# Betelgeuse losing brightness **URL:** \ **Category:** Factual Questions\ **Created:** [December 27, 2019, 11:34pm UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478 "2019-12-27T23:34:54Z")\ **Posts on this page:** 20\ **Page:** 10
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [December 24, 2023, 1:58pm UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/182 "2023-12-24T13:58:09Z")
I found multiple records and different types of measurements, but, considering the error bars, “about 1000 Solar radii” is indeed the estimate for the size of Betelgeuse right now. Which makes it a Class M supergiant. You don’t want to be nearby when it blows. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [December 24, 2023, 3:38pm UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/183 "2023-12-24T15:38:43Z")
> [@DPRK](#): > > You don’t want to be nearby when it blows. You don’t want to be nearby when it doesn’t. It is rather bright, plus you might run into Ford Prefect. ---
**Author:** ![PastTense](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pasttense/32/14550_2.png) [@PastTense](https://boards.straightdope.com/u/PastTense)\ **Post date:** [March 18, 2024, 12:08am UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/184 "2024-03-18T00:08:27Z")
> Radio measurements of its changing light suggested it was rotating at [5 kilometers](https://www.aanda.org/articles/aa/full_html/2018/01/aa31761-17/aa31761-17.html) (3.1 miles) per second. > > The big problem with that is that stars of Betelgeuse’s vintage should, theoretically, have a maximum rotation speed at least two orders of magnitude lower. So, astronomers wonder, what the heck gives? > > Well, according to new research, it may have been a big old tricksy-doodle. A team led by astrophysicist Jing-Ze Ma of the Max Planck Institute for Astrophysics in Germany has found that Betelgeuse’s boiling surface could be so riotous that it generates an illusion of fast rotation. > **[Betelgeuse's Wild Surface Seems to Be Baffling Our Telescopes](https://www.sciencealert.com/betelgeuses-wild-surface-seems-to-be-baffling-our-telescopes)** > > There's something peculiar about dying star Betelgeuse. Open access study at: [https://iopscience.iop.org/article/10.3847/2041-8213/ad24fd/pdf](https://iopscience.iop.org/article/10.3847/2041-8213/ad24fd/pdf) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 18, 2024, 12:35am UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/185 "2024-03-18T00:35:49Z")
I hadn’t realized there was anything left unknown about Betelgeuse’s rotation. I had thought that that was pretty well pinned down. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [March 18, 2024, 2:45am UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/186 "2024-03-18T02:45:02Z")
> [@PastTense](#): > > > Radio measurements of its changing light suggested it was rotating at 5 kilometers (3.1 miles) per second. I find it baffling when an author (not @PastTense ) describes a rotation rate as a linear speed. WTF is that supposed to mean? Doubly so for an amorphous gas ball whose constituent regions are all rotating at different rates. ---
**Author:** ![74westy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/74westy/32/3950_2.png) [@74westy](https://boards.straightdope.com/u/74westy)\ **Post date:** [March 18, 2024, 3:42pm UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/187 "2024-03-18T15:42:22Z")
I was puzzled too and, naturally, the pop science article is no use and its author probably didn’t even woder about it. I was therefore forced to read the first paragraph of the study. > [@PastTense's second link](#): > > 1. Introduction > Cool evolved stars are not expected to be rotating fast, at > least not at their surfaces. As the stars evolve, their envelopes > expand by 1–2 orders of magnitude. The outer layers thus slow > down as a result of angular momentum conservation and may > be further reduced by, e.g., mass loss due to stellar winds (e.g., > Maeder & Meynet 2000; Smith 2014), possibly inward > convective transport of angular momentum (e.g., Brun & > Toomre 2002; Brun & Palacios 2009), and magnetic braking > (Mestel 1968). The theory of single-star evolution therefore > predicts slow surface rotation rates, less than about 1 km s−1 for > stars at the tip of the red giant branch (e.g., Privitera et al. > 2016a) and less than about 0.1 km s−1 for red supergiants > (RSGs; Wheeler et al. 2017; Chatzopoulos et al. 2020), which > are the cool giant descendants of massive stars. The way I read it is, the linear speed of the surface seems to be the relevant metric. That means large stars with the same linear surface speed would have larger angular velocities (at the surface) but are considered “the same” in terms of the effect being discussed. ---
