This ALMA image shows the submillimeter surface of Betelgeuse, revealing its irregular shape and regions of hotter gas. The brightest hotspot, toward the northeast of the star, appears at nearly the same location in ALMA observations separated by more than seven years, suggesting that some structures in Betelgeuse’s atmosphere can survive considerably longer than predicted by current models of stellar convection. Credit: ALMA (ESO/NAOJ/NRAO)/W. Dent et al.
Betelgeuse looks less like a perfect sphere than a boiling, misshapen blob. In some of the sharpest submillimeter images yet made of the famous red supergiant, its edge ripples in and out, bright patches break up its face, and clumps of molecular gas crowd the atmosphere around it.
The new study of Betelgeuse with the ALMA telescope draws attention to one prominent hot region that has remained in almost the same place for at least 7.3 years — far longer than the giant convective structures thought to churn through a star like this. This may ultimately force astronomers to rethink how convection, shocks and perhaps Betelgeuse’s recently detected companion shape a star approaching the end of its life.
Betelgeuse sits roughly 600 light-years away and is simply immense, with a radius about 800 times that of the Sun. In August 2023, researchers used the Atacama Large Millimeter/submillimeter Array, or ALMA, in its most extended configuration, with antennas separated by as much as 16 kilometers. That gave them a resolution as fine as seven milliarcseconds, enough to distinguish structures only about 17 percent as wide as Betelgeuse itself.
ALMA mapped radiation from a thick layer of Betelgeuse’s inner atmosphere just above the optical photosphere (stars are hot blobs of plasma and have no surface). Across most of this layer, the temperature was about 2,300 kelvins, or roughly 2,000 degrees Celsius.
A hotspot that refuses to disappear
Two hotter regions stood out, one northeast of the star’s center and a weaker one to the southwest. The northeastern patch was about 500 to 800 kelvins hotter than the surrounding gas and carried roughly 1 percent of the star’s luminosity at these wavelengths.
Astronomers had seen something similar before. A 2017 ALMA study of Betelgeuse’s atmosphere, using observations made in 2015, found a prominent hot region in the northeastern quadrant. The new team reanalyzed those data and compared them directly with the 2023 observations. The hotspot’s position differed by less than about two milliarcseconds, while its contrast had barely changed.
And that is difficult to square with current models. Simulations generally give coherent, large convective structures lifetimes ranging from months to a few years. Near-infrared observations from 2008 and optical and ultraviolet observations have also found bright structures in similar parts of the star. If they are manifestations of the same phenomenon — something the researchers cannot yet establish — the structures could have persisted for closer to two decades.
Astronomers have long suspected that a handful of enormous convection cells transport energy through red supergiants. Hot gas rises, overshoots the stellar surface and can launch shocks into the atmosphere. Those shocks may also help explain one of the big unsolved questions about red supergiants: how they lose so much mass before they die.
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Betelgeuse changes shape even when the hotspot stays put
The hotspot may be stubborn, but the rest of Betelgeuse is not.
Its submillimeter “surface” deviates from a circle by roughly plus or minus 6 percent in radius, with particularly large corrugations toward the southeast. Unlike the northeastern hotspot, these ripples changed noticeably between 2015 and 2023.
The gas above them changed even more. Silicon monoxide and carbon monoxide revealed a molecular atmosphere extending several stellar radii outward. In the new images, that gas is extremely clumpy, with differences in brightness approaching 10 to 1 between some regions. Its distribution looks substantially different from 2015.
These two ALMA observations also happen to fall on opposite sides of Betelgeuse’s famous Great Dimming. Between late 2019 and early 2020, the star faded dramatically, briefly drawing speculation that it might be preparing to explode. A 2021 Nature study of the Great Dimming instead found evidence that a cool patch on Betelgeuse helped form a cloud of dust that obscured part of the star.
The new ALMA data contain a weak molecular region in roughly the same direction, along with a detached clump of gas. If that clump was expelled during the dimming, its present position would imply an average outward speed of about 10 kilometers per second. But with observations from only two epochs, the researchers cannot yet connect it securely to the 2020 event.
And then there is Betelgeuse’s companion
Using ESO’s Very Large Telescope (VLT), astronomers have obtained the clearest image ever of what likely is Betelgeuse B, a star orbiting Betelgeuse.
Betelgeuse is a red supergiant star in the Orion constellation. Its brightness changes periodically, and for decades astronomers had suspected that some of these variations could be due to a companion star orbiting around it. Credit:ESO/M. Montargès et al. Background: N. Rissinger
Another possibility has become harder to ignore.
Astronomers recently obtained their strongest evidence yet that Betelgeuse has a close stellar companion. The proposed orbital axis of that companion lies close to the line connecting Betelgeuse’s two hot regions. Its predicted position during the 2023 ALMA observations was also near one of the most distorted parts of the star’s atmosphere.
That geometry is intriguing, but the researchers stop well short of claiming the companion causes the hotspots. Tides, rotation or a preference for long-lived convection near the stellar poles could all potentially play roles. The current ALMA data also show no clear signature of the rotation previously proposed for Betelgeuse.
Repeated observations should help distinguish between different explanations. If the hotspots remain fixed while the surrounding molecular gas and corrugated edge continue to change, astronomers may be looking at a surprisingly stable structure embedded inside an otherwise violently dynamic star.
“Its eventual fate as a supernova makes it fascinating to know what it actually looks like now,” lead author Bill Dent, an astronomer at ESO, said in the ESO release accompanying the new ALMA image.
