Editing Betelgeuse (section)

== Observational history ==
Betelgeuse and its red coloration have been noted since [[Classical antiquity|antiquity]]; the classical astronomer [[Ptolemy]] described its color as {{Transliteration|grc|hypókirrhos}} ({{wikt-lang|grc|ὑπόκιρρος}}, 'more or less orange-tawny'), a term later described by a translator of [[Ulugh Beg]]'s ''[[Zij-i Sultani]]'' as {{wikt-lang|la|rubedo}}, [[Latin]] for 'ruddiness'.<ref name=allen/>{{efn|
''Stella lucida in umero dextro, quae ad rubedinem vergit.''<ref name=allen/>
:
"Bright star in right shoulder, which inclines to ruddiness."
}} In the 19th century, before modern systems of [[stellar classification]], [[Angelo Secchi]] included Betelgeuse as one of the prototypes for his [[Secchi classes|Class&nbsp;III]] (orange to red) stars.<ref>
{{cite conference
 | last=Brück | first=H. A.
 | date=11–15 July 1978
 | title=P. Angelo Secchi, S.J. 1818–1878
 | conference=Spectral Classification of the Future
 | editor1-last=McCarthy | editor1-first=M.F. 
 | editor2-last=Philip | editor2-first=A.G.D.
 | editor3-last=Coyne | editor3-first=G.V. 
 | book-title=Proceedings of the IAU Colloquium&nbsp;47
 | pages=7–20
 | publication-date=1979
 | location=Vatican City, IT
 | bibcode=1979RA......9....7B
}}</ref> Three centuries before Ptolemy, in contrast, Chinese astronomers observed Betelgeuse as yellow; such an observation, if accurate, could suggest the star was in a [[yellow supergiant]] phase around this time,<ref name=NEWSCIENTIST1>{{cite magazine
 | title=Ancient Chinese suggest Betelgeuse is a young star
 | date=22 October 1981
 | via=Reed Business Information
 | magazine=[[New Scientist]]
 | volume=92
 | issue=1276
 | page=238
 | url=https://books.google.com/books?id=L4NTyHivbV8C&pg=PA238
 }}{{Dead link|date=April 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref name=Neuhauser2022>{{Cite journal |last1=Neuhäuser |first1=R |last2=Torres |first2=G |last3=Mugrauer |first3=M |last4=Neuhäuser |first4=D L |last5=Chapman |first5=J |last6=Luge |first6=D |last7=Cosci |first7=M |date=October 2022 |title=Colour evolution of Betelgeuse and Antares over two millennia, derived from historical records, as a new constraint on mass and age |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=516 |issue=1 |pages=693–719 |bibcode=2022MNRAS.516..693N |doi=10.1093/mnras/stac1969 |issn=0035-8711 |doi-access=free |arxiv=2207.04702 |hdl-access=free |hdl=10278/5003332}}</ref> a credible possibility, given current research into these stars' complex circumstellar environment.<ref name=LEVESQUE1/>

=== Nascent discoveries ===
Aboriginal groups in South Australia have shared oral tales of the variable brightness of Betelgeuse for an unknown period.<ref>
{{cite news
 |last1=Boutsalis |first1=Kelly
 |date=10 August 2020
 |title=Teaching indigenous star stories
 |website=The Walrus
 |url=https://thewalrus.ca/space-teaching-indigenous-star-stories/
 |access-date=6 July 2021
}}
</ref><ref>
{{cite journal
 |last1=Hamacher |first1=Duane W.
 |year=2018
 |title=Observations of red-giant variable stars by Aboriginal Australians
 |journal=[[The Australian Journal of Anthropology]]
 |volume=29 |page=89
 |doi=10.1111/taja.12257 |arxiv=1709.04634
 |s2cid=119453488 |bibcode=2018AuJAn..29...89H
}}
</ref>

[[File:John Herschel 1846 (cropped).png|thumb|left|upright=0.8| [[John Herschel|Sir&nbsp;John Herschel]] in 1846]]
The variation in Betelgeuse's brightness was described in 1836 by [[John Herschel|Sir John Herschel]] in ''Outlines of Astronomy''. From 1836 to 1840, he noticed significant changes in magnitude when Betelgeuse outshone [[Rigel]] in October 1837 and again in November 1839.<ref name=wilk99/> A 10-year quiescent period followed; then in 1849, Herschel noted another short cycle of variability, which peaked in 1852. Later observers recorded unusually high [[Maxima and minima|maxima]] with an interval of years, but only small variations from 1957 to 1967. The records of the [[American Association of Variable Star Observers]] (AAVSO) show a maximum [[apparent magnitude|brightness]] of 0.2 in 1933 and 1942, and a minimum of 1.2, observed in 1927 and 1941.<ref name="AAVSO">
{{cite web
 | last=Davis |first=Kate
 | date=December 2000
 | title=Variable star of the month: Alpha Orionis
 | publisher=[[American Association of Variable Star Observers]] (AAVSO)
 | url=http://www.aavso.org/vsots_alphaori
 | access-date=10 July 2010
}}
</ref><ref name=BURNHAM>
{{cite book
 | first=Robert Jr. | last=Burnham | author-link=Robert Burnham Jr.
 | year=1978
 | title=Burnham's Celestial Handbook: An observer's guide to the universe beyond the Solar system
 | volume=2
 | publisher=Courier Dover Publications
 | location=New York, NY
 | isbn=978-0-486-23568-4
 | url=https://archive.org/details/burnhamscelestia02robe
 | url-access=registration
 | page=[https://archive.org/details/burnhamscelestia02robe/page/1290 1290]
}}
</ref> This variability in brightness may explain why [[Johann Bayer]], with the publication of his ''[[Uranometria]]'' in 1603, designated the star ''alpha'', as it probably rivaled the usually brighter Rigel (''beta'').<ref name="100greatest">
{{cite book
 | last=Kaler |first= James B.
 | year=2002
 | title=The Hundred Greatest Stars
 | publisher=Copernicus Books
 | location=New York, NY
 | isbn=978-0-387-95436-3
 | page=33
}}
</ref> From Arctic latitudes, Betelgeuse's red colour and higher location in the sky than Rigel meant the [[Inuit]] regarded it as brighter, and one local name was ''Ulluriajjuaq'' ("large star").<ref name=inuit>
{{cite book
 |last = MacDonald |first = John
 |year = 1998
 |title = The Arctic sky: Inuit astronomy, star lore, and legend
 |publisher = Royal Ontario Museum / Nunavut Research Institute
 |location = Toronto, Ontario / Iqaluit, NWT
 |isbn = 978-0-88854-427-8
 |url = https://archive.org/details/arcticskyinuitas0000macd/page/52
 |url-access = registration
 |pages = [https://archive.org/details/arcticskyinuitas0000macd/page/52 52–54, 119]
}}
</ref>

