Editing Betelgeuse (section)

===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>