**Author:** ![TemporalFix](https://avatars.discourse-cdn.com/v4/letter/t/c6cbf5/32.png) [@TemporalFix](https://boards.straightdope.com/u/TemporalFix)\ **Post date:** [March 21, 2024, 7:13am UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/188 "2024-03-21T07:13:21Z")
The reason for the linear speed could be that it’s determined by studying doppler shifts. This gives you linear speeds. To convert this to angular, you need the star’s radius which is not precisely known. And the rotation is generally not homogenous. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [March 21, 2024, 11:30am UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/189 "2024-03-21T11:30:13Z")
I had assumed there was an observed broadening of spectral adsorption lines that would would, assuming a rotating sphere, provide an estimate of rotation - so indeed Doppler shifts causing the broadening. What is interesting is that the observation below was done in the radio spectrum 340GHz. Which is impressive. Chasing down the [reference for measuring rotation,](https://arxiv.org/abs/1711.07983) from the wikipedeia page for Betelgeuse, the abstract includes this: > [@](#): > > We observed Betelgeuse using ALMA’s extended configuration in band 7 (f~340 GHz, {\lambda}~0.88 mm), resulting in a very high angular resolution of 18 mas. > Using a solid body rotation model of the 28SiO(v=2,J=8-7) line emission, we show that the supergiant is rotating with a projected equatorial velocity of v\_eq sin i = 5.47 +/- 0.25 km/s at the equivalent continuum angular radius R\_star = 29.50 +/- 0.14 mas. So the answer is - this is the rotational speed at the equator, assuming the current estimates for distance to the star and the observed angular diameter. ---
**Author:** ![PastTense](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pasttense/32/14550_2.png) [@PastTense](https://boards.straightdope.com/u/PastTense)\ **Post date:** [September 11, 2024, 10:11pm UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/190 "2024-09-11T22:11:27Z")
Interesting new theory: it may actually be two stars. > If you train a telescope on Betelgeuse for weeks, you’ll see it dimming, then brightening, then dimming again. These pulsations stretch over roughly 400 days, although the 2020 [“Great Dimming”](https://www.livescience.com/betelgeuse-star-dimming-explained.html) event reveals such periodicity may occasionally go awry. But if you plotted Betelgeuse’s light intensity over years, you’d find these 400-day-long heartbeats superimposed on a much larger, slower heartbeat. Technically called a long secondary period (LSP), this second type of heartbeat lasts about six years, or 2,170 days, in Betelgeuse’s case… > Periodic changes in a star’s brightness typically occur when the star swells and then shrinks, again and again. This happens because gas near the star’s core gets super-heated and rises to the surface, where it expands ― causing the star’s size to increase ― but then cools and settles back towards the interior, shrinking the star. The general consensus among astronomers is that Betelgeuse’s 400-day-long pulsations arise from such cycling. But the cause of the star’s 2,170-day-long LSP had remained elusive, despite several possible theories, including the presence of gigantic dust clouds. The two star theory is proposed as an explanation of the long secondary period. > **[Betelgeuse, Betelgeuse? One of the brightest stars in the sky may actually be...](https://www.livescience.com/space/astronomy/betelgeuse-may-have-a-sunlike-companion-study-suggests)** > > Betelgeuse, one of the brightest stars in the sky, may have a secret sunlike companion that drives the star’s mysterious six-year-long "heartbeat," new research suggests. Here is the preprint: [https://arxiv.org/pdf/2408.09089](https://arxiv.org/pdf/2408.09089) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 13, 2024, 1:10am UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/191 "2024-09-13T01:10:49Z")
I don’t buy it. Stellar companions are not a subtle thing. If Betelgeuse had a companion, we’d have detected it long ago. ---
**Author:** ![Precambrianmollusc](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/precambrianmollusc/32/63_2.png) [@Precambrianmollusc](https://boards.straightdope.com/u/Precambrianmollusc)\ **Post date:** [September 13, 2024, 1:43am UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/192 "2024-09-13T01:43:59Z")
The paper addresses the ability to detect a companion ( called Betelbuddy) and suggest it is about 1-2 solar mass and orbiting fairly close , and given the luminosity of Betelgeuse and the max possible temperature of the companion means it would be very difficult to detect. ( section 5.2 of the paper) ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [September 13, 2024, 3:20pm UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/193 "2024-09-13T15:20:18Z")
