BONNEAU1973/><ref name=WEINER/><ref name=BALEGA>
{{cite journal
 | last1=Balega   |first1=Iu.       | last2=Blazit   |first2=A.
 | last3=Bonneau  |first3=D.        | last4=Koechlin |first4=L.
 | last5=Labeyrie |first5=A.        | last6=Foy      |first6=R.
 | title=The angular diameter of Betelgeuse
 | journal=Astronomy and Astrophysics
 |date=November 1982
 | volume=115
 | issue= 2
 | pages=253–56
 | bibcode=1982A&A...115..253B
}}
</ref>
Combining these data with historical distance estimates of 180 to {{val|815|u=ly}} yields a projected radius of the stellar disk of anywhere from 1.2 to {{val|8.9|u=AU}}. Using the Solar System for comparison, the orbit of [[Solar System#Mars|Mars]] is about {{val|1.5|u=AU}}, [[Solar System#Ceres|Ceres]] in the [[asteroid belt]] {{val|2.7|u=AU}}, [[Solar System#Jupiter|Jupiter]] {{val|5.5|u=AU}}—so, assuming Betelgeuse occupying the place of the Sun, its photosphere might extend beyond the Jovian orbit, not quite reaching [[Solar System#Saturn|Saturn]] at {{val|9.5|u=AU}}.
[[File:Betelgeuse radio wavelengths.jpg|thumb|right|upright=1.35 |Radio image from 1998 showing the size of Betelgeuse's photosphere (circle) and the effect of convective forces on the star's atmosphere]]

The precise diameter has been hard to define for several reasons:

# Betelgeuse is a pulsating star, so its diameter changes with time;
# The star has no definable "edge" as limb darkening causes the optical emissions to vary in color and decrease the farther one extends out from the center;
# Betelgeuse is surrounded by a circumstellar envelope composed of matter ejected from the star—matter which absorbs and emits light—making it difficult to define the photosphere of the star;<ref name=UCBERKELEY2009/>
# Measurements can be taken at varying [[wavelengths]] within the [[electromagnetic spectrum]] and the difference in reported diameters can be as much as 30–35%, yet comparing one finding with another is difficult as the star's apparent size differs depending on the wavelength used.<ref name=UCBERKELEY2009/> Studies have shown that the measured angular diameter is considerably larger at ultraviolet wavelengths, decreases through the visible to a minimum in the near-infrared, and increase again in the mid-infrared spectrum;<ref name=GILLILAND1/><ref name=PERRIN2>
{{cite journal
 |last1=Perrin |first1=G. |last2=Ridgway |first2=S.T.
 |last3=Coudé du Foresto |first3=V. |last4=Mennesson |first4=B.
 |last5=Traub |first5=W.A. |last6=Lacasse |first6=M.G.
 |year=2004
 |title=Interferometric observations of the supergiant stars {{nobr|{{mvar|α}} Orionis}} and {{nobr|{{mvar|α}} Herculis}} with FLUOR at IOTA
 |journal=[[Astronomy and Astrophysics]]
 |volume=418 |issue=2 |pages=675–685
 |doi=10.1051/0004-6361:20040052
 |arxiv=astro-ph/0402099  |s2cid=119065851
 |bibcode=2004A&A...418..675P
 |quote=Assuming a distance of {{val|197|45|u=pc}}, an angular distance of {{val|43.33|0.04|u=mas}} would equate to a radius of {{val|4.3|u=AU}} or {{solar radius|920}}
}}
</ref><ref name=YOUNG>
{{cite press release
 | last=Young  | first=John
 | date=24 November 2006
 | title=Surface imaging of Betelgeuse with COAST and the WHT
 | publisher=[[University of Cambridge]]
 | quote=Images of hotspots on the surface of Betelgeuse taken at visible and infra-red wavelengths using high resolution ground-based [[astronomical interferometer|interferometers]]
 | url=http://www.mrao.cam.ac.uk/telescopes/coast/betel.html
 | access-date=21 June 2007
 | url-status=dead
 | archive-url=https://web.archive.org/web/20070614111315/http://www.mrao.cam.ac.uk/telescopes/coast/betel.html
 | archive-date=14 June 2007
 | df=dmy-all
}}
</ref>
# [[Scintillation (astronomy)|Atmospheric twinkling]] limits the resolution obtainable from ground-based telescopes since turbulence degrades angular resolution.<ref name=BUSCHER/>

