A new study making the rounds predicts that supergiant Betelgeuse will explode as a supernova sooner rather than later, but others are urging caution.

The roiling surface of Betelgeuse
This artist’s impression shows the supergiant star Betelgeuse.
ESO / L. Calçada

If astronomers had to guess the next, nearby star to go supernova in the Milky Way, their bets might go to Betelgeuse. The bright-red supergiant star that marks Orion’s shoulder is nearing the end of its life, and it’s less than 1,000 light-years from Earth. But how close is it to going supernova? And will we be around to see it?

Typically, astronomers suggest it might explode within the next 100,000 years — that is, “soon” on a cosmic timeframe, not a human one. But a new study posted June 1st on the arXiv has been making the rounds, in which Hideyuki Saio (Tohoku University, Japan) and colleagues claim that the star might be further along in its evolution, and that much closer to exploding, than we thought. However, others are taking issue with that result.

The claim comes down to the star’s pulsations. Betelgeuse is unstable, “breathing” in and out regularly, with overlapping overtones. Following its brightness over the past century (thanks in part to data from the American Association for Variable Star Observers), astronomers have noted changes over periods of 2,200 days, 420 days, 230 days, and 185 days.

Usually, astronomers treat the 420-day up-and-down as the primary in-and-out pulsation, with the shorter cycles as overtones. The 2,200-day (or 6-year) period isn’t generally considered part of these ins and outs, and is instead dubbed a long secondary period, a feature of unknown origin common to one-third of supergiant stars.

If the 420-day period is the primary one, then Betelgeuse would have the diameter of 800 to 900 Suns lined up in a row. Placed in the solar system, it would almost reach the orbit of Jupiter. Saio and colleagues, however, think that might be an underestimate. If the 2,200-day cycle is the primary one, and all the rest are overtones, then the star would be even more supergiant, spanning 1,200 Suns — even wider than Jupiter’s orbit.

Stellar beast
This image, made with the Atacama Large Millimeter/submillimeter Array (ALMA), shows the red supergiant Betelgeuse placed at the center of our solar system. This shows one estimate of the star's size, which isn't yet well understood.

In line with its larger size, the star would be even further along in its lifecycle. Stars like Betelgeuse live fast and large. Like the Sun, they first light up by fusing hydrogen into helium within their cores, but they quickly move on to helium, fusing it into carbon. Carbon then burns to make other, heavier elements. Around the core, lighter elements burn in shells, causing the star to billow outward like a hot plasma balloon.

Saio and his colleagues use computer simulations to watch stars evolve from birth to old age, then they calculate the pulsations they ought to see at each stage. They find that all four pulsations — from the 2,200-day cycle through the 185-day cycle — can be explained by a “breathing” star in the late stages of carbon-burning. “After carbon is exhausted in the core, a core-collapse leading to a supernova explosion is expected in a few tens years,” the researchers write.

But when will the carbon run out? It’s hard to tell because the pulsation periods don’t change much at this late stage. “It’s not possible to exactly estimate how much carbon is left in the core at present,” Saio says. “We just guess the time to the carbon exhaustion is probably less than a few hundred years.”

So, to put some headlines in perspective, Saio’s group isn’t saying Betelgeuse will blow tomorrow or even in the next decade. The researchers’ claim is that Betelgeuse would blow within 1,000 years rather than 10,000 or 100,000.

Notes of Caution

Other researchers are expressing qualms about the new calculations. Morgan MacLeod (Center for Astrophysics, Harvard & Smithsonian), a theorist who studies pulsating stars, says the new results don’t mesh with other observations of the star. The problem with taking the 2,200-day cycle to be part of Betelgeuse’s “breathing” is that it makes the supergiant too giant.

Betelgeuse is so big and so nearby that we can compare the size estimate from Saio’s team to actual interferometric measurements of its diameter. Saio’s team quotes several of these measurements, each of which finds a larger size between 1,000 and 1,500 times the Sun’s girth.

However, these are all taken at infrared wavelengths. Stars aren’t solid objects, and different wavelengths penetrate to different depths. Visible-light observations, which penetrate to the visible surface (or photosphere), give a smaller size. “The sizes mentioned in the paper (55 milli-arcseconds) are measured at several microns,” MacLeod says, “and are larger than the optical photosphere size of about 42 milli-arcseconds.” László Molnár and colleagues detail this contradiction further in the Research Notes of the AAS.

The 2,200-day pulsation, if radial, creates other problems, too. Spectroscopic measurements show that the star’s surface expands and contracts at some 1.5 kilometers per second (3,300 mph). If the star is “breathing” at this rate over the span of 2,200 days, its total diameter would be changing by 180 times the Sun’s size every cycle. Even for astronomers, that’s a lot.

