ALMA Reveals Betelgeuse's Stubborn Hotspot Defies Stellar Models

Betelgeuse does not look like a star should. Through the sharpest submillimeter images ever captured of the famous red supergiant, it appears as a boiling, lopsided mass—its edges rippling inward and outward, bright patches scattered across its face, clumps of molecular gas crowding the space around it. The Atacama Large Millimeter/submillimeter Array, or ALMA, pointed its most extended configuration at Betelgeuse in August 2023, with antennas spread across 16 kilometers, achieving a resolution fine enough to see structures only about 17 percent as wide as the star itself. What astronomers found there has begun to trouble the models they use to understand how stars like this one work.

The most striking discovery was a hotspot—a region roughly 500 to 800 kelvins hotter than the surrounding atmosphere—that has refused to move or fade. Located northeast of the star's center, this patch carries about 1 percent of Betelgeuse's luminosity at submillimeter wavelengths. When researchers compared the 2023 ALMA data with observations from 2015, the hotspot's position had shifted by less than two milliarcseconds, and its brightness had barely changed. That stability is the problem. Current models of stellar convection predict that large, coherent structures like this should persist for only months to a few years. If the hotspot is the same phenomenon captured in near-infrared observations from 2008 and optical and ultraviolet data from around the same era, it may have been burning in the same location for nearly two decades.

Betelgeuse sits about 600 light-years away and is simply enormous—its radius roughly 800 times that of the Sun. When ALMA mapped radiation from the thick inner layer of its atmosphere just above the optical photosphere, it found most of that region hovering around 2,300 kelvins, or about 2,000 degrees Celsius. The northeastern hotspot stood out sharply against this baseline, a persistent anomaly that forces a reckoning with how astronomers understand energy transport in red supergiants. Theorists have long believed that a handful of enormous convection cells move heat through these stars, with hot gas rising, overshooting the stellar surface, and launching shocks into the atmosphere. Those shocks may help explain one of the enduring mysteries about red supergiants: how they shed so much mass before they explode.

Yet the rest of Betelgeuse tells a different story. Its submillimeter surface deviates from a perfect circle by roughly plus or minus 6 percent in radius, with particularly pronounced ripples toward the southeast. Unlike the stubborn northeastern hotspot, these corrugations changed noticeably between 2015 and 2023. The molecular atmosphere above them—traced by silicon monoxide and carbon monoxide—proved even more volatile. In the new images, that gas is extremely clumpy, with brightness differences approaching 10 to 1 between some regions. Its overall distribution looks substantially different from eight years earlier, as if the star's outer layers are in constant flux while something deeper holds firm.

The two ALMA observations bracket one of the most dramatic events in Betelgeuse's recent history. Between late 2019 and early 2020, the star underwent its famous Great Dimming, fading so dramatically that some astronomers wondered if it was preparing to explode. A 2021 study in Nature found that a cool patch on Betelgeuse had helped form a cloud of dust that obscured part of the star. The new ALMA data show 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 current position would suggest an outward speed of about 10 kilometers per second. But with only two epochs of observation, researchers cannot yet securely connect it to the 2020 event.

Another factor has become harder to dismiss. Astronomers recently gathered 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, and its predicted position during the 2023 ALMA observations was near one of the most distorted parts of the star's atmosphere. The geometry is intriguing, though researchers carefully avoid claiming the companion causes the hotspots. Tides, rotation, or a preference for long-lived convection near the stellar poles could all play roles. The current ALMA data show no clear signature of the rotation previously proposed for Betelgeuse.

What emerges from these observations is a portrait of a star approaching the end of its life in a state of profound contradiction. The hotspot remains locked in place, defying the predictions of current models and suggesting that something about stellar convection in red supergiants remains fundamentally misunderstood. Meanwhile, the atmosphere churns and shifts, the molecular gas clumps and redistributes, the edge ripples and warps. Repeated observations in the coming years should help distinguish between competing explanations—whether the hotspot is anchored by a companion star, stabilized by rotation, or simply a manifestation of convection physics that astronomers have yet to fully grasp. For now, Betelgeuse keeps its secrets, a boiling, misshapen blob that refuses to fit neatly into the models built to explain it.