ALMA Watched Betelgeuse's Surface for a Decade. Hot Spots 800 Degrees Above Their Surroundings Are Not Wandering at Random.

Astronomers using the Atacama Large Millimeter Array have produced the sharpest look yet at the inner atmosphere of Betelgeuse, and the picture is no longer symmetric. Since 2015 the red supergiant's surface has grown visibly patchy, with hot spots running roughly 800 degrees Celsius above the material around them.

The work was led by Bill Dent at the University of Manchester and published in Astronomy & Astrophysics. ALMA, a 66-antenna array on the Chajnantor plateau in Chile, resolved features at 7 milliarcseconds. That is the angular equivalent of picking out something 43 feet across on the surface of the moon.

What makes the result more than a pretty image is that one of the hot spots has stayed put. "The brightest hot spot was also seen in the 2015 data, suggesting that convective cells are actually rather persistent," Dent said. Red supergiants are supposed to boil. Their outer layers are so tenuous and so hot that convection cells the size of the inner solar system rise, radiate and collapse, and the textbook expectation is that they churn on timescales of months to a few years. A cell that holds its position across a decade is a different kind of object.

The hot spots also line up. Rather than scattering across the disk, the brightest regions cluster along a preferred direction, which points to something imposing an axis on the star. In July 2026, astronomers confirmed what had been suspected for a century: Betelgeuse has a close companion, a much smaller star orbiting inside the supergiant's extended envelope. If that companion's orbit lies in Betelgeuse's equatorial plane, it would stir the equatorial regions while leaving the poles comparatively undisturbed — and stable polar convection is exactly where the persistent bright spots appear to sit.

That connection matters for the question everyone actually asks about Betelgeuse, which is when it will explode and what it will look like when it does. The star's 2019–2020 Great Dimming, when it faded by more than a magnitude and briefly dropped out of the top ten brightest stars in the sky, is now generally attributed to a dust cloud thrown off by the star itself and a cool patch on the surface. Understanding how mass leaves the star — in random puffs, or in structured outflows organized by a companion — changes the models of how much material a red supergiant sheds before core collapse.

Betelgeuse is roughly 550 light years away and somewhere between 10 and 20 times the mass of the sun. It will end as a Type II supernova, though the timescale astronomers give runs from tomorrow to 100,000 years from now, which is a way of saying nobody knows. What the ALMA data provide is a decade-long baseline on the layer where the star's mass loss begins.

Further observations are planned as the companion moves through its orbit. If the hot-spot pattern shifts in step with it, the case for a companion-driven atmosphere gets much stronger.