The generally reported radii of large cool stars are [[Rosseland optical depth|Rosseland radii]], defined as the radius of the photosphere at a specific optical depth of two-thirds. This corresponds to the radius calculated from the effective temperature and bolometric luminosity. The Rosseland radius differs from directly measured radii, with corrections for [[limb darkening]] and the observation wavelength.<ref name=dyck>{{cite journal |doi=10.1086/300453 |title=Radii and effective temperatures for K and M&nbsp;giants and supergiants. II |journal=The Astronomical Journal |volume=116 |issue=2 |page=981 |year=1998 |last1=Dyck |first1=H.M. |last2=van&nbsp;Belle |first2=G.T. |last3=Thompson |first3=R.R. |bibcode=1998AJ....116..981D |citeseerx=10.1.1.24.1889 |s2cid=16674990 }}</ref> For example, a measured angular diameter of 55.6&nbsp;[[milliarcsecond|mas]] would correspond to a Rosseland mean diameter of 56.2&nbsp;mas, while further corrections for the existence of surrounding dust and gas shells would give a diameter of {{val|41.9|u=mas}}.<ref name=dolan2016/>

To overcome these challenges, researchers have employed various solutions. Astronomical interferometry, first conceived by [[Hippolyte Fizeau]] in 1868, was the seminal concept that has enabled major improvements in modern telescopy and led to the creation of the [[Michelson interferometer]] in the 1880s, and the first successful measurement of Betelgeuse.<ref name=PERRIN1>
{{cite journal
 | last1=Perrin | first1=Guy | last2=Malbet | first2=Fabien
 | title=Observing with the VLTI
 | journal=EAS Publications Series
 | year=2003
 | volume= 6
 | bibcode=2003EAS.....6D...3P
 | page=3
 | doi=10.1051/eas/20030601| doi-access=free
 }}
</ref>
Just as human [[depth perception]] increases when two eyes instead of one perceive an object, Fizeau proposed the observation of stars through two [[apertures]] instead of one to obtain [[Interference (wave propagation)|interferences]] that would furnish information on the star's spatial intensity distribution. The science evolved quickly and multiple-aperture interferometers are now used to capture [[Speckle imaging|speckled images]], which are synthesized using [[Fourier analysis]] to produce a portrait of high resolution.<ref name=APOD1>
{{cite APOD
 | title=3 ATs
 | date=21 April 2012
 | access-date=17 August 2012
 | quote=Photograph showing three of the four enclosures which house 1.8&nbsp;meter Auxiliary Telescopes (ATs) at the [[Paranal Observatory]] in the [[Atacama Desert]] region of [[Chile]].}}</ref> It was this methodology that identified the hotspots on Betelgeuse in the 1990s.<ref name=WORDEN>
{{cite magazine
 | last=Worden | first=S.
 | title=Speckle Interferometry
 | magazine=[[New Scientist]]
 | year=1978
 | volume=78
 | pages=238–40
 | bibcode=1978NewSc..78..238W
}}
</ref>
Other technological breakthroughs include [[adaptive optics]],<ref name=RODDIER1>
{{cite conference
 | last=Roddier | first=F.
 | year=1999
 | title=Ground-based interferometry with adaptive optics
 | book-title=Working on the Fringe: Optical and IR interferometry from ground and space 
 | series=ASP Conference
 | volume=194 | page=318
 | publisher = [[Astronomical Society of the Pacific]]
 | isbn=978-1-58381-020-0
 | bibcode=1999ASPC..194..318R
}}
</ref> [[Space observatory|space observatories]] like Hipparcos, [[Hubble Space Telescope|Hubble]] and [[Spitzer Space Telescope|Spitzer]],<ref name=GILLILAND1/><ref name=NASA1>
{{cite press release
 | title=Top five breakthroughs from Hubble's workhorse camera
 | date=4 May 2009
 | series=[[NASA Jet Propulsion Laboratory]]
 | publisher = [[California Institute of Technology]]
 | place = Pasadena, CA
 | url=http://www.jpl.nasa.gov/news/features.cfm?feature=2132
 | access-date=28 August 2007 | url-status=dead
 | archive-url=https://web.archive.org/web/20090507102643/http://www.jpl.nasa.gov/news/features.cfm?feature=2132
 | archive-date=7 May 2009
}}
</ref>
and the [[AMBER (Very Large Telescope)|Astronomical Multi-BEam Recombiner (AMBER)]], which combines the beams of three telescopes simultaneously, allowing researchers to achieve milliarcsecond [[spatial resolution]].<ref name=MELNICK>
{{cite press release
 | last1=Melnick | first1=J. | last2=Petrov | first2=R.
 | last3=Malbet | first3=F.
 | date=23 February 2007
 | url=http://www.eso.org/public/news/eso0706/
 | title=The Sky Through Three Giant Eyes, AMBER Instrument on VLT Delivers a Wealth of Results
 | publisher=[[European Southern Observatory]]
 |access-date=29 August 2007
}}
</ref><ref name=WITTKOWSKI>
{{cite journal
 |last = Wittkowski |first = M.
 |date = 23 February 2007
 |journal = [[New Astronomy Reviews]]
 |title = MIDI and AMBER from the user's point of view
 |volume = 51  |issue = 8–9  |pages = 639–649
 |bibcode = 2007NewAR..51..639W
 |doi= 10.1016/j.newar.2007.04.005
 |url = http://www.vlti.org/events/assets/1/proceedings/2.4_Wittkowski.pdf
 |access-date = 29 August 2007 |url-status = dead
 |archive-url = https://web.archive.org/web/20110728164017/http://www.vlti.org/events/assets/1/proceedings/2.4_Wittkowski.pdf
 |archive-date = 28 July 2011
 |df = dmy-all
}}
</ref>

