The red supergiant marking Orion's shoulder seems to be spinning too fast. Did it get a boost when merged with a smaller companion star 100,000 years ago?

Betelgeuse and Rigel in Orion
Orion, the Hunter, is a hallmark of northern winter skies. The red supergiant star Betelgeuse marks one of his shoulders.
Akira Fujii

This much is clear: Someday Betelgeuse will explode as a supernova. It's roughly 650 light-years away — and when it goes, it'll be spectacular.

But astronomers can't estimate when that might happen, because virtually everything known about this star is uncertain. Its surface temperature, mass, luminosity, and even its distance aren't pinned down very well.

Those loose ends have nagged at J. Craig Wheeler, a colorful theorist and supernova specialist at the University of Texas, who has long been "obsessed with the uncertainty in the evolutionary state of Betelgeuse." But to narrow the date range for the star's eventual demise, he needs tighter constraints on its basic characteristics. Wheeler and an international team of undergraduate students have mounted an effort called the "Betelgeuse Project" to try to do that, and their results appear in an article published December 19th in the Monthly Notices of the Royal Astronomical Society.

Best guesses, as put forward earlier this year by Michelle Dolan (University of Notre Dame) and others, are that Betelgeuse has 19 times the Sun's mass (somewhat larger than previously assumed), 126,000 times its energy output, a surface temperature of 3500 kelvin, and a diameter of at least 1.2 billion kilometers — big enough to gobble up everything to the outer fringe of our asteroid belt and maybe even Jupiter too.

Size of Betelgeuse
Astronomers first determined the diameter of Betelgeuse in 1920, but it wasn't until 1995 that the Hubble Space Telescope recorded the supergiant star's swollen disk.
A. Dupree / R. Gilliland / NASA / ESA

These guesstimates are all tied to how far away the star is, and for now we don't know that more accurately than to about ±25%. Another complication is that astronomers can resolve the disk of Betelgeuse (it's about 0.05 arcsecond across) — but, again, the true value is uncertain.

And don't expect the European Space Agency's Gaia astrometry mission to pinpoint this star's distance. Betelgeuse is too bright for the spacecraft's sensitive sensors. Gaia can use a small filter to dim the light of stars brighter than magnitude 3, explains project scientist Timo Prusti, but the stars' centers still saturate. The "extremely challenging" analysis and reduction of these small images will take time, Prusti says, with "quite some patience needed."

Another key characteristic is how fast Betelgeuse's bloated outer envelope spins. The blue- and redshifting of its spectrum (one limb is moving toward us, while the one on the opposite side is moving away), combined with a tilt to our line of sight of about 20°, yields a rotation speed along the equator of about 15 km per second. In its youth Betelgeuse must have spun much faster, perhaps as fast as 250 km/s. But like a spinning figure skater who flings her arms outward to slow down, the swelling of Betelgeuse as it became a red supergiant and the gradual loss of angular momentum via outflowing stellar winds have slowed the rotation rate dramatically.

Betelgeuse is Spinning (Too) Fast

Now, 15 km/s might not seem all that fast, but for comparison the Sun's equator crawls along at just 2 km/s. Having modeled the star's evolution with various masses and spin rates, Wheeler and his student team find the 15 km/s spin to be a particularly challenging constraint that only fits "at a very special, short-lived point in the evolution" lasting a mere 1,000 years. Given that this soon-to-die star is perhaps 8 or 8½ million years old, it's highly unlikely we're seeing it at such a short blip in its lifetime.

According to Wheeler's team, one "out" could be that Betelgeuse formed as part of a binary system and that it gobbled up a 1-solar-mass companion while ballooning to its present size.

“Suppose Betelgeuse had a companion when it was first born," he muses in a University of Texas press release. "And let’s just suppose it is orbiting around Betelgeuse at an orbit about the size that Betelgeuse is now. And then Betelgeuse turns into a red supergiant and absorbs it — swallows it."

Actually, this hypothesis isn't far-fetched. Most beefy stars, those in spectral classes O or B, do form as binaries, and there's a 1-in-5 chance that the massive, solitary stars seen today are actually mergers of two paired suns into one.

