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update: (August 2020) With all the newest news about including what the Dr. Becky video discusses as linked in How do magnetic fields mess with astronomers' observations? I'll bet this question can now be answered!

update: (June 2023) (much!) more at Dr. Becky's New study claims Betelgeuse supernova IMMINENT (decades not centuries!) | Night Sky News June 2023 (links to papers & preprints in the notes below the video)


The Astronomers Telegrams #13341 The Fainting of the Nearby Red Supergiant Betelgeuse and #13365 Updates on the “Fainting” of Betelgeuse (both in December 2019) discuss the importance of measuring color temperature and spectral features of the needed observations.

Did this fainting episode show the same kinds of spectral changes behavior as the irregular but more limited variation in brightness, or did it contain a dimming component that affects it differently, for example as intervening material (e.g. dust, debris, broken planets) might do?


Betelgeuse, Betelgeuse, Betelgeuse, there, I said it, now let's wait 700 years to see what happens.

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    $\begingroup$ Fainting? I didn't realize Betelgeuse was blacking out & losing consciousness. I think Dimming would be a better word. ;-) $\endgroup$
    – Fred
    Dec 31, 2019 at 11:40
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    $\begingroup$ @Fred a quick check for "variable star fainting" in scholar.google.com returns many results. It seems to be a well-established term. I don't know if it has a technical definition or if its use is similar to "fading", so I've just asked When we say a variable star is "fainting" does it mean something more or different than "dimming" or "fading"? $\endgroup$
    – uhoh
    Dec 31, 2019 at 14:06
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    $\begingroup$ Newly published Spectroscopic evidence for a large spot on the dimming Betelgeuse open access in Nature may answer this. $\endgroup$
    – uhoh
    Aug 15, 2021 at 1:02
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    $\begingroup$ With my poor neglected body I don't think I've got several decades left to see Betelgeuse go supernova. :-( If it's only going to be a few decades, I might just make it to see Betelgeuse expire before I do. I think it would a show worth watching. At least I got to see some guys walk & drive on the Moon, some decades back. By the way, ... even Dr Becky said "the dimming event" in the video you link to. :-) $\endgroup$
    – Fred
    Jun 18, 2023 at 7:03
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    $\begingroup$ @Fred 1) I guess we're in the same general category. At least we can make "gedankenobservations" by asking questions like "If we could look up and see Betelgeuse go supernova - what would it look like? Who/which observations would notice first? Would there be warnings? Soon enough for news alerts to circle the globe ahead of time?" etc. 2) Oh, thanks! for picking up on Dr. Becky's dimming :-) $\endgroup$
    – uhoh
    Jun 18, 2023 at 7:07

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It's now nearly four years after "the great dimming of Betelgeuze" - and the matter has not yet been finally settled; as such this answer will necessarily include some personal judgement and weighting of the existing literature - and might be proven wrong as research progresses and things evolve.

Most evidence from modelling, radial velocity measurements and spectroscopy seems to suggest that it indeed was (quite a lot) ejected matter from the stellar atmosphere (into the direction of Earth) which cooled and in doing so obstructed the view from Earth onto the hot(ter) surface of Betelgeuze (images by E. Wheatly (STScl) and Montarges et al (2021) respectively):

Outburst of Betelgeuze

Image of Betelgeuze taken with the VLT/SPHERE-ZIMPOS as published in Montarges (2021)

Betelgeuze is known to be a semi-regular variable subject to radial puslations of about 400 days and a longer frequency one of a bit more than 2000 days. These variations can be understood and modelled. These models are e.g. presented in Montarges et al (Nature, 2021) or Macleod et al (ApJ, 2023), and also supported spectroscopically e.g. by Dupree et al (ApJ, 2020), Dupree et al (2022) or Alexeeva et al (Nature Communications, 2021). Prior to the dimming Betelgeuze showed an unusually long period of radial expansion, and subsequent spectroscopy is compatible with ejected matter cooling.

E.g. spatially resolved from Dupree et al (2020) show an asymmetry in the maximum emission at different wavelenth, supposedly indicative of a localized, unusually large outburst:

Off-center flux maxima prior / during the great dimming, Dupree et al (2020)

The radial motion is about anti-cyclic to the brightness of Betelgeuze. It has been unusually long been outward prior to the big dimming (Dupree et al (2022)): Anti-correlation of brightness and radial velocity

Interesting in this context is IMHO also the paper by Neuhäuser et al (MNRAS, 2022) which study the colour of Betelgeuze based on historic records over the last few millenia; even when that is to be treated with caution and has big uncertainties, it hints at a recent redding, and thus also at Betelgeuze being in the beginning of its red giant phase rather than the end - thus making mass ejections as typical to these kind of red giants also a likely explanation.

Studies concerning chemical composition and modelling the pulsations (e.g. Saio et al (2023, in review)) are not really conclusive (e.g. see also this comment by Molnar et al on that preprint) as for the time till supernova the inner chemical composition is important, but only the atmospheric composition is accessible to spectroscopy and pulsation alone not compatible with spectroscopy. Thus even with optimistic assumptions in regard to a late stage of Betelgeuze, an outburst hypothesis is compatible with what is modelled and not an indicator of any other process.

Related sidenote: Notworthy is for all studies, that the mass of Betelgeuze is relatively hard to determine due to its rather big disc size (50mas) compared to its parallax motion (6mas) - that makes parallax measurements inaccurate. Thus luminosity is inaccurate due to distance inaccuracy, and thus also the mass determination. And basically every model requires mass as an important input parameter; this results in masses ranging from 12 to 20 solar masses being compatible with these observations, with the likely masses being somewhere in the middle or slightly above.

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