Betelgeuse unusual dimming - what's up?

[Astronuc]

TL;DR Summary

The red giant, and semi-regular variable, Betelgeuse is the dimmest seen in years, prompting some speculation that the star is about to explode.

Betelgeuse is making news after some astronomers noticed unusual dimming. It's the dimmest in quite awhile.

https://www.nationalgeographic.com/...ange-astronomers-are-buzzing-about-supernova/
E.F. Guinan, R. J. Wasatonic (Villanova Univ.) and T. J. Calderwood (AAVSO)
http://www.astronomerstelegram.org/?read=13341

V-band and Wing TiO-band and Near-IR photometry of the semi-regular variable red supergiant, Betelgeuse (alpha Ori; M1.5 - M2.5 Iab) has been carried out over last 25+ years. This photometry was joined by complementary B & V photometry from T. Calderwood (AAVSO). Betelgeuse and Antares are the two nearest red supergiant core-collapse Type-II supernova (SN II) progenitors. Photometry from this season shows the star has been declining in brightness since October 2019, now reaching a modern all-time low of V = +1.12 mag on 07 December 2019 UT. Betelgeuse undergoes complicated quasi-periodic brightness variations with a dominant period of ~420 +/-15 days. But also Betelgeuse has longer-term (5 - 6 years) and shorter term (100 - 180 days) smaller brightness changes. Currently this is the faintest the star has been during our 25+ years of continuous monitoring and 50 years of photoelectric V-band observations. The light variations are complicated and arise from pulsations as well from the waxing and waning of large super-granules on the star's convective surface. . . .

https://www.aavso.org/vsots_alphaori
https://en.wikipedia.org/wiki/Betelgeuse#Variability
https://phys.org/news/2019-12-betelgeuse-tempestuous-star.html

Earlier this year - https://Earth'sky.org/brightest-stars/betelgeuse-will-explode-someday

 

[Vanadium 50]

I don't see any link between dimming (especially as its brightness is in the trough of both cycles) and a supernova about to occur.

 

[Astronuc]

Besides being interesting, I see this as a teaching moment.

Another article on the current dimming - EarthSky, Dec 23, 2019 - Betelgeuse is ‘fainting’ but (probably) not about to explode
https://Earth'sky.org/space/betelgeuse-fainting-probably-not-about-to-explode

I went looking for Red Giant to Supernova in the Milky Way, and we haven't seen one recently.

Discovery of Most Recent Supernova in Our Galaxy (2008)

WASHINGTON -- The most recent supernova in our galaxy has been discovered by tracking the rapid expansion of its remains. This result, using NASA's Chandra X-ray Observatory and the National Radio Astronomy Observatory's Very Large Array, will help improve our understanding of how often supernovae explode in the Milky Way galaxy.

The supernova explosion occurred about 140 years ago, making it the most recent in the Milky Way. Previously, the last known supernova in our galaxy occurred around 1680, an estimate based on the expansion of its remnant, Cassiopeia A.

https://www.nasa.gov/home/hqnews/2008/may/HQ_08126_Chandra_Supernova.html

From EarthSky, May 2018

What about Betelgeuse? Another star often mentioned in the supernova story is Betelgeuse, one of the brightest stars in our sky, part of the famous constellation Orion. Betelgeuse is a supergiant star. It is intrinsically very brilliant.

Such brilliance comes at a price, however. Betelgeuse is one of the most famous stars in the sky because it’s due to explode someday. Betelgeuse’s enormous energy requires that the fuel be expended quickly (relatively, that is), and in fact Betelgeuse is now near the end of its lifetime. Someday soon (astronomically speaking), it will run out of fuel, collapse under its own weight, and then rebound in a spectacular Type II supernova explosion. When this happens, Betelgeuse will brighten enormously for a few weeks or months, perhaps as bright as the full moon and visible in broad daylight.

When will it happen? Probably not in our lifetimes, but no one really knows. It could be tomorrow or a million years in the future. When it does happen, any beings on Earth will witness a spectacular event in the night sky, but earthly life won’t be harmed. That’s because Betelgeuse is 430 light-years away.

https://Earth'sky.org/astronomy-essentials/supernove-distance
More recent articles have the distance of Betelgeuse ~640-650 ly.