Observations in different regions of the electromagnetic spectrum—the visible, near-infrared ([[Infrared#Astronomy division scheme|NIR]]), mid-infrared (MIR), or radio—produce very different angular measurements. In 1996, Betelgeuse was shown to have a uniform disk of {{val|56.6|1.0|u=mas}}. In 2000, a [[Space Sciences Laboratory]] team measured a diameter of {{val|54.7|0.3|u=mas}}, ignoring any possible contribution from hotspots, which are less noticeable in the mid-infrared.<ref name="WEINER" /> Also included was a theoretical allowance for limb darkening, yielding a diameter of {{val|55.2|0.5|u=mas}}. The earlier estimate equates to a radius of roughly {{val|5.6|u=AU}} or {{solar radius|1,200}}, assuming the 2008 Harper distance of {{val|197.0|45|u=pc}},<ref name=SMITH2009>
{{cite journal
 |last1=Smith  |first1 = Nathan
 |last2=Hinkle |first2 = Kenneth H.
 |last3=Ryde   |first3 = Nils
 |date=March 2009
 |title=Red supergiants as potential type&nbsp;IIn supernova progenitors: Spatially resolved 4.6&nbsp;μm CO emission around VY&nbsp;CMa and Betelgeuse
 |journal=The Astronomical Journal
 |volume=137 |issue=3 |pages=3558–3573
 |arxiv=0811.3037 |doi=10.1088/0004-6256/137/3/3558
 |bibcode=2009AJ....137.3558S |s2cid=19019913
}}
</ref>
a figure roughly the size of the Jovian orbit of {{val|5.5|u=AU}}.<ref name=ASTRONOMYMAG2009>
{{cite news
 | title=Red giant star Betelgeuse in the constellation Orion is mysteriously shrinking
 | date=June 2009
 | magazine=[[Astronomy Magazine]]
 | url=http://www.astronomy.com/en/News-Observing/News/2009/06/Red%20giant%20star%20Betelgeuse%20in%20the%20constellation%20Orion%20is%20mysteriously%20shrinking.aspx
 | access-date=14 September 2012
}}
</ref><ref name=APOD2>
{{cite APOD
 |title=The spotty surface of Betelgeuse
 |date=6 January 2010
 |access-date=18 July 2010
}}
</ref>

In 2004, a team of astronomers working in the near-infrared announced that the more accurate photospheric measurement was {{val|43.33|0.04|u=mas}}. The study also put forth an explanation as to why varying wavelengths from the visible to mid-infrared produce different diameters: The star is seen through a thick, warm extended atmosphere. At short wavelengths (the visible spectrum) the atmosphere scatters light, thus slightly increasing the star's diameter. At near-infrared wavelengths ([[K band (infrared)|K]] and [[L band (infrared)|L bands]]), the scattering is negligible, so the classical photosphere can be directly seen; in the mid-infrared the scattering increases once more, causing the thermal emission of the warm atmosphere to increase the apparent diameter.<ref name=PERRIN2/>
[[File:Orion's Big Head Revealed in Infrared.jpg|thumb|right|Infrared image of Betelgeuse, [[Meissa]] and [[Bellatrix]] with surrounding [[nebula]]e]]