In 1920, [[Albert A. Michelson]] and [[Francis G. Pease]] mounted a six-meter [[Interferometry|interferometer]] on the front of the 2.5-meter telescope at [[Mount Wilson Observatory]], helped by [[John August Anderson]]. The trio measured the angular diameter of Betelgeuse at 0.047[[arcsecond|″]], a figure that resulted in a diameter of {{val|3.84|e=8|u=km}} ({{val|2.58|ul=AU}}) based on the [[stellar parallax|parallax]] value of {{val|0.018|ul="}}.<ref name=MICHELSON>
{{cite journal
 |last1=Michelson |first1=A.A. |author1-link=Albert Abraham Michelson
 |last2=Pease |first2=F.G. |author2-link=Francis G. Pease
 |year=1921
 |title=Measurement of the diameter of Alpha Orionis with the interferometer
 |journal=[[Astrophysical Journal]]
 |volume=53 |issue=5 |pages=249–259
 |bibcode=1921ApJ....53..249M
 |doi=10.1086/142603 |pmid=16586823
 |pmc=1084808 |s2cid=21969744
|quote=The 0.047&nbsp;arcsecond measurement was for a uniform disk. In the article Michelson notes that limb darkening would increase the angular diameter by about 17%, hence 0.055&nbsp;arcseconds.
}}
</ref> But limb darkening and measurement errors resulted in uncertainty about the accuracy of these measurements.

The 1950s and 1960s saw two developments that affected stellar [[convection]] theory in red supergiants: the [[Stratoscope]] projects and the 1958 publication of ''Structure and Evolution of the Stars'', principally the work of [[Martin Schwarzschild]] and his colleague at [[Princeton University]], Richard Härm.<ref name=BRUCEMEDAL>
{{cite web
 | last=Tenn |first = Joseph S.
 | date=June 2009
 | title=Martin Schwarzschild 1965
 | series=The Bruce Medalists
 | publisher=[[Astronomical Society of the Pacific]] (ASP)
 | url=http://www.phys-astro.sonoma.edu/BruceMedalists/
 | access-date=28 September 2010
}}
</ref><ref name=SCHWARZSCHILD1958>
{{cite book
 | last=Schwarzschild |first=M. |author-link=Martin Schwarzschild
 | year=1958
 | title=Structure and Evolution of the Stars
 | publisher=Princeton University Press
 | bibcode=1958ses..book.....S
 | isbn=978-0-486-61479-3
 | url=https://archive.org/details/StructureAndEvolutionOfTheStars
 | url-access=registration
}}
</ref>
This book disseminated ideas on how to apply computer technologies to create stellar models, while the Stratoscope projects, by taking balloon-borne telescopes above the Earth's [[Wave turbulence|turbulence]], produced some of the finest images of [[solar granule]]s and [[sunspot]]s ever seen, thus confirming the existence of convection in the solar atmosphere.<ref name=BRUCEMEDAL/>
{{clear}}