The National Geographic article mentions "some speculation" that the star is about to explode, but doesn't attribute the speculation. From an interview with one scientist: “The biggest question now is when it will explode in a supernova,” UC Berkeley’s Sarafina Nance, who studies Betelgeuse and stellar explosions, said on Twitter. “Disclaimer: I don't think it's going to explode any time soon,” she added during an interview with National Geographic.

I was looking for closest SN in recent history, but they've been outside our galaxy.

https://en.wikipedia.org/wiki/History_of_supernova_observation#1970–1999
(An odd statement - "Although no supernova has been observed in the Milky Way since 1604, it appears that a supernova exploded in the constellation Cassiopeia about 300 years ago, around the year 1667 or 1680.")
SN1987A is the closest, in the Large Magellanic Cloud.
https://en.wikipedia.org/wiki/History_of_supernova_observation#2000_to_present

 

[mfb]

So far I haven't seen any astronomer or astrophysicist thinking it is "about to explode" on human timescales. It has been variable for as long as we observed it. It will probably become brighter again starting early next year.

A supernova in our lifetime would be really interesting, of course. Betelgeuse is so close that neutrino detectors might pick up the silicon burning phase, giving us more warning time than the usual neutrino warning system based on supernova neutrinos. The supernova then would lead to a huge number of events in many detectors, and starting a few hours afterwards we can observe the supernova live with every telescope in the right hemisphere.

 

[Vanadium 50]

If we are going to discuss SNe, great, but I would title the thread something besides "Betelgeuse Unusual Dimming", So far as I know, there is no scientific link between the two. Also, "the dimmest its been in 50 years" (assuming that's the case) is not very long on stellar time scales.

Betelgeuse is an old (relative to its lifetime), red star. But there are lots of old, red stars out there. There are plenty of other good progenitor SN candidates out there - including Eta Carinae and Sher 25, both blue and in some respects resembling Sanduleak -69 202, the SN1987A progenitor.

 

[stefan r]

Suppose we roll an artillery shell or similar explosive down a very long flight of stairs (do not try at home). I would wager an explosion is more likely to start at the moment the can hits the concrete. I am not suggesting that Betelgeuse is similar to a steel shell. However, cycles in the star's radius could be similar to a type of bounce. It might be possible to predict which part of a cycle a star will most likely be in when the final collapse occurs.

 

[OmCheeto]

Besides being interesting, I see this as a teaching moment.
...

Over the top interesting.

Here's a quote I ran across that I completely agree with:

Jason Major; "Astronomers **know** that this is not a likely event on any given day. But the idea of it is so enticing, it’s the equivalent of talking about what you’d do with your money if you won the lottery. ​

​

Have fun with it. ​

​

#Betelgeuse #Betelgeuse #Betelgeuse"​

A lot of chatter on Twitter under #Betelgeuse:

Miguel Montargès; "I cannot remain silent about #Betelgeuse. After all, it was the main subject of my PhD. I offer you below a recap of my favorite results on this star.​

..."​

...
resembling Sanduleak -69 202
...

Thanks for mentioning that star. Is it possible that watching Betelgeuse might yield a similar wiki entry?

Sanduleak -69 202​

The discovery that a blue supergiant was a supernova progenitor contradicted all known theories at the time and produced a flurry of new ideas about how such a thing might happen, but it is now accepted that blue supergiants are a normal progenitor for some supernovae.​

Or was that just Wikipedian hyperbole?

 

[mfb]

Suppose we roll an artillery shell or similar explosive down a very long flight of stairs (do not try at home). I would wager an explosion is more likely to start at the moment the can hits the concrete. I am not suggesting that Betelgeuse is similar to a steel shell. However, cycles in the star's radius could be similar to a type of bounce. It might be possible to predict which part of a cycle a star will most likely be in when the final collapse occurs.

I don't see how this analogy would work at all. Let's try this: If you walk from side to side on the roof is the boiler in the basement more likely to explode when you turn around at one end?

----

If Betelgeuse will become a supernova within the next million years then our chance to see it in our lifetime is better than the chance to win the jackpot in a lottery drawing.

 

[Vanadium 50]

Is it possible that watching Betelgeuse might yield a similar wiki entry?

"The discovery that a blue supergiant was a supernova progenitor contradicted all known theories at the time "

Probably not.

