ALMA shows Betelgeuse’s lumpy atmosphere and a hotspot that has lasted years

New images of Betelgeuse from the Atacama Large Millimeter/submillimeter Array, taken in 2023 and circulating this week, show a star that is not round and not even. The average temperature of the atmosphere in the ALMA band is about 2,020 degrees Celsius. Two hotter patches sit on the disk. The brightest is about 526 degrees Celsius hotter than the gas around it. The star's apparent radius varies by up to about 6 percent. The outline is bumpy because the surface is a field of rising and falling gas, not a smooth photosphere like the Sun's.

The hotspot is the problem. Large convective cells in standard models of red supergiants should form and fade on timescales shorter than the seven-plus years this feature has now been seen. ALMA's 2023 map lines up with earlier images well enough that the same bright region is still there. That endurance is what made astronomers sit up. Either the models underestimate how long a cell can live, or something else is pinning the heat in place.

The companion in the same frame of mind

In July a team led by Miguel Montargès at Paris Observatory published the clearest direct image yet of a faint companion, provisionally Betelgeuse B, using the SPHERE instrument on ESO's Very Large Telescope. The December 2024 frames were timed for predicted maximum separation. The companion comes out at about two to three solar masses, heavier than the Sun-like object earlier papers had assumed. Montargès called it the end of a century-long question: Betelgeuse is not a single star.

The proposed orbital axis of that companion sits close to the line that joins the two hot regions on the ALMA disk. The companion's predicted position in 2023 was also near one of the most distorted parts of the atmosphere. The ALMA team does not claim the companion causes the hotspot. Tides, rotation or a preference for long-lived convection near the poles could all do the work. The geometry is now hard to ignore.

Why a lumpy shoulder of Orion matters

Betelgeuse is about 800 times the Sun's radius and sits roughly 600 to 724 light-years away, depending on the distance scale used. It is the nearest red supergiant that can be resolved as a disk. Its Great Dimming in 2019 and 2020 set off a public countdown to a supernova that has not arrived. A companion and a stubborn hotspot change the clock. Mass transfer, extra mixing and a second set of orbital dynamics all feed into how and when the core collapses.

ALMA sees millimetre light from the extended atmosphere. SPHERE sees a point source next to a glare that has to be subtracted with tools built for exoplanets. Together they turn Betelgeuse from a single famous variable into a small system. The next useful measurement is a second-epoch image of B at a different point in its orbit, and another ALMA map to test whether the hotspot has moved. Until then the shoulder of Orion is a star with a partner and a burn that will not fade on schedule.