Studies with the [[Infrared Optical Telescope Array|IOTA]] and VLTI published in 2009 brought strong support to the idea of dust shells and a molecular shell (MOLsphere) around Betelgeuse, and yielded diameters ranging from 42.57 to {{val|44.28|u=mas}} with comparatively insignificant margins of error.<ref name=HAUBOIS>
{{cite journal
 | last1=Haubois  | first1=X.        | last2=Perrin    | first2=G.
 | last3=Lacour   | first3=S.        | last4=Verhoelst | first4=T.
 | last5=Meimon   | first5=S.        | last6=Mugnier   | first6=L.
 | last7=Thiébaut | first7=E.        | last8=Berger    | first8=J.P.
 | last9=Ridgway  | first9=S.T.
 | display-authors=6
 | year=2009
 | title=Imaging the spotty surface of Betelgeuse in the H&nbsp;band
 | journal=[[Astronomy & Astrophysics]]
 | volume=508 | issue=2  | pages=923–32
 | arxiv = 0910.4167 | s2cid=118593802
 | bibcode=2009A&A...508..923H
 | doi=10.1051/0004-6361/200912927
}}
</ref><ref name=HERNANDEZ>
{{cite journal
 | last1=Hernandez Utrera |first1=O.
 | last2=Chelli |first2=A
 | title=Accurate Diameter Measurement of Betelgeuse Using the VLTI/AMBER Instrument
 | journal=Revista Mexicana de Astronomía y Astrofísica, Serie de Conferencias
 | year=2009
 | volume=37
 | pages=179–80
 | url=http://www.astroscu.unam.mx/rmaa/RMxAC..37/PDF/RMxAC..37_ohernandez.pdf
 | bibcode=2009RMxAC..37..179H
 | access-date=16 August 2010
 | archive-date=15 July 2011
 | archive-url=https://web.archive.org/web/20110715041557/http://www.astroscu.unam.mx/rmaa/RMxAC..37/PDF/RMxAC..37_ohernandez.pdf
 | url-status=dead
 }}</ref> In 2011, a third estimate in the near-infrared corroborating the 2009 numbers, this time showing a limb-darkened disk diameter of {{val|42.49|0.06|u=mas}}.<ref name=OHNAKA2011>{{cite journal|doi=10.1051/0004-6361/201016279|title=Imaging the dynamical atmosphere of the red supergiant Betelgeuse in the CO first overtone lines with VLTI/AMBER|journal=Astronomy & Astrophysics |volume=529 |pages=A163 |year=2011 |last1=Ohnaka |first1=K.|last2=Weigelt |first2=G. |last3=Millour |first3=F. |last4=Hofmann |first4=K.-H. |last5=Driebe |first5=T. |last6=Schertl |first6=D. |last7=Chelli |first7=A. |last8=Massi |first8=F. |last9=Petrov |first9=R. |last10=Stee |first10=Ph. |bibcode=2011A&A...529A.163O |arxiv=1104.0958 |s2cid=56281923 }}</ref>{{efn|
"We derive a uniform-disk diameter of {{val|42.05|0.05|u=mas}} and a power-law-type limb-darkened disk diameter of {{val|42.49|0.06|u=mas}} and a limb-darkening parameter of {{val|9.7|0.5|e=−2}}"<ref name=OHNAKA2011/>
}} The near-infrared photospheric diameter of {{val|43.33|u=mas}} at the Hipparcos distance of {{val|152|20|u=pc}} equates to about {{val|3.4|u=AU}} or {{solar radius|730}}.<ref name="KERVELLA2011">{{cite journal|doi=10.1051/0004-6361/201116962|title=The close circumstellar environment of Betelgeuse|journal=Astronomy & Astrophysics |volume=531 |pages=A117 |year=2011 |last1=Kervella |first1=P. |last2=Perrin |first2=G. |last3=Chiavassa |first3=A. |last4=Ridgway |first4=S. T. |last5=Cami |first5=J. |last6=Haubois |first6=X. |last7=Verhoelst |first7=T.|arxiv=1106.5041|s2cid=119190969}}</ref> A 2014 paper derives an angular diameter of {{val|42.28|u=mas}} (equivalent to a {{val|41.01|u=mas}} uniform disc) using H and K band observations made with the VLTI AMBER instrument.<ref name=montarges>{{cite journal |last1=Montargès |first1=M. |last2=Kervella |first2=P. |last3=Perrin |first3=G. |last4=Ohnaka |first4=K. |last5=Chiavassa |first5=A. |last6=Ridgway |first6=S. T. |last7=Lacour |first7=S. |date=2014 |title=Properties of the CO and H2O MOLsphere of the red supergiant Betelgeuse from VLTI/AMBER observations |journal=Astronomy & Astrophysics |volume=572 |pages=id.A17 |bibcode=2014A&A...572A..17M |doi=10.1051/0004-6361/201423538 |arxiv=1408.2994 |s2cid=118419296}}</ref>