In January 1987 (the supernova was in February) we had zero observed progenitor stars. Now we have around forty. We had no models that gave both an explosion and a remnant. Now we do. I won't say we won't learn a lot from a nearby SN, but we've also learned a great deal in the last third of a century.

 

[Astronuc]

There are three topics in the thread: 1) communication of science to the public, 2) red supergiants (RSGs), particularly Betelgeuse, which are variable, and 3) supernovae.

With respect to 1) communication of science to the public, I was interested in articles by institutions such as National Geographic about the recent 'unusual' dimming of Betelgeuse, and a comment about some speculation that it was about to become a supernova. I think it is unfortunate that such comments are made if there is no evidence to support such a comment or claim. I can understand the sensationalization of news in order to attract attention of the public, but it seems to me a disservice to the public. I'd rather see objective reporting that informs the public. PF prefers the use of peer-reviewed literature such as journal articles from reputable journals, or textbooks from reputable publishers.

I 'd prefer to focus on topic 2, red supergiants and their behavior. Topic 3, while interesting is a bit premature with respect to Betelgeuse and its 'unusual' dimming.

I went looking for any example, particularly light curves, of red giant stars including Betelgeuse, and especially those of 10-15 solar masses. Certainly there are some, and light curves of Betelgeuse are readily found. I found the following article concerning "Variability in red supergiant stars" (2006):
https://academic.oup.com/mnras/article/372/4/1721/1189711
From the abstract: "The full sample contains 48 red semiregular or irregular variable stars, with a mean time-span of observations of 61 yr."
UniverseToday published a plot of the magnitude of Betelgeuse from 1970 to present.
https://www.universetoday.com/144465/waiting-for-betelgeuse-whats-up-with-the-tempestuous-star/

Certainly, because of its proximity and size, Betelgeuse is well-studied. So when it becomes a supernova, we'll have a lot of data. And there are plenty of other opportunities as Vanadium 50 indicated, just not too many within 1000 ly of earth.

As far as I can tell, of the supernovaes we have observed within the last several decades, it was after the fact, that is the SN is observed (and most in another galaxy) and then scientists review older records to see what there was before, i.e., the progenitor.

From a Wikipedia article, "Red supergiant photospheres contain a relatively small number of very large convection cells compared to stars like the Sun. This causes variations in surface brightness that can lead to visible brightness variations as the star rotates." The comment cites: https://ui.adsabs.harvard.edu/abs/1975ApJ...195..137S/abstract
https://en.wikipedia.org/wiki/Red_supergiant_star

Antares is another RSG of interest.

Aldebaran, designated α Tauri, about 65 light-years from the Sun, is also interesting but it is only a red giant with mass estimated to be ~1.7 solar masses, and it varies slowly in brightness between magnitudes 0.75 and 0.95.

Another interesting article with respect to topic 2: Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars, https://arxiv.org/abs/1103.5805

 

[Vanadium 50]

On topic #1, I agree. It's a pity that National Geographic is resorting to sensationalism and clickbait. Their authority on this is a 1st year grad student who studied Betelgeuse as an undergraduate project. In the old days, they probably would have reached out to a professor somewhere. Today one just copies down what's in a blog.

It turns out that the prediction "she's going to blow any minute now" was popular in 2012 and 2010. I guess every few years it happens.

Our best models tell us that Betelgeuse has ~10^5 years left. Those same models tell us that it will have a few 10^3 years left after it starts carbon burning, which hasn't happened yet. So to argue that "it's going to blow!" requires one to accept half of the models' predictions and reject the other half. Could be right, but it seems that the people doing this don't know their Paxtons from their Haxtons.

On topic #2, in some ways Betelgeuse is great because it's so close. Because it's surface can be imaged (sort-of) the are opportunities to study things impossible to study with more distant stars, like starspots. On the negative, we don't know how far away it is (very well), so we don't know its radius very well, and that's a key parameter in the modelling.

The reason its distance is poorly known is that it is too bright for Gaia. More generally, the estimates of its distance read like a cautionary tale on uncertainties. Paper after paper comes out saying "you'd think that the error on this technique is x%, but it's really x/2%" and they all disagree with each other.