What’s more, the 2,200-day pulsation would also affect the pulsation of the overtones. So, for example, the 400-day cycle wouldn’t always be 400 days. When the star puffs out to its full size, this overtone would lengthen; likewise, when the star shrinks, it would shorten. “These changes would be expected to repeat systematically every 2,200-day cycle,” MacLeod says, “and I don't think we see evidence for that in Betelegeuse's long-term light curve, which varies more randomly around the 400-day typical cycle.”

“I think the interpretation presented by Saio and colleagues isn't at all ruled out,” he adds. “But there are some potential questions that it raises, that, at face value, seem in tension with what the data indicate.”




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June 11, 2023 at 8:15 am

This intriguing article made me wonder how bright Betelgeuse will be when it explodes. The peak absolute magnitude of Type II supernovae is about -16. Absolute magnitude is the luminosity at a distance of 10 parsecs or 32.6 light-years, but the star is about 170 parsecs from Earth. Applying the inverse-square law of light indicates 6 magnitudes of dimming due to its distance. So, the Betelgeuse supernova will max out near magnitude -10 as seen from Earth. That’s more than 100 times brighter than Venus.

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June 12, 2023 at 9:56 am

Or in other words about as bright as the Half Moon (First Quarter/Last Quarter, 50% illuminated --according to the Mallama magnitude curves 😮 ). Impressive! And visible in daylight easily. And so bright that it would ruin "deep sky" observing at night for many weeks.

There will be a big difference, of course, comparing a Half Moon and a Betelgeuse supernova. All that brilliance will be concentrated in a tiny point of light. At night it will cast shadows, but unlike shadows cast by the Moon, there will be no penumbra, no smooth gradation of shading over a range of half a degree from fully dark shadow to fully illuminated surface. Instead, Betelgeuse will cast razor-sharp shadows presumably outlined with impressive diffraction fringes. Even casual observers will notice and be amazed... 🙂

A pinpoint of such brilliance might also be "blinding". Not enough to do any real retinal damage (by my estimate, which could be optimistic) but certainly enough to leave annoying "streaks" in an observer's vision for a few minutes. A Betelgeuse supernova would be a spectacular event, but it will "own" the sky for weeks. And poor Orion will never be the same again when it's all done. 🙁

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June 12, 2023 at 1:14 pm

How does one "like" a comment?Asking for a friend.

It will be be interesting if there are any total solar eclipses during May thru July while Betelguese is in its supernova phase.
Would it make the sky around it bright enough to partly diminish the "night sky during daytime" of totality?

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June 18, 2023 at 12:19 pm

You asked:
"How does one "like" a comment? Asking for a friend."

I think you just did. 🙂

You suggested the possibility of a total solar eclipse with Supernova Betelgeuse in the sky nearby. How would they compare? Let's go with the usual statement that the Sun's corona is comparable in brightness to the Full Moon, which seems reasonable. Given that Betelgeuse as a supernova would be about as bright as a Half Moon, that's several magnitudes fainter. So it would be impressive to see, but my guess is that it probably wouldn't disturb the "night for day" effect. 🙂

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June 16, 2023 at 4:39 pm

I calculate (assuming the SN reaches ~0.5" in diameter during its "kaboom" phase (lol - a star-shattering kaboom!), its brightness is -11.7 MPSAS.

A 1/2 moon at average distance should be in the ~5.0 MPSAS regime, so Betelgeuse will be BRILLIANT in comparison!!! [Using: MPSAS = M +2.5*log(A*f), where M is magnitude at time of observation, A is area in arc sec^2, and f is illuminated fraction of disk (0.5 for 1/2 or quarter phase). ]

FYI, Moon is around Mag -10.3 or so or in this range. Using the sun as another example, Sol would be: MPSAS = -26.7+2.5*(1.00*A); so using d(sol) as ~1890", we get -10.7 MPSAS!!!

So Betelgeuse will be in the range of Sol for sky brightness, albeit smaller. So use your sunglasses! lol. Should cast some nice shadows... I hope I am around for the big show...

Cheers, and clear skies all!


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June 18, 2023 at 11:19 am

DH, The tricky part here is that this is true of all stars all the time. They're sending "hot" photons at us, and photons don't get tired.

Suppose I look at a G2 main sequence star in my telescope. Let's say it's eta Cassiopeiae, for the sake of argument. Photons don't lose energy as they travel, so the intensity of light or "surface brightness" of eta Cas is identical to the surface brightness of our own Sun. So why doesn't it blind me when I look at eta Cas casually in the night sky or through a telescope?

The surface brightness of a star like eta Cas, which can be calculated as "magnitudes per square arc second" (your MPSAS) is basically identical to the Sun. So as with eclipse blindness issues from "Bailly's Beads" during a solar eclipse, this sounds like it could be dangerous (similar case: from the surface of Pluto or another Kuiper Belt object, the Sun would be fainter by about 8 magnitudes, but its small disk would still be blinding). But in this case, the star's size is far below the resolution limit of human vision. If I look at Betelgeuse when it goes supernova, I'll be looking at a resolution-limited disk. By naked eye, the resolution limit is about a minute of arc across. That's over 100x bigger than your 0.5" so more than 10 magnitudes fainter. Still dazzling and likely to leave temporary "streaks" in vision, but not blindingly bright.