In 2009 it was announced that the radius of Betelgeuse had shrunk from 1993 to 2009 by 15%, with the 2008 angular measurement equal to {{val|47.0|u=mas}}.<ref name=TOWNES1/><ref name=COWEN>{{cite web
 | url=http://www.sciencenews.org/view/generic/id/44573/title/Betelgeuse_shrinks
 | title=Betelgeuse Shrinks: The Red Supergiant has Lost 15 Percent of its Size
 | date=10 June 2009
 | first=Ron
 | last=Cowen
 | quote=
 | archive-date=29 June 2011
 | archive-url=https://web.archive.org/web/20110629205838/http://www.sciencenews.org/view/generic/id/44573/title/Betelgeuse_shrinks
 | access-date=11 June 2009 | url-status=dead
}}
</ref>{{efn|
The shrinkage corresponds to the star contracting by a distance equal to that between Venus and the Sun, researchers reported June 9 at an American Astronomical Society meeting and in the June&nbsp;1 Astrophysical Journal Letters.<ref name=COWEN/>
}}
Unlike most earlier papers, this study used measurements at one specific wavelength over 15&nbsp;years. The diminution in Betelgeuse's [[apparent size]] equates to a range of values between {{val|56.0|0.1|u=mas}} seen in 1993 to {{val|47.0|0.1|u=mas}} seen in 2008— a contraction of almost {{val|0.9|u=AU}} in {{val|15|u=years}}.<ref name=TOWNES1/> The observed contraction is generally believed to be a variation in just a portion of the extended atmosphere around Betelgeuse, and observations at other wavelengths have shown an increase in diameter over a similar period.<ref name=montarges/>

The latest models of Betelgeuse adopt a photospheric angular diameter of around {{val|43|u=mas}}, with multiple shells out to 50–{{val|60|u=mas}}.<ref name=kervella2018/> Assuming a distance of {{val|197|u=pc}}, this means a stellar diameter of {{val|887|203|u=R_solar}}.<ref name=dolan2016/>

Once considered as having the largest angular diameter of any star in the sky after the [[Sun]], Betelgeuse lost that distinction in 1997 when a group of astronomers measured [[R Doradus]] with a diameter of {{val|57.0|0.5|u=mas}}, although R Doradus, being much closer to Earth at about {{val|200|u=ly}}, has a linear diameter roughly one-third that of Betelgeuse.<ref name=BEDDING>
{{cite journal
 | last1=Bedding |first1=T.R.
 | last2=Zijlstra  | first2=A.A.  | last3=von der Luhe  | first3=O.
 | last4=Robertson | first4=J.G.  | last5=Marson | first5=R.G.
 | last6=Barton    | first6=J.R.  | last7=Carter | first7=B.S.
 | year=1997
 | title=The angular diameter of R&nbsp;Doradus: A nearby Mira-like star
 | journal=[[Monthly Notices of the Royal Astronomical Society]]
 | volume=286  | issue=4  | pages=957–62
 | bibcode=1997MNRAS.286..957B | s2cid=15438522
 | arxiv = astro-ph/9701021
 | doi=10.1093/mnras/286.4.957  | doi-access=free
}}
</ref>