So far as I know, all asymptotic and red giant branch stars are variable. I can't think of an exception. This isn't surprising when you consider the structure - a sun-sized core that's fusing and a solar-system (at least the inner solar system) envelope of incandescent gas. There's a lot of dynamics in the envelope - heat, mass and light transfer.

 

[Astronuc]

On the negative, we don't know how far away it is (very well), so we don't know its radius very well, and that's a key parameter in the modelling.

That's one of the things that surprised me, although I probably shouldn't be surprised. Earlier articles had a distance of ~430-450 ly, then more recent articles have the distance ~640 - 700 ly, depending on rounding.

For example, "Betelgeuse lies some 430 light-years from Earth. (Note: determining distances, especially to red supergiant stars, is a vexing problem in astronomy. Estimates vary and are often revised, with some as high as 650 light-years.) Yet it’s already one of the brightest stars in Earth’s sky. The reason is that Betelgeuse is a supergiant star. It is intrinsically very brilliant." https://Earth'sky.org/brightest-stars/betelgeuse-will-explode-someday

 

[Vanadium 50]

depending on rounding.

Somehow this subject attracts people who don't understand rounding: I keep seeing numbers like 12.34567890123 +/- 3.4567890123456. Makes me want to revoke their PhDs.

I think it's fair to say that parallax fits are poor. Given that, the question is what to do about it? Inflating the error is certainly one thing that can be done, but given that it is likely a systematic effect and thus non-Gaussian, it's not ideal.

Betelgeuse suffers from being bright. There are only 8 (on average) brighter stars, so surveys like Gaia are optimized for less light. It's also far (if it were far and dim we wouldn't have a name for it) and that means that the parallax is small: a few milliarcseconds. Canopus is brighter, but it's 10x closer. Deneb is farther, but it's also dimmer.

 

[Orodruin]

I keep seeing numbers like 12.34567890123 +/- 3.4567890123456.

Better than just 12.34567890123 ...

 

[OmCheeto]

That's one of the things that surprised me
...

Unless you were some Betelgeuse nerd, who knew all about it, I think anyone would be surprised how bizarre it is.

Seeing it referred to as a "hot vacuum" confused me quite a bit.
Being a notascientist, I immediately merged heliopause with neon signs science, and decided PF would definitely ban me if I postulated such nonsense, or even asked the question; "What the hell is this thing!"

So I googled some more and found an article published kind of yesterday, and was most enchanted:

December 30, 1981
Ancient China sheds 'light' on a red star
By Robert C. Cowen

The "red star" being of course Betelgeuse.

Anyone have Roberts email address?

 

[Astronuc]

That's one of the things that surprised me, although I probably shouldn't be surprised. Earlier articles had a distance of ~430-450 ly, then more recent articles have the distance ~640 - 700 ly, depending on rounding.

One of the papers discussing the measurement of distance to stars, in this case Betelgeuse. A NEW VLA–HIPPARCOSDISTANCE TO BETELGEUSE AND ITS IMPLICATIONS, by Graham M. Harper, Alexander Brown, and Edward F. Guinan, 2008
https://casa.colorado.edu/~gmh/pubs/aj_135_4_1430.pdf
"These new solutions indicate a smaller parallax, and hence greater distance (197±45 pc), than that given in the original Hipparcos Catalogue (131±30 pc) and in the revised Hipparcos reduction." 197 pc = 642 ly (~640 ly), 131 pc = 427 ly (~430 ly).

Guinan was a co-author on the recent Astronomer's Telegram (23 Dec 2019) about the dimming of Betelgeuse.
http://www.astronomerstelegram.org/?read=13365

At the time, there were plans at NASA for the Space Interferometry Mission, but it was canceled in 2010. One goal was to improve the measurements of distances to stars like Betelgeuse.
https://en.wikipedia.org/wiki/Space_Interferometry_Mission

 

[Vanadium 50]

Anyone have Roberts email address?

The article is 40 years old. Bob, if he's still alive, is about 92.

It's also nonsense. Th4e Li-Zhi paper, which is about many things and not a scientific argument for this, says that it takes 2000 years for a star to evolve from a yellow supergiant to a red supergiant, and the Chinese text is ~2000 years old. Problem is, Ptolemy reported Betelgeuse as red in about 150 AD. Furthermore, the Chinese text places Betelgeuse (M2) between Bellatrix (B2) and Antares (M1). That's where it belongs - it's certainly more yellow than either of the two reference stars. But the most important fact is that "yellow stars" do not look yellow, especially to the naked eye. They look white. Capella looks white. Procyon looks white. Polaris looks white.

