Betelgeuse’s atmosphere has gone blotchy and a hidden companion star may be the reason

Astronomers pointing the Atacama Large Millimeter Array at Betelgeuse have found that the red supergiant’s inner atmosphere has grown noticeably spottier and more lopsided since the star’s dramatic dimming event in 2020. The new observations, led by Bill Dent of the University of Manchester, mark the first time the instrument has resolved that inner layer of the star since the dimming, and the team says the uneven pattern may be connected to a much smaller companion star that other researchers have recently proposed orbits close to Betelgeuse.

A First Look Since the Great Dimming

Betelgeuse, the bright red star marking Orion’s shoulder, became a global talking point in late 2019 and early 2020 when it dimmed far more than usual, prompting speculation about whether the aging supergiant might be approaching a supernova. That event faded, and the star’s brightness eventually recovered, but the episode left astronomers with a strong interest in tracking what its outer layers were doing next.

The new ALMA observations represent the first time researchers have imaged Betelgeuse’s inner atmosphere at this resolution since that 2020 event. Because Betelgeuse is one of the closest and largest stars visible from Earth, ALMA can resolve structure across its surface in a way that is not possible for almost any other star outside the solar system, giving astronomers a rare direct look at how a red supergiant’s outer layers change over time.

Betelgeuse sits roughly 550 light-years from Earth, close enough on an astronomical scale that its apparent size in the sky, though still a single point to the naked eye, is large enough for powerful arrays like ALMA to resolve as an actual disk with visible surface features. That combination of proximity and sheer physical size, Betelgeuse is estimated to be hundreds of times the diameter of the sun, makes it one of only a handful of stars beyond the solar system where astronomers can study surface-level detail rather than relying purely on brightness and spectral measurements.

What ALMA Found in the Inner Atmosphere

According to the team’s findings, Betelgeuse’s inner atmosphere has become increasingly uneven and asymmetric since 2015, developing distinct hot spots rather than a smoothly varying surface. Some of those spots run roughly 800 degrees hotter than the surrounding atmosphere, a substantial temperature differential for a single stellar surface and a sign of significant ongoing turbulence beneath the visible layers.

That kind of patchiness is not entirely new in giant stars, whose surfaces are known to churn with large convective cells, but the degree of asymmetry recorded in these observations stands out. The pattern suggests processes beneath Betelgeuse’s surface are organizing heat and material in ways current models of stellar convection do not fully predict.

Hotspots That Refuse to Move

One of the more striking results involves a single hotspot toward the northeast of the star that has stayed in nearly the same position across ALMA observations separated by more than seven years. ALMA’s own account of the findings describes this persistence as unusual, since convective hot spots on giant stars are generally expected to shift and dissolve over periods of months or a few years, not remain fixed across nearly a decade.

That longevity is part of what makes the companion star hypothesis worth taking seriously. A feature anchored to roughly the same location for years is more consistent with an external gravitational influence, something orbiting nearby and periodically reinforcing the pattern, than with the essentially random churn of ordinary stellar convection.

The Companion Star Hypothesis

Separate research in recent months has proposed that Betelgeuse may host a much smaller, fainter companion star orbiting close enough to interact with its outer atmosphere. Dent’s team notes that the orientation of the long-lived hotspots aligns intriguingly with that proposed companion’s expected position, raising the possibility that the smaller star’s gravity is shaping how material and heat distribute across Betelgeuse’s surface.

A companion star, even a small one, orbiting inside or near a supergiant’s extended atmosphere could plausibly carve out consistent structure by disturbing the gas around it in a repeating pattern tied to the orbit, rather than the essentially random turbulence that drives ordinary convective hot spots.

What Remains Unconfirmed

Researchers have been careful to describe the connection between the hotspots and the proposed companion as suggestive rather than established. Coverage of the study notes that the team stops well short of claiming the companion directly causes the hotspot pattern, and no direct detection of the companion itself has been confirmed through these observations.

Confirming or ruling out the companion star will likely require additional observing campaigns, potentially combining ALMA’s resolution with other instruments capable of detecting a faint object so close to one of the brightest stars in the sky. Until then, the persistent asymmetry stands as one of the more unusual features documented on Betelgeuse since its widely watched 2020 dimming, with the underlying cause still an open question for the astronomers studying it.

The findings are expected to appear in the journal Astronomy & Astrophysics, adding a peer-reviewed record to a body of Betelgeuse research that has expanded rapidly since the 2020 dimming drew renewed attention to the star. Because Betelgeuse is close enough and bright enough to observe in detail with instruments on the ground, researchers expect it to remain one of the most closely tracked stars in the sky for years, regardless of whether the companion hypothesis is eventually confirmed.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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