=== Imaging breakthroughs ===
[[File:Betelgeuse pulsating UV (HST).jpg|thumb|right|1998/9 [[ultraviolet|UV]] [[Hubble Space Telescope|HST]] images of Betelgeuse showing asymmetrical pulsations with corresponding [[spectral line]] profiles]]
Astronomers saw some major advances in astronomical imaging technology in the 1970s, beginning with [[Antoine Émile Henry Labeyrie|Antoine Labeyrie]]'s invention of [[speckle interferometry]], a process that significantly reduced the blurring effect caused by [[astronomical seeing]]. It increased the [[optical resolution]] of ground-based [[telescope]]s, allowing for more precise measurements of Betelgeuse's photosphere.<ref name=LABEYRIE1970>
{{cite journal
 | last=Labeyrie |first=A. | author-link=Antoine Émile Henry Labeyrie
 | date=May 1970
 | title=Attainment of diffraction-limited resolution in large telescopes by Fourier analysing speckle patterns in star images
 | journal=[[Astronomy and Astrophysics]]
 | volume=6 | page=85
 | bibcode=1970A&A.....6...85L
}}
</ref><ref name=BONNEAU1973>
{{cite journal
 | last1=Bonneau |first1=D.
 | last2=Labeyrie |first2=A. | author2-link=Antoine Émile Henry Labeyrie
 | year=1973
 | title=Speckle interferometry: Color-dependent limb darkening evidenced on Alpha Orionis and Omicron Ceti
 | journal=[[Astrophysical Journal]]
 | volume= 181 | page=L1
 | bibcode=1973ApJ...181L...1B
 | doi=10.1086/181171
}}
</ref> With improvements in [[infrared telescopy]] atop [[Mount Wilson Observatory|Mount Wilson]], [[McDonald Observatory|Mount Locke]], and [[Mauna Kea Observatories|Mauna Kea]] in Hawaii, astrophysicists began peering into the complex circumstellar shells surrounding the supergiant,<ref name=SUTTON1977>
{{cite journal
 | last1=Sutton |first1=E.C. | last2=Storey |first2=J.W.V.
 | last3=Betz |first3=A.L. | last4=Townes |first4=C.H.
 | last5=Spears |first5=D.L.
 | year=1977
 | title=Spatial heterodyne tnterferometry of VY&nbsp;Canis Majoris, Alpha Orionis, Alpha Scorpii, and R&nbsp;Leonis at 11&nbsp;microns
 | journal=[[Astrophysical Journal Letters]]
 | volume=217 | pages=L97–L100
 | doi=10.1086/182547 | bibcode=1977ApJ...217L..97S
| doi-access=free
 }}
</ref><ref name=BERNAT1975>
{{cite journal
 | last1=Bernat |first1=A.P.
 | last2=Lambert |first2=D.L.
 | date=November 1975
 | title=Observations of the circumstellar gas shells around Betelgeuse and Antares
 | journal=[[Astrophysical Journal]]
 | volume=201 | pages=L153–L156
 | doi=10.1086/181964 | bibcode=1975ApJ...201L.153B
| doi-access=free
 }}
</ref><ref name=DYCK1975>
{{cite journal
 | last1=Dyck |first1=H.M.
 | last2=Simon |first2=T.
 | date=February 1975
 | title=Circumstellar dust shell models for Alpha Orionis
 | journal=[[Astrophysical Journal]]
 | volume=195 | pages=689–693
 | doi=10.1086/153369 | bibcode=1975ApJ...195..689D
}}
</ref> causing them to suspect the presence of huge gas bubbles resulting from convection.<ref name=BOESGAARD1975>
{{cite journal
 | last1=Boesgaard |first1=A.M.
 | last2=Magnan |first2=C.
 | date=June 1975
 | title=The circumstellar shell of alpha Orionis from a study of the Fe&nbsp;II emission lines
 | journal=[[Astrophysical Journal]]
 | volume=198 | issue=1 | pages=369–371, 373–378
 | doi=10.1086/153612 | bibcode=1975ApJ...198..369B
}}
</ref> However, it was not until the late 1980s and early 1990s, when Betelgeuse became a regular target for [[aperture masking interferometry]], that breakthroughs occurred in visible-light and [[Infrared photography|infrared imaging]]. Pioneered by [[John E. Baldwin|J.E.&nbsp;Baldwin]] and colleagues of the [[Cavendish Astrophysics Group]], the new technique employed a small mask with several holes in the telescope pupil plane, converting the [[aperture]] into an ad hoc interferometric array.<ref name=BERNAT2008>
{{cite web
 | last=Bernat |first=David
 | year=2008
 | title=Aperture masking interferometry
 | website=Ask an Astronomer
 | publisher=[[Cornell University]]
 | series=Astronomy department
 | url=http://astro.cornell.edu/~dbernat/apm.html#
 | access-date=15 October 2012
}}
</ref> The technique contributed some of the most accurate measurements of Betelgeuse while revealing bright spots on the star's photosphere.<ref name=BUSCHER>
{{cite journal
 | last1=Buscher |first1=D.F. | last2=Baldwin |first2=J.E.
 | last3=Warner |first3=P.J. | last4=Haniff |first4=C.A.
 | year=1990
 | title=Detection of a bright feature on the surface of Betelgeuse
 | journal=[[Monthly Notices of the Royal Astronomical Society]]
 | volume=245 | page=7
 | bibcode=1990MNRAS.245P...7B
}}
</ref><ref name=WILSON1>
{{cite journal
 | last1=Wilson |first1=R.W.
 | last2=Dhillon |first2=V.S.
 | last3=Haniff |first3=C.A.
 | year=1997
 | title=The changing face of Betelgeuse
 | journal=[[Monthly Notices of the Royal Astronomical Society]]
 | volume=291 | issue=4 | page=819
 | bibcode=1997MNRAS.291..819W
 | doi=10.1093/mnras/291.4.819 | doi-access=free
 }}
</ref><ref name=BURNS>
{{cite journal
 | last1=Burns | first1=D. | last2=Baldwin | first2=J.E.
 | last3=Boysen | first3=R.C. | last4=Haniff | first4=C.A.
 | last5=Lawson | first5=P.R. | last6=MacKay | first6=C.D.
 | last7=Rogers | first7=J. | last8=Scott | first8=T.R.
 | last9=Warner | first9=P.J.
 | display-authors=6
 | date=September 1997
 | title=The surface structure and limb-darkening profile of Betelgeuse
 | journal=[[Monthly Notices of the Royal Astronomical Society]]
 | volume=290 | issue=1 | pages=L11–L16
 | bibcode=1997MNRAS.290L..11B
 | doi=10.1093/mnras/290.1.l11 | doi-access=free
}}
</ref> These were the first optical and infrared images of a stellar disk other than the [[Sun]], taken first from ground-based interferometers and later from higher-resolution observations of the [[Cambridge Optical Aperture Synthesis Telescope|COAST telescope]]. The "bright patches" or "hotspots" observed with these instruments appeared to corroborate a theory put forth by Schwarzschild decades earlier of massive [[Convection zone|convection]] cells dominating the stellar surface.<ref name=TUTHILL>
{{cite journal
 | last1=Tuthill | first1=P.G.
 | last2=Haniff | first2=C.A.
 | last3=Baldwin | first3=J.E.
 | date=March 1997
 | title=Hotspots on late-type supergiants
 | journal=Monthly Notices of the Royal Astronomical Society
 | volume=285 | issue=3 | pages=529–39
 | bibcode=1997MNRAS.285..529T
 | doi=10.1093/mnras/285.3.529 | doi-access=free
}}
</ref><ref name=SCHWARZSCHILD1975>
{{cite journal
 | last=Schwarzschild | first=M. |author-link=Martin Schwarzschild
 | year=1975
 | title=On the scale of photospheric convection in red giants and supergiants
 | journal=[[Astrophysical Journal]]
 | volume=195 | issue=1 | pages=137–44
 | bibcode=1975ApJ...195..137S | doi=10.1086/153313
| doi-access=free}}
</ref>