Now suppose I look at "Supernova Betelgeuse" with a magnification of 100x through a telescope! 🙂 Would that do retinal damage? Or without UV (blocked by glass lenses), are we safe?? When Betelgeuse blows, I'm sure many of us will have the opportunity to try this experiment. Any volunteers? You go first... 🙂

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June 17, 2023 at 2:31 am

I can hardly wait ! I assume there will be indications that supernova is imminent (order-of-magnitude luminosity increases). But years, months, days or hours?

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June 18, 2023 at 11:47 am

Well, at least a few hours notice! That's because a supernova's energy is almost entirely carried away in a colossal blast of invisible neutrinos, and neutrinos fly faster than light (actually they fly just a tiny bit slower than light, but unlike photons, they blast out of a supernova with almost no delay and arrive before the great flash of light). You can check out the "SuperNova Early Warning System" here: https://snews.bnl.gov/
There's also a Wikipedia page for this, and it references "Sky & Telescope magazine's AstroAlert service" which seems to have moved on from S&T many years ago (someone should perhaps edit the Wikipedia page and the pages on the S&T website for "AstroAlert" which Google finds).

The odds that Betelgeuse will go supernova are still probably below 0.1% annually and quite possibly a hundred times smaller. I'm of two minds on this: I would love to see a brilliant naked-eye supernova... But I would be saddened to see Orion so broken when the remnant of Betelgeuse fades away...

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June 17, 2023 at 7:30 am

Regardless of when it happens, I predict NE Ohio will be cloudy for the entire time.

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June 17, 2023 at 11:27 pm

Cool article. The graphic of its size compared to the solar system, and the speed of its "breathing", are worth taking some time with, to pause and think about and try to wrap your brain around the magnitude of things in the universe! 🙂

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June 18, 2023 at 10:41 am

It looks huge, yes, but it's a "barely there" sort of giant... The size of this star is like the size of a cloud in the sky. Clouds can look like towering, solid mountains, but birds (and airplanes!) fly right through them. 🙂 Betelgeuse is "inflated", and most of it is thinner than any cloud. If Betelgeuse had the same mean density as our Sun, it would be less than three times bigger in diameter than our Sun. That's hundreds of times smaller than astronomers estimate and tens of millions of times difference in density. The star's outer layers have been puffed up to create a vast, thin glowing cloud around it. This is the "visible" size of the star, but the gas is so thin that a planet could fly through it. It would emerge a bit worse for wear, sure, but it wouldn't be stopped by passing through... 🙂

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June 18, 2023 at 9:23 am

If Betelgeuse is less than 1.000 LY away, and it is expected to blow within 1.000 years, it is already exploded now. It's just a matter of how long before the light comes.

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June 18, 2023 at 10:54 am

Yes and no. 🙂
Even stranger than the idea that there are events that have "already" occurred that we can't see yet is one of the less well-known features of the theory of relativity, called "relativity of simultaneity". The "present" --the set of events that are occurring "right now"-- depends on your relative speed. So to say that Betelgeuse has "already" exploded before we the light from it turns out to have no real meaning in astrophysics. Some observers moving in one direction at some speed will say, yes, it already exploded centuries ago while others moving in the opposite direction will say, no, its explosion is centuries in our future. The very concept of being "in the past" or events having "already happened" is relative to motion for events that are farther away than the light travel limit! Strange, right? The only thing that is "absolute" in a relativity sense is the light (and other phenomena travelling at the speed of light) that we are able to see, detect, and measure. When we see it explode, it has exploded. Until then? It's all relative. 🙂

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June 18, 2023 at 3:43 pm

You are probably right from a universal point of view.

But so "down to earth":

At one time or another – we can call it T0 – Betelgeuse explodes, and if you stand right next to it, you experience the explosion at the same time as it happens.

We here on Earth are 1,000 light-years from Betelgeuse and therefore experience it later.

One day/evening, here on earth, we will see Betelgeuse light up very strongly, and we will say that it has just exploded, say on 22 September 2058 at 15.30 UT.

Since Betelgeuse is 1,000 light-years away from us, and the light therefore takes 1,000 light-years to reach us, when we see the explosion, we can determine that T0 must be no later than September 22, 1958. (unless we're moving directly towards Betelgeuse at the same speed, then it will be faster, I guess. I haven't looked into this...)

That people elsewhere in the galaxy/universe don't see it yet - or have seen it - doesn't change the fact that T0 must occur before we see anything, I guess?

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June 19, 2023 at 2:55 pm

Yes, September 22, 1958 should be September 22, 1058...

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