ALMA finds long-lived hotspot on Betelgeuse

The images from the Atacama Desert show not a calm point of light, but an uneven stellar atmosphere: Betelgeuse's brightest hotspot is up to about 800 Kelvin hotter than the surrounding gas. The images were captured in 2023 with the Atacama Large Millimeter/Submillimeter Array (ALMA), which is operated by the European Southern Observatory (ESO) and international partners.

Betelgeuse is about 600 light-years from Earth. Its radius is roughly 800 times that of the Sun—enough for ALMA to make structures in its atmosphere visible that remain hidden on most other stars. The atmosphere's average temperature is around 2300 Kelvin; at least two hotter regions lie in the northeast and southwest of the stellar disk.

One possible explanation from ESO researchers points inside the star. Hot gas rises from deeper layers and can generate shock waves when it reaches the outer atmosphere. This could produce the bright, irregular features. The crucial comparison comes from the archive: Compared with similar ALMA data from 2015, the