In 1995, the [[Hubble Space Telescope]]'s [[Faint Object Camera]] captured an [[Ultraviolet astronomy|ultraviolet image]] with a resolution superior to that obtained by ground-based interferometers—the first conventional-telescope image (or "direct-image" in NASA terminology) of the disk of another star.<ref name=GILLILAND1>
{{cite journal
 | last1=Gilliland | first1=Ronald L.
 | last2=Dupree | first2=Andrea K.
 | date=May 1996
 | title=First image of the surface of a star with the Hubble Space Telescope
 | journal=[[Astrophysical Journal Letters]]
 | volume=463 | issue=1 | pages=L29
 | bibcode=1996ApJ...463L..29G
 | doi=10.1086/310043 | doi-access=free
 | quote=The yellow/red "image" or "photo" of Betelgeuse commonly seen is not a picture of the red supergiant, but a mathematically generated image based on the photograph. The photograph was of much lower resolution: The entire Betelgeuse image fit within a 10×10 pixel area on the [[Hubble Space Telescope]]s [[Faint Object Camera]]. The images were oversampled by a factor of 5 with bicubic spline interpolation, then deconvolved.
}}
</ref>
Because [[ultraviolet]] light is absorbed by the [[Atmosphere of Earth|Earth's atmosphere]], observations at these wavelengths are best performed by [[space telescope]]s.<ref name=cox2000>
{{cite book
 | editor=Cox, A.N.
 | title=Allen's Astrophysical Quantities
 | year=2000
 | publisher=Springer-Verlag
 | location=New York, NY
 | isbn=978-0-387-98746-0
}}
</ref>
This image, like earlier pictures, contained a bright patch indicating a region in the southwestern quadrant {{val|2000|ul=K|fmt=commas}} hotter than the stellar surface.<ref>
{{cite book
 | last1=Petersen | first1=Carolyn Collins
 | last2=Brandt | first2=John C.
 | year=1998 | orig-year=1995
 | title=Hubble Vision: Further adventures with the Hubble Space Telescope | edition=2nd
 | publisher=Cambridge University Press
 | location=Cambridge, UK
 | isbn=978-0-521-59291-8
 | url=https://archive.org/details/hubblevisionfurt0000pete
 | url-access=registration
 | pages=[https://archive.org/details/hubblevisionfurt0000pete/page/91 91–92]
}}
</ref>
Subsequent ultraviolet spectra taken with the [[Goddard High Resolution Spectrograph]] suggested that the hot spot was one of Betelgeuse's poles of rotation. This would give the rotational axis an inclination of about 20° to the direction of Earth, and a [[position angle]] from [[Celestial pole|celestial North]] of about 55°.<ref name=UITENBROEK>
{{cite journal
 | last1=Uitenbroek | first1=Han
 | last2=Dupree | first2=Andrea K.
 | last3=Gilliland | first3=Ronald L.
 | year=1998
 | title=Spatially resolved Hubble Space Telescope spectra of the chromosphere of {{nobr|{{mvar|α}} Orionis}}
 | journal=The Astronomical Journal
 | volume=116 | issue=5 | pages=2501–2512
 | doi=10.1086/300596 | bibcode=1998AJ....116.2501U
 | s2cid=117596395
| doi-access=free
 }}
</ref>

=== 2000s studies ===
In a study published in December 2000, the star's diameter was measured with the [[Infrared Spatial Interferometer]] (ISI) at mid-infrared wavelengths producing a limb-darkened estimate of {{val|55.2|0.5|ul=mas}} – a figure entirely consistent with Michelson's findings eighty years earlier.<ref name=MICHELSON/><ref name=WEINER>
{{cite journal
 | last1=Weiner | first1 = J. | last2=Danchi | first2=W.C.
 | last3=Hale | first3=D.D.S. | last4=McMahon | first4=J.
 | last5=Townes | first5=C.H. | last6=Monnier | first6=J.D.
 | last7=Tuthill | first7=P.G.
 | date=December 2000
 | title=Precision measurements of the diameters of {{nobr|{{mvar|α}} Orionis}} and ο&nbsp;Ceti at 11&nbsp;microns
 | journal=The Astrophysical Journal
 | volume=544 | issue=2 | pages=1097–1100
 | bibcode=2000ApJ...544.1097W
 | doi=10.1086/317264 | doi-access=free
}}
</ref>
At the time of its publication, the estimated parallax from the [[Hipparcos]] mission was {{val|7.63|1.64|u=mas}}, yielding an estimated radius for Betelgeuse of {{val|3.6|u=AU}}. However, an infrared interferometric study published in 2009 announced that the star had shrunk by 15% since 1993 at an increasing rate without a significant diminution in magnitude.<ref name=UCBERKELEY2009>
{{cite press release
 | last=Sanders | first=Robert
 | date=9 June 2009
 | title=Red giant star Betelgeuse mysteriously shrinking
 | website=UC Berkeley News
 | publisher=UC Berkeley
 | url=http://www.berkeley.edu/news/media/releases/2009/06/09_betelim.shtml
 | access-date=18 April 2010
}}
</ref><ref name=TOWNES1/>
Subsequent observations suggest that the apparent contraction may be due to shell activity in the star's extended atmosphere.<ref name=RAVI1>
{{cite journal
 | last1=Ravi | first1=V. | last2=Wishnow | first2=E.
 | last3=Lockwood | first3=S. | last4=Townes | first4=C.
 | date=December 2011
 | title=The many faces of Betelgeuse
 | journal=[[Publications of the Astronomical Society of the Pacific]]
 | volume=448 | page=1025
 | bibcode=2011ASPC..448.1025R | arxiv = 1012.0377
}}
</ref>