 

[Vanadium 50]

dimming of Betelgeuse.

It points out that Betelgeuse is near the minimum of both the 2335 day cycle and the 420 day cycle.

 

[OmCheeto]

The article is 40 years old. Bob, if he's still alive, is about 92.

It's also nonsense. Th4e Li-Zhi paper, which is about many things and not a scientific argument for this, says that it takes 2000 years for a star to evolve from a yellow supergiant to a red supergiant, and the Chinese text is ~2000 years old. Problem is, Ptolemy reported Betelgeuse as red in about 150 AD. Furthermore, the Chinese text places Betelgeuse (M2) between Bellatrix (B2) and Antares (M1). That's where it belongs - it's certainly more yellow than either of the two reference stars. But the most important fact is that "yellow stars" do not look yellow, especially to the naked eye. They look white. Capella looks white. Procyon looks white. Polaris looks white.

Well, ok, it was nonsense. But did they know enough then to know it was nonsense?

I watched a 90 minute video yesterday where the speaker said* it takes about 100,000 years to transition from yellow to red. So when I saw that NASA APOD said today;

"Betelgeuse is also recognized as a nearby red supergiant star that will end its life in a core collapse supernova explosion sometime in the next 1,000 years, though that cosmic cataclysm will take place a safe 700 light-years or so from our fair planet.", ​

it made me wonder if they know something, or they are capable of typos.

Betelgeuse was also yesterdays APOD: Betelgeuse Imagined

-------------
*
Weirdest Stars in the Universe, including Thorne-Żytkow Objects: Emily Levesque, CU Boulder
00:04:45 It takes about 100,000 years to go from Deneb to Betelgeuse stage. (paraphrased)

ps.
Most entertaining moment in the video, IMHO:

00:48:00 TZOs Kip Thorne & Anna Żytkow Objects​

"...Years ago, Kip Thorne and myself 'invented'​

theoretical models of stars...​

Please let me know if there may be some interest​

in pursuing these lines of enquiry." - Anna Żytkow​

 

[mfb]

Probably a typo. It might happen in the next 1000 years but it is not very likely.

 

[Vanadium 50]

But did they know enough then to know it was nonsense?

You mean did they know about Ptolemy? Yes. For about 1800 years.

sometime in the next 1,000 years

Highly unlikely. Because...

Our best models tell us that Betelgeuse has ~10^5 years left. Those same models tell us that it will have a few 10^3 years left after it starts carbon burning, which hasn't happened yet.

 

[mfb]

How do we know it didn't start carbon burning yet?
This website is based on a probabilistic argument. Helium burning is a much longer phase, but that argument doesn't lead to a minimal remaining lifetime of course.

 

[Vanadium 50]

How do we know it didn't start carbon burning yet?

Apparently the models show [unobserved] structural changes when carbon burning starts. I am not an expert i these models, but will say you can't pick and choose model predictions.

 

[Nik_2213]

https://phys.org/news/2016-12-famous-red-star-betelgeuse-faster.html

Okay, a few years old, but Open Access :
Astronomer J. Craig Wheeler of The University of Texas at Austin thinks that Betelgeuse, the bright red star marking the shoulder of Orion, the hunter, may have had a past that is more interesting than meets the eye. Working with an international group of undergraduate students, Wheeler has found evidence that the red supergiant star may have been born with a companion star, and later swallowed that star. The research is published today in the journal Monthly Notices of the Royal Astronomical Society.
https://academic.oup.com/mnras/article/465/3/2654/2454753
/
So, they reckon that Betelgeuse had a binary partner, but swallowed it about ~100 ky BP.
==
If such an Alpha Ori_B sort of endured within big Alpha Ori_A's vog, albeit diminished, and now with a ~5.9 yr elliptical orbit, would this account for some of the variations ??

 

[Vanadium 50]

With respect to 1) communication of science to the public, I was interested in articles by institutions such as National Geographic about the recent 'unusual' dimming of Betelgeuse, and a comment about some speculation that it was about to become a supernova.

And now PF is doing the exact same thing in its featuring of this thread.