===Occultations===
[[File:Leona Betelgeuse.png|thumb|left|upright=1.35|Predicted path using SOLEX]]
Betelgeuse is too far from the ecliptic to be occulted by the major planets, but occultations by some [[asteroid]]s (which are more wide-ranging and much more numerous) occur frequently. A partial occultation by the 19th magnitude asteroid {{mp|(147857) 2005 UW|381}} occurred on 2 January 2012. It was partial because the angular diameter of the star was larger than that of the asteroid; the brightness of Betelgeuse dropped by only about 0.01 magnitudes.<ref name="occult">{{Cite web |last=Denissenko |first=Denis |date=3 October 2004 |title=Unique occultations |url=http://hea.iki.rssi.ru/~denis/special.html |archive-url=https://web.archive.org/web/20121216061951/http://hea.iki.rssi.ru/~denis/special.html |archive-date=16 December 2012}}</ref><ref>{{cite book |last=Hanslmeier |first=Arnold |title=Introduction to Astronomy and Astrophysics |year=2023 |isbn=978-3-662-64636-6 |pages=303–335 |chapter=State Variables of Stars |doi=10.1007/978-3-662-64637-3_8}}</ref>

The 14th magnitude asteroid [[319 Leona]] was predicted to occult on 12 December 2023, 01:12 UTC.<ref name="AT-20231209">{{cite news |last=Sigismondi |first=Costantino |date=9 December 2023 |title=The occultation of Betelgeuse by Leona: recovering the stellar surface brightness of a red supergiant, with a diffuse telescope, on Dec 12 1:12 UT |url=https://www.astronomerstelegram.org/?read=16374 |work=[[The Astronomer's Telegram]] |url-status=live |archiveurl=https://archive.today/20231212022447/https://www.astronomerstelegram.org/?read=16374 |archivedate=12 December 2023 |accessdate=11 December 2023 }}</ref> Totality was at first uncertain, and the occulation was projected to only last approximately twelve seconds (visible on a narrow path on Earth's surface, the exact width and location of which was initially uncertain due to lack of precise knowledge of the size and path of the asteroid).<ref>{{cite journal |last=Sigismondi |first=Costantino |year=2020 |title=The partial asteroidal occultation of Betelgeuse on Jan 2, 2012 |journal=Gerbertvs |volume=13 |page=25 |arxiv=1112.6398 |bibcode=2020Gerb...13...25S}}</ref> Projections were later refined as more data were analyzed for<ref>{{Cite web |title=IOTA-ES |url=https://www.iota-es.de/betelgeuse2023.html |access-date=2023-12-08 |website=www.iota-es.de}}</ref> a totality ("ring of fire") of approximately five seconds and a 60&nbsp;km wide path stretching from Tajikistan, Armenia, Turkey, Greece, Italy, Spain, the Atlantic Ocean, Miami, Florida and the [[Florida Keys]] to parts of Mexico.<ref>{{Cite news|url=https://www.theguardian.com/science/2023/dec/08/eclipse-betelgeuse-star-asteroid|title=Astronomers brace for rare eclipse as asteroid to pass in front of bright star|agency=Associated Press|date=8 December 2023|newspaper=The Guardian}}</ref> (The serendiptous event would also afford detailed observations of 319 Leona itself.)<ref>{{cite web |last=Hernandez |first=Joe |date=December 10, 2023 |title=A massive star called Betelgeuse will be briefly obscured by an asteroid Monday night |publisher=NPR |url=https://www.npr.org/2023/12/10/1218448190/a-massive-star-called-betelgeuse-will-be-briefly-obscured-by-an-asteroid-monday-}}</ref> Among other programmes 80 [[citizen science|amateur astronomers]] in Europe alone have been coordinated by astrophysicist [[Miguel Montargès]], et al. of the [[Paris Observatory]] for the event.<ref>{{cite web |last=Guenot |first=Marianne |date=Dec 7, 2023 |title=Betelgeuse, one of the brightest stars in the sky, will almost disappear next week. Here's how to see it. |website=[[MSN]] |url=https://www.msn.com/en-us/travel/news/betelgeuse-one-of-the-brightest-stars-in-the-sky-will-almost-disappear-next-week-heres-how-to-see-it/ar-AA1l9jJK |archive-url=https://web.archive.org/web/20231227010556/https://www.msn.com/en-us/travel/news/betelgeuse-one-of-the-brightest-stars-in-the-sky-will-almost-disappear-next-week-heres-how-to-see-it/ar-AA1l9jJK |archive-date=2023-12-27 |url-status=live}}</ref>