In addition to the star's diameter, questions have arisen about the complex dynamics of Betelgeuse's extended atmosphere. The mass that makes up galaxies is recycled as [[life cycle of stars|stars are formed and destroyed]], and red supergiants are major contributors, yet the process by which mass is lost remains a mystery.<ref name=BERNAT>
{{cite journal
 | last=Bernat | first=Andrew P.
 | year=1977
 | title=The circumstellar shells and mass-loss rates of four M&nbsp;supergiants
 | journal=[[Astrophysical Journal]]
 | volume=213 | pages=756–66
 | bibcode=1977ApJ...213..756B | doi=10.1086/155205
 | s2cid=121146305
 }}
</ref>
With advances in interferometric methodologies, astronomers may be close to resolving this conundrum. Images released by the [[European Southern Observatory]] in July 2009, taken by the ground-based [[Very Large Telescope]] Interferometer (VLTI), showed a vast plume of gas extending {{val|30|u=AU}} from the star into the surrounding atmosphere.<ref name=KERVELLA2009>
{{cite journal
 | last1=Kervella | first1=P. | last2=Verhoelst | first2=T.
 | last3=Ridgway | first3=S.T. | last4=Perrin | first4=G.
 | last5=Lacour | first5=S. | last6=Cami | first6=J.
 | last7=Haubois | first7=X.
 | date=September 2009
 | title=The close circumstellar environment of Betelgeuse. Adaptive optics spectro-imaging in the near-IR with VLT/NACO
 | journal=Astronomy and Astrophysics
 | volume=504 | issue=1 | pages=115–25
 | bibcode=2009A&A...504..115K | s2cid=14278046
 | doi=10.1051/0004-6361/200912521 |arxiv = 0907.1843
}}
</ref>
This mass ejection was equal to the distance between the Sun and [[Neptune]] and is one of multiple events occurring in Betelgeuse's surrounding atmosphere. Astronomers have identified at least six shells surrounding Betelgeuse. Solving the mystery of mass loss in the late stages of a star's evolution may reveal those factors that precipitate the explosive deaths of these stellar giants.<ref name="UCBERKELEY2009" />

===2019–2020 fading===
[[File:Betelgeuse AAVSO 2019.jpg|thumb|upright=1.4|[[AAVSO]] [[apparent magnitude|V-band magnitude]] of Betelgeuse, between September&nbsp;2016 and August&nbsp;2023]]
[[File:Eso2003c.jpg|thumb|upright=1.4|alt=Two orange blobs side by side on black backgrounds, one caption "Jan&nbsp;2019" and the other captioned "Dec&nbsp;2019"|Comparison of [[SPHERE]] images of Betelgeuse taken in January&nbsp;2019 and December&nbsp;2019, showing the changes in brightness and shape]]

A pulsating [[semiregular variable star]], Betelgeuse is subject to multiple cycles of increasing and decreasing brightness due to changes in its size and temperature.<ref name=dolan2016/> The astronomers who first noted the dimming of Betelgeuse, [[Villanova University]] astronomers Richard Wasatonic and [[Edward Guinan]], and amateur Thomas Calderwood, theorize that a coincidence of a normal 5.9&nbsp;year light-cycle minimum and a deeper-than-normal 425&nbsp;day period are the driving factors.<ref name=AT-20191223>
{{cite news
 |last1=Guinan |first1=Edward F. |author1-link = Edward Guinan
 |last2=Wasatonic |first2=Richard J.
 |last3=Calderwood |first3=Thomas J.
 |date=23 December 2019
 |title=Updates on the "fainting" of Betelgeuse
 |website=[[The Astronomer's Telegram]]
 |id=ATel #13365
 |url=http://www.astronomerstelegram.org/?read=13365
 |access-date=27 December 2019
}}
</ref>
Other possible causes hypothesized by late 2019 were an eruption of gas or dust or fluctuations in the star's surface brightness.<ref name=earthsky>
{{cite web
 |first=Deborah |last=Byrd
 |date=2019-12-23 
 |title=Betelgeuse is 'fainting' but (probably) not about to explode
 |website=Earth & Sky
 |url=https://earthsky.org/space/betelgeuse-fainting-probably-not-about-to-explode
 |access-date=2020-01-04 |df=dmy-all
}}
</ref>

By August&nbsp;2020, long-term and extensive studies of Betelgeuse, primarily using [[Ultraviolet astronomy|ultraviolet observations]] by the [[Hubble Space Telescope]], had suggested that the unexpected dimming was probably caused by an immense amount of superhot material ejected into space. The material cooled and formed a dust cloud that blocked the starlight coming from about a quarter of Betelgeuse's surface. Hubble captured signs of dense, heated material moving through the star's atmosphere in September, October and November before several telescopes observed the more marked dimming in December and the first few months of 2020.<ref name=NYT-20200814>
{{cite news
 |last=Overbye |first=Dennis |author-link=Dennis Overbye
 |date=14 August 2020
 |title=This star looked like it would explode. Maybe it just sneezed
 |newspaper=[[The New York Times]]
 |url=https://www.nytimes.com/2020/08/14/science/betelgeuse-star-supernova.html
 |access-date=15 August 2020
 |quote=The mysterious dimming of the red supergiant Betelgeuse is the result of a stellar exhalation, astronomers say.
}}
</ref><ref>
{{cite press release
 |title=Hubble finds that Betegeuse's mysterious dimming is due to a traumatic outburst
 |date=13 August 2020 
 |publisher=[[Hubble Space Telescope]]
 |url=https://hubblesite.org/contents/news-releases/2020/news-2020-44
}}
</ref><ref name=APJ-20200813>
{{cite journal 
 |last1=Dupree | first1=Adrea K. 
 |display-authors=etal 
 |date=13 August 2020 
 |title=Spatially resolved ultraviolet spectroscopy of the great dimming of Betelgeuse 
 |journal=[[The Astrophysical Journal]]
 |volume=899 |number=1 |page=68
 |doi=10.3847/1538-4357/aba516 |arxiv=2008.04945
 |bibcode=2020ApJ...899...68D |s2cid=221103735
|doi-access=free 
 }}
</ref>