 

[Astronuc]

And now PF is doing the exact same thing in its featuring of this thread.

Edit/update: I just noticed the title has been revised. I would not have written "ready to supernova?", but rather "so what?"

I don't see any speculation about Betelgeuse imminently becoming a supernova. I do see a discussion on various aspects of Betelgeuse, and red supergiants in general, and a lot of citations from peer-reviewed articles concerning the various aspects of Betelgeuse and other giant/supergiants, which are also variable stars. There is a lot we don't know, and some things we know but with relatively high uncertainty (e.g., distance from earth).

Another aspect is age: I've seen 6 My, 8 or 8.5 My, or less than 10 My (so it might go supernova in 1 My +/- 0.5 My, or is 2 My +/- 1 My), and that variance greatly exceeds 100 Ky, or 0.1 My. Or put another way, how old do we estimate Betelgeuse to be, and what age to we expect such stars to achieve at the point they transition to a supernova.

Or how about mass, which would be relevant to age: 11.6 +5.0/−3.9 M☉ (from paper: https://ui.adsabs.harvard.edu/abs/2011ASPC..451..117N/abstract, or https://arxiv.org/pdf/1109.4562.pdf)
Another estimate: Betelgeuse, "thought to be between 13 and 17 solar masses, . . . " from https://www.atnf.csiro.au/outreach/education/senior/astrophysics/stellarevolution_postmain.html

From Jim Kaler (1998, updated 2011) - "Betelgeuse is clearly a highly evolved star, one whose central hydrogen fuel supply has run out. As a result, the core has contracted into a hot dense state, and the outer portions swelled outward. We do not really know the star's condition at the moment, but the odds are that it is now in the process of fusing helium into carbon and oxygen in its core. From theory, its initial mass should have fallen somewhere around 18 or 19 times that of the Sun. Starting life as hot, blue, class O star only around 10 million years ago, Betelgeuse will fuse elements through neon, magnesium, sodium, and silicon all the way to iron. The core will then collapse, causing the star to blow up as a supernova, most likely leaving a compact neutron star about the size of a small town behind."

I think we are trying to make sense of what is reported in the general media compared to what is reported in the scientific literature.

Slates attempt: https://slate.com/technology/2014/0...-give-it-100000-years-before-it-explodes.html

Then in one of the citations from the Slate article: Evolutionary Tracks for Betelgeuse (https://arxiv.org/pdf/1406.3143v1.pdf) "The EG and MESA models give almost identical results for the interior thermodynamic properties. In both models the star is characterized at the present time by the presence of a developing carbon-oxygen core up to the bottom of the helium burning shell at ∼3−4 M⊙ (r ≈ 0.20R where R is the photospheric radius of the star)." The article summarizes different measurements of various parameters of the star, and some currently accepted values.

The light curves, or plot of magnitude as a function of time, gives a signature of the stellar evolution. One question is then, "what are the indications in that signature that the star is about to become a supernova?" Apparently that is unknown, because we haven't had the current instruments long enough to have observed a nearby (in this galaxy, and within several hundred ly) supernova as it happened.

 

[mfb]

Greg changed the title.

The article summarizes different measurements of various parameters of the star, and some currently accepted values.

It is a reference for the expectation that it didn't start carbon burning yet. As far as I understand figure 4 it will grow larger and redder (keeping its overall luminosity) during that. They don't expect the star to live that much longer:

Our best guess is that the star will supernova in less than∼100,000 yrs (even longer in the EG model). We note, however, that there error ellipse encompasses the entire track so that the star could be further along in its evolution.

 

[Vanadium 50]

I see this thread is no longer featured, so this is moot.

There is no good way to measure Betelgeuse's mass. If it were a multiple star system, one could look at the orbit of its companion, but it's not, so we can't. The best we can do is infer the mass based on radius and luminosity, and now we're back to the same problem as measuring its distance.

If that weren't bad enough, Betelgeuse sits in a cloud of gas and maybe a little dust that it's ejected. The amount of gas is related to how long it's been on the RGB, but is also dependent on distance. Furthermore, what we measure is not the star's surface but the light that's gone through this cloud.

You can run all this together in models and look at what comes out. But the models can't do better than their input parameters.