Bubble around Betelgeuse
This 2012 infrared image of Betelgeuse by the orbiting Herschel telescope shows shells of matter on one side of the star.
Leen Decin / ESA

There might even be circumstantial evidence for Betelgeuse's gluttonous act. We've known for two decades that this star is surrounded by a double-rimmed shell of matter that lies about 7 arcminutes away. Some astronomers think it's a shock front created as the star plows through the interstellar medium. But Wheeler calculates that the shell is just about where it should be if Betelgeuse had "burped" while devouring its companion roughly 100,000 years ago.

The next step for the Betelgeuse Project will be to be to probe the star's interior using a technique called asteroseismology. This might reveal, for instance, whether a relatively dense, undissolved remnant of the sacrificial companion lies within the star's enormous volume. Stay tuned!


Image of Jim-Baughman


December 23, 2016 at 8:19 pm

Surely the spin rate for the outer layers of Betelgeuse calls for some context, particularly when its 15 km/s rotation speed is specifically compared with the Sun’s 2 km/s rate in the article.

Since the circumference of Betelgeuse is almost 1400 times that of the Sun, its rate of rotation, in terms of the entire star, lags feebly behind that of the Sun. If the two rotated at the same rate, Betelgeuse’s typical rotation would be comparable to a real crawl on the Sun—ten meters per second (.01 km instead of 2km/s). Likewise if the Sun’s current speed were reflected comparably on Betelgeuse, the massive star’s outer layers would whip around at a zippy 2775 km per second.

So while it appears possible to tease out evidence of a merger in Betelgeuse’s past from its spin, it does not help your argument to compare its current spin rate with that of the Sun, characterizing one as “too fast” and the other as “crawl[ing] along”. If anything the situation is reversed.

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Image of J. Kelly Beatty

J. Kelly Beatty

December 23, 2016 at 10:24 pm

Jim, well, I certainly see your point, but a lot of people don't consider 15 km/s "fast" by cosmic standards, so I was looking for a convenient comparison. bottom line is that the outer envelope of Betelgeuse shouldn't be spinning so fast (should be much slower, allowing for all the angular momentum issues).

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Image of Jim-Baughman


December 23, 2016 at 10:49 pm

Thanks Kelly!

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Image of John-Murrell


January 2, 2017 at 2:40 pm

In Prof. Gerry Gilmore's lecture to the Royal Astronomical Siciety about Gaia he stated that Gaia should be able to measure the position and proper motion of all bright stars includig Sirius so that it is likley that Betelgeuse will have it's position measured as well. The only problem other than reducing the observations as stated in the talk is that Gaia may not see Betelgeuse as a point source due to it's size.

The talk is available on line at, it's well worth listening to to show what Gaia has delivered and what will be in future data releases.

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February 10, 2017 at 3:47 pm

With great respect to Kelly Beatty, thanks for writing on Betelgeuse and its possible demise. Regarding its "higher than unexpected" rotational speed perhaps we can use the imperfect analogy of a beautiful ice dancer ending a performance with a powerful display that combines rotational speed and angular velocity in the form of the skaters spin. The skater spins and pulls their arms in toward their core while manipulating their body to accomplish the desired effect. Having seen several "crude" photoshop manipulated images (ESA space telescope) that claim to show extreme events taking place on that sun as well as protuberances or hot spots it begs the question is Betelgeuse in the process behind its ejected dust shroud in the process of core collapse. Though my eyes are old and approaching their seventh decade of use I have noticed since I was a boy with a small refractor that Betelgeuse has both noticeably dimmed and has changed in color from a neon like red to more of an orange iron magnesium shade. I cannot help but wonder if Betelgeuse did not eat a possible companion star so much as it has a nice little black hole near it pulling mass off it thus "cracking the skaters whip". It is my hope that NASA and the Folks at ESA and China are actively studying Betelgeuse less we have an unexpected end that shows up like the proverbial biblical "thief in the night" or a badly abused riddle from Nostradamus presenting earth with a repeat demonstration of the Leschamp event and the true power of the cosmos. Warmest regards, David Pawlowski

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