By January&nbsp;2020, Betelgeuse had dimmed by a factor of approximately 2.5 from magnitude&nbsp;0.5 to 1.5 and was reported still fainter in February in ''[[The Astronomer's Telegram]]'' at a record minimum of +1.614, noting that the star is currently the "least luminous and coolest" in the 25&nbsp;years of their studies and also calculating a decrease in radius.<ref name=AT-20200201>
{{cite news
 |last1=Guinan |first1=Edward F.
 |last2=Wasatonic |first2=Richard J.
 |date=1 February 2020
 |title=Betelgeuse Updates – 1 February 2020; 23:20 UT
 |id=ATel #13439
 |website=[[The Astronomer's Telegram]]
 |url=http://www.astronomerstelegram.org/?read=13439
 |access-date=2 February 2020
}}
</ref>
''[[Astronomy (magazine)|Astronomy]]'' magazine described it as a "bizarre dimming",<ref name = Dec2019-AstroMag>
{{cite news
 |first = Erika K. |last = Carlson
 |date = 27 December 2019
 |title = Betelguese's bizarre dimming has astronomers scratching their heads
 |magazine = [[Astronomy (magazine)|Astronomy]]
 |url = http://www.astronomy.com/news/2019/12/betelgueses-bizarre-dimming-has-astronomers-scratching-their-heads
 |access-date = 28 December 2019
}}
</ref>
and popular speculation inferred that this might indicate an imminent [[supernova]].<ref>
{{cite news
 |last=Griffin |first=Andrew
 |date= 29 December 2019
 |title=Betelgeuse: Star is behaving strangely and could be about to explode into a supernova, say astronomers
 |newspaper=[[The Independent]]
 |url=https://www.independent.co.uk/life-style/gadgets-and-tech/news/betelgeuse-star-night-sky-supernova-explosion-a9263626.html
 |access-date=2019-12-30 |df=dmy-all
}}
</ref><ref>
{{cite news
 |last=Mack |first=Erick 
 |date=2019-12-27
 |title=Betelgeuse star acting like it's about to explode, even if the odds say it isn't
 |website=[[CNET]]
 |url=https://www.cnet.com/news/betelgeuse-star-acting-like-its-about-to-explode-even-if-odds-say-its-not/
 |access-date=2019-12-30 |df=dmy-all
}}
</ref>
This dropped Betelgeuse from one of the top 10 [[List of brightest stars|brightest stars in the sky]] to outside the top&nbsp;20,<ref name=AT-20191223/> noticeably dimmer than its near neighbor [[Aldebaran]].<ref name=earthsky/> Mainstream media reports discussed speculation that Betelgeuse might be about to explode as a supernova,<ref name=NG-20191226>
{{cite news
 |last=Drake |first=Nadia |author-link=Nadia Drake
 |date=26 December 2019
 |title=A giant star is acting strange, and astronomers are buzzing
 |website=[[National Geographic Society]]
 |url=https://www.nationalgeographic.com/science/2019/12/betelgeuse-is-acting-strange-astronomers-are-buzzing-about-supernova/
 |archive-url=https://web.archive.org/web/20191226131847/https://www.nationalgeographic.com/science/2019/12/betelgeuse-is-acting-strange-astronomers-are-buzzing-about-supernova/
 |url-status=dead
 |archive-date=26 December 2019
 |access-date=26 December 2019
 |quote=The red giant Betelgeuse is the dimmest seen in years, prompting some speculation that the star is about to explode. Here's what we know.
}}
</ref><ref name = Dec2019-WaPo>
{{cite news
 |first = Sarah |last = Kaplan
 |date = 27 December 2019
 |title = Is Betelgeuse, one of the sky's brightest stars, on the brink of a supernova?
 |newspaper = [[The Washington Post]]
 |url = https://www.washingtonpost.com/science/2019/12/27/is-betelgeuse-one-skys-brightest-stars-brink-supernova/
 |access-date = 28 December 2019
}}
</ref><ref name = Dec2019-ABC>
{{cite news
 |first = Kelsie |last = Iorio
 |date = 27 December 2019
 |title = Is Betelgeuse, the red giant star in the constellation Orion, going to explode?
 |website = [[ABC News (Australia)|ABC News]]
 |place = Australia
 |access-date = 28 December 2019
 |url = https://www.abc.net.au/news/2019-12-27/is-betelgeuse-about-to-explode3f/11828624
}}
</ref><ref name = Dec2019-NYP/>
but astronomers note that the supernova is expected to occur within approximately the next 100,000&nbsp;years and is thus unlikely to be imminent.<ref name=NG-20191226/><ref name = Dec2019-ABC/>