As far as predicting the end, you don't run a model that says "Star X lasts 10 million years, and its present age is 9.9 so you have 100,000 to go". Subtracting two large numbers is numerically unstable. Instead one fits the other way, e.g. "this star will start carbon burning in less than X years". Once carbon burning starts, there's a millennium, perhaps two left. Once neon burning starts, it's only a few years.

Finally, although we have ~40 identified progenitor stars, we don't have long-term detailed observations of them. How would we know to monitor this star and not that one? There are a very few lensed galaxies where we know a supernova will take place (the first such prediction was in 2015) but they so distant that one can barely make out the progenitor galaxy, much less the progenitor star.

 

[mfb]

How would we know to monitor this star and not that one?

Well, monitor all of them. Gaia, LSST. Sky surveys don't reach the precision of dedicated observations but at least we get tens of observations in the years before a supernova.
With increasing size of neutrino detectors we can also get a bit more warning time for supernovae very close to us. Instead of hours we might have days in the future.

 

[Ibix]

With increasing size of neutrino detectors we can also get a bit more warning time for supernovae very close to us. Instead of hours we

Why is that? Do we expect heavy element fusion to emit lots of neutrinos?

 

[Vanadium 50]

Well, monitor all of them.

But what you'd like is spectroscopy.

Instead of hours we might have days in the future.

Well, maybe a day. But suppose you detect the onset of silicon burning. It doesn't help you see the onset of oxygen or neon burning, much less carbon.

 

[Astronuc]

Why is that? Do we expect heavy element fusion to emit lots of neutrinos?

On February 23 1987 detectors deep underground that where designed to detect proton decay suddenly detected a huge number of neutrinos (8 in 5 seconds). Scientist where perplexed by this influx in neutrinos at first until on February 24 a grad student named Ian Shelton announced his observation of a supernova in the Large Magellanic Cloud. This was a core collapse supernova. When the core of a massive star collapses, it crushes the protons and electrons together and neutrinos form.

p + e → n + ν.

The neutrinos pass straight through the collapsing star before the explosion takes place. This is why the neutrinos where detected before the supernova was visibly observed. When the neutrinos leave, they also take energy away from the star and the star continues to collapse and rebounds out in an explosion that can outshine the brightness of the entire galaxy. Neutrinos are very important to the study of supernovas because they provide an early warning signal and allow scientists to be looking in the right direction before the supernova even takes place.

From http://www.astro.wisc.edu/~larson/Webpage/neutrinos.html

Given the proximity of Betelgeuse compared with the SN1987A, I'd expected the number of neutrinos would be much greater from Betelgeuse than from SN1987A.

 

[Vanadium 50]

I'd expected the number of neutrinos would be much greater from Betelgeuse than from SN1987A.

Yes.

But.

We actually saw mostly antineutrinos from SN 1987a. The DUNE experiment will have ~40 kilotons of liquid argon underground, and is sensitive to neutrinos. So even SNe in the LMC will be detected with hundreds of events at DUNE, and because DUNE sees neutrinos it will be a probe of early-time behavior. (Neutrinos come early; Antineutrinos only come late)

 

[mfb]

Why is that? Do we expect heavy element fusion to emit lots of neutrinos?

Not the main fusion reactions directly, but some side reactions produce neutrinos or nuclei doing beta decays, and the overall reaction rates increase massively in the days leading to a supernova. Eventually the temperature gets so hot that the process electron+positron->neutrino+antineutrino becomes relevant (that's also an important process that cools neutron stars initially).
Here is a discussion.

Neutrino detectors can pick up this signal if the star is near enough. Betelgeuse should produce a noticeable signal in Super-Kamiokande (and the larger Hyper-Kamiokande has been approved already). The supernova will flood it with neutrino events afterwards. Slide 21 here has expected warning times. KamLAND might achieve a longer warning time (slide 22). The combination of both should make the signal clearer.

Even dark matter detectors might pick up some neutrinos from the silicon burning phase in the future: Discussion for ARGO

But what you'd like is spectroscopy.

Gaia can detect the temperature, at least. It also does radial velocity measurements. I wonder if a star that varies in size would be registered as change in the radial velocity.

 

[Vanadium 50]

I wonder if a star that varies in size would be registered as change in the radial velocity.

Probably not. Radial motion is 22 km/s. Pulsation period is 200 million seconds. For even large pulsations it's a small effect.