If BeetleBuddy existed, wouldn’t we have noticed the periodic dimming well before now? ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [September 13, 2024, 3:49pm UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/194 "2024-09-13T15:49:13Z")
Yes, the Long Secondary Period has been observed for many years. The Betelbuddy theory is a proposed explanation for this well known phenomenon. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 14, 2024, 12:11am UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/195 "2024-09-14T00:11:17Z")
Dimming due to eclipsing has a very distinctive and well-recognized light curve. And I’m not sure how else a companion would cause periodic luminosity changes. ---
**Author:** ![Peter\_Morris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peter_morris/32/359_2.png) [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)\ **Post date:** [September 14, 2024, 12:25am UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/196 "2024-09-14T00:25:33Z")
> [@Chronos](#): > > Dimming due to eclipsing has a very distinctive and well-recognized light curve. For clarification, does Betelgeuse exhibit this or not? ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [September 14, 2024, 2:23am UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/197 "2024-09-14T02:23:36Z")
I just skimmed the paper, but apparently the hypothesis is not a simple eclipsing situation. It involves nearby dust clouds which are affected by the revolution of the hypothesized companion. See section 4.8.3: > Thus we propose a new scenario: a companion whose orbital period sets the timescale of the LSP > and RV fluctuations, and whose presence modulates, removes, eliminates, or changes the optical depth of any > potentially dusty material in its vicinity, rather than > dragging a trailing cloud of dusty material. There are > multiple ways in which this might occur, including, for > example, dynamically perturbing the potential of the > dusty medium or through irradiation. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 14, 2024, 1:17pm UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/198 "2024-09-14T13:17:03Z")
> [@Peter\_Morris](#): > > For clarification, does Betelgeuse exhibit this or not? No. If it did, then it would have been known many decades ago. ---
**Author:** ![GoodZuckerbergs](https://avatars.discourse-cdn.com/v4/letter/g/ba8739/32.png) [@GoodZuckerbergs](https://boards.straightdope.com/u/GoodZuckerbergs)\ **Post date:** [September 18, 2024, 11:13pm UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/199 "2024-09-18T23:13:34Z")
> [@PastTense](#): > > Interesting new theory: it may actually be two stars. So six months after the last post in this topic you resurrected the thread to tell us this. And you did so on September 11th, exactly five days after the second Beetlejuice movie premiered in theaters. So, to verify: Yes, there are two Beetlejuices now, and the second one is slightly less radiant than the first, although I actually like Jenna Ortega despite her lack of emotional range, and Winona Ryder and Catherine O’Hara are still smokin’ hot all these decades later. ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [July 22, 2025, 12:52pm UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/200 "2025-07-22T12:52:14Z")
Astronomers say they have found Betelgeuse’s small companion: a blue A or B star, hot but not yet on the main sequence, orbiting Betelgeuse at about 4 AU. Believed to be the explanation for the six year dimming cycle, caused as it orbits Betelgeuse. > **[Betelgeuse’s companion star revealed in new images](https://www.sciencenews.org/article/betelgeuse-companion-star-revealed)** > > Revealed by advanced imaging, the long-sought "Betelbuddy" is much smaller and fainter than Betelgeuse and orbits within the supergiant’s atmosphere. > **[Astronomers crack 1,000-year-old Betelgeuse mystery with 1st-ever sighting of...](https://www.space.com/astronomy/astronomers-crack-1-000-year-old-betelgeuse-mystery-with-1st-ever-sighting-of-secret-companion-photo-video)** > > "Papers that predicted Betelgeuse's companion believed that no one would likely ever be able to image it." > **[It's official. Betelgeuse has a companion star. But it won't last long – one...](https://www.skyatnightmagazine.com/news/betelgeuse-companion-star)** > > Betelgeuse's companion star was found in a tight orbit using the ‘Alopeke instrument on Gemini North at the International Gemini Observatory. ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [July 22, 2025, 12:59pm UTC](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478/201 "2025-07-22T12:59:01Z")
Query: the articles say the companion star has not yet started fusing hydrogen at its core. Then what is it doing as a source of heat and light? [Previous page](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478.md?page=9) [Next page](https://boards.straightdope.com/t/betelgeuse-losing-brightness/845478.md?page=11)