By 17&nbsp;February 2020, Betelgeuse's brightness had remained constant for about 10&nbsp;days, and the star showed signs of rebrightening.<ref>
{{cite news
 |first=Bruce |last=Dorminey
 |date=17 February 2020
 |title=Betelgeuse has finally stopped dimming, says astronomer
 |magazine=[[Forbes]]
 |url=https://www.forbes.com/sites/brucedorminey/2020/02/17/betelgeuse-has-finally-stopped-dimming-says-astronomer/#80156477945b
 |access-date=19 February 2020
}}
</ref>
On 22&nbsp;February 2020, Betelgeuse may have stopped dimming altogether, all but ending the dimming episode.<ref name=AT-20200222>
{{cite news
 |last1=Guinan |first1=Edward |last2=Wasatonic |first2=Richard
 |last3=Calderwood |first3=Thomas |last4=Carona |first4=Donald
 |date=22 February 2020
 |title=The fall and rise in brightness of Betelgeuse
 |id=ATel #13512
 |website=[[The Astronomer's Telegram]]
 |url=http://www.astronomerstelegram.org/?read=13512
 |access-date=22 February 2020
}}
</ref>
On 24&nbsp;February 2020, no significant change in the infrared over the last 50 years was detected; this seemed unrelated to the recent visual fading and suggested that an impending core collapse may be unlikely.<ref name=AT-20200224>
{{cite news
 |last1=Gehrz |first1=R.D.
 |display-authors=etal
 |date=24 February 2020
 |title=Betelgeuse remains steadfast in the infrared
 |website=[[The Astronomer's Telegram]]
 |id=ATel #13518 
 |url=http://www.astronomerstelegram.org/?read=13518
 |access-date=24 February 2020
}}
</ref>
Also on 24&nbsp;February 2020, further studies suggested that occluding "large-grain [[circumstellar dust]]" may be the most likely explanation for the dimming of the star.<ref name=EA-20200306>{{cite press release
 |title=Dimming Betelgeuse likely isn't cold, just dusty, new study shows
 |date=6 March 2020
 |publisher=[[University of Washington]]
 |website=[[EurekAlert!]]
 |url=https://www.eurekalert.org/pub_releases/2020-03/uow-dbl030520.php
 |access-date=6 March 2020
 |archive-date=28 February 2021
 |archive-url=https://web.archive.org/web/20210228020242/https://www.eurekalert.org/pub_releases/2020-03/uow-dbl030520.php
 |url-status=dead
 }}</ref><ref name=ARX-20200224>
{{cite journal
 |last1=Levesque |first1=Emily M.
 |last2=Massey |first2=Philip
 |date=24 February 2020
 |title=Betelgeuse just isn't that cool: Effective temperature alone cannot explain the recent dimming of Betelgeuse
 |journal=The Astrophysical Journal Letters
 |volume=891 |issue=2 |page=L37
 |doi=10.3847/2041-8213/ab7935 |doi-access=free
 |bibcode=2020ApJ...891L..37L |bibcode-access=free
 |arxiv=2002.10463 |s2cid=211296241 
}}
</ref>
A study that uses [[observation]]s at [[Submillimetre astronomy|submillimetre]] wavelengths rules out significant contributions from dust absorption. Instead, large [[starspot]]s appear to be the cause for the dimming.<ref>
{{cite journal
 |last1=Dharmawardena |first1=Thavisha E. |last2=Mairs |first2=Steve
 |last3=Scicluna |first3=Peter |last4=Bell |first4=Graham
 |last5=McDonald |first5=Iain |last6=Menten |first6=Karl
 |last7=Weiss |first7=Axel |last8=Zijlstra |first8=Albert
 |date=2020-06-29 |df=dmy-all
 |title=Betelgeuse fainter in the submillimeter too: An analysis of JCMT and APEX monitoring during the recent optical minimum
 |journal=[[The Astrophysical Journal]]
 |volume=897 |issue=1 |page=L9
 |doi=10.3847/2041-8213/ab9ca6 |issn=2041-8213
 |arxiv=2006.09409 |bibcode=2020ApJ...897L...9D
 |s2cid=219721417
|doi-access=free }}
</ref>
Followup studies, reported on 31&nbsp;March 2020 in ''The Astronomer's Telegram'', found a rapid rise in the brightness of Betelgeuse.<ref name=AT-20200331>
{{cite news
 |last=Sigismondi |first=Costantino
 |date=31 March 2020
 |title=Rapid rising of Betelgeuse's luminosity
 |id=ATel #13601 
 |website=[[The Astronomer's Telegram]]
 |url=http://www.astronomerstelegram.org/?read=13601
 |access-date=1 April 2020
}}
</ref>

Betelgeuse is almost unobservable from the ground between May and August because it is too close to the Sun. Before entering its 2020 [[Conjunction (astronomy)|conjunction]] with the Sun, Betelgeuse had reached a brightness of +0.4&nbsp;. Observations with the [[STEREO|STEREO-A]] spacecraft made in June and July&nbsp;2020 showed that the star had dimmed by 0.5 since the last ground-based observation in April. This is surprising, because a maximum was expected for August/September&nbsp;2020, and the next minimum should occur around April&nbsp;2021. However Betelgeuse's brightness is known to vary irregularly, making predictions difficult. The fading could indicate that another dimming event might occur much earlier than expected.<ref>
{{cite news
 |last1=Dupree |first1=Andrea
 |last2=Guinan |first2=Edward 
 |last3=Thompson |first3=William T. 
 |collaboration=STEREO/SECCHI/HI consortium
 |date=28 July 2020
 |title=Photometry of Betelgeuse with the STEREO Mission while in the glare of the Sun from Earth
 |id=ATel #13901 
 |website=[[Astronomer's Telegram]]
 |url=http://astronomerstelegram.org/?read=13901
 |access-date=28 July 2020
}}
</ref>
On 30&nbsp;August 2020, astronomers reported the detection of a second dust cloud emitted from Betelgeuse, and associated with recent substantial dimming (a secondary minimum on 3&nbsp;August) in luminosity of the star.<ref name=AT-20200830>
{{cite news 
 |last1=Sigismondi |first1=Costantino
 |display-authors=etal
 |date=30 August 2020
 |title=Second dust cloud on Betelgeuse
 |website=[[The Astronomer's Telegram]]
 |id=ATel #13982
 |url=http://www.astronomerstelegram.org/?read=13982
 |access-date=31 August 2020
}}
</ref>

In June 2021, the dust was explained as possibly caused by a cool patch on its photosphere<ref>
{{cite journal
 |last1=Montargès |first1=M.
 |last2=Cannon    |first2=E.
 |last3=Lagadec   |first3=E.
 |display-authors=etal
 |date=16 June 2021
 |title=A dusty veil shading Betelgeuse during its Great Dimming
 |journal=Nature
 |volume=594 |issue=7863 |pages=365–368
 |doi=10.1038/s41586-021-03546-8 |pmid=34135524
 |arxiv=2201.10551 |bibcode=2021Natur.594..365M
 |s2cid=235460928
}}
</ref><ref>
{{cite journal
 |last=Levesque |first=E.
 |date=16 June 2021
 |title=Great dimming of Betelgeuse explained
 |journal=[[Nature (journal)|Nature]]
 |volume=594 |issue=7863 |pages=343–344
 |doi=10.1038/d41586-021-01526-6 |pmid=34135515
 |bibcode=2021Natur.594..343L |s2cid=235459976
}}
</ref><ref>
{{cite journal
 |last=Montargès |first=M. 
 |date=16 June 2021
 |journal=[[Nature (journal)|Nature]]
 |title=Imaging the great dimming of Betelgeuse
 |url=https://astronomycommunity.nature.com/posts/imaging-the-great-dimming-of-betelgeuse
}}
</ref><ref name=NYT-20210617>
{{cite news
 |last=Overbye |first=Dennis |authorlink=Dennis Overbye
 |date=17 June 2021
 |title=Betelgeuse merely burped, astronomers conclude
 |newspaper=[[The New York Times]]
 |url=https://www.nytimes.com/2021/06/17/science/betelgeuse-montarges-star-supernova.html
 |access-date=17 June 2021
 |quote=The dramatic dimming of the red supergiant in 2019 was the product of dust, not a prelude to destruction, a new study has found.
}}
</ref>
and in August a second independent group confirmed these results.<ref>
{{cite journal
 |last1=Alexeeva |first1=Sofya |last2=Zhao |first2=Gang
 |last3=Gao |first3=Dong-Yang |last4=Du |first4=Junju
 |last5=Li |first5=Aigen |last6=Li |first6=Kai
 |last7=Hu |first7=Shaoming
 |date=2021-08-05 |df=dmy-all
 |title=Spectroscopic evidence for a large spot on the dimming Betelgeuse
 |journal=Nature Communications
 |volume=12 |issue=1 |page=4719
 |doi=10.1038/s41467-021-25018-3 |doi-access=free
 |pmid=34354072 |pmc=8342547 |arxiv=2108.03472
 |bibcode=2021NatCo..12.4719A |issn=2041-1723 |language=en
}}
</ref><ref>
{{Cite web
 |last=Harris |first=Margaret
 |date=2021-08-06
 |title=New evidence supports dark-spot theory for Betelgeuse's 'great dimming'
 |website=Physics World
 |url=https://physicsworld.com/new-evidence-supports-dark-spot-theory-for-betelgeuses-great-dimming/
 |access-date=2021-08-07 |df=dmy-all
 |language=en-GB
}}
</ref> The dust is thought to have resulted from the cooling of gas ejected from the star. An August 2022<ref>
{{cite journal
 |last1=Dupree    |first1=Andrea K.   |last2=Strassmeier |first2=Klaus G.
 |last3=Calderwood |first3=Thomas     |last4=Granzer     |first4=Thomas
 |last5=Weber     |first5=Michael     |last6=Kravchenko  |first6=Kateryna
 |last7=Matthews  |first7=Lynn D.     |last8=Montarges   |first8=Miguel
 |last9=Tappin    |first9=James       |last10=Thompson   |first10=William T.
 |display-authors=6
 |date=2022-08-02 |df=dmy-all
 |title=The great dimming of Betelgeuse: A surface mass ejection and its consequences
 |journal=[[The Astrophysical Journal]]
 |volume=936 |issue=1 |page=18
 |doi=10.3847/1538-4357/ac7853 |arxiv=2208.01676
 |bibcode=2022ApJ...936...18D |s2cid=251280168
|doi-access=free }}
</ref><ref>
{{cite web
 |last=Garner |first=Rob
 |date=2020-08-13
 |title=Hubble finds Betelgeuse's mysterious dimming due to traumatic outburst
 |website=[[NASA]]
 |url=http://www.nasa.gov/feature/goddard/2020/hubble-finds-that-betelgeuses-mysterious-dimming-is-due-to-a-traumatic-outburst
 |access-date=2022-08-22 |df=dmy-all
}}
</ref><ref>
{{cite web
 |title=How Betelgeuse blew its top and lost its rhythm
 |date=2022-08-22
 |website=[[Physics World]]
 |url=https://physicsworld.com/how-betelgeuse-blew-its-top-and-lost-its-rhythm/
 |access-date=2022-08-22 |df=dmy-all
 |language=en-GB
}}
</ref>
study using the [[Hubble Space Telescope]] confirmed previous research and suggested the dust could have been created by a surface mass ejection. It conjectured as well that the dimming could have come from a short-term minimum coinciding with a long-term minimum producing a grand minimum, a 416-day cycle and 2010&nbsp;day cycle respectively, a mechanism first suggested by astronomer [[Leo Goldberg|L.&nbsp;Goldberg]].<ref>
{{cite journal
 |last=Goldberg |first=L.
 |date=May 1984
 |title=The variability of alpha Orionis
 |journal=Publications of the Astronomical Society of the Pacific
 |language=en
 |volume=96 |pages=366
 |doi=10.1086/131347 |bibcode=1984PASP...96..366G
 |s2cid=121926262 |issn=0004-6280
|doi-access=free
 }}
</ref>
In April&nbsp;2023, astronomers reported the star reached a peak of 0.0 visual and 0.1 V-band magnitude.<ref name=AT-20230422>
{{cite news
 |last1=Sigismondi |first1=Constantino
 |display-authors=etal
 |date=22 April 2023
 |title=Monitoring Betelgeuse at its brightest
 |id=Atel #16001
 |website=[[The Astronomer's Telegram]]
 |url=https://www.astronomerstelegram.org/?read=16001
 |access-date=22 April 2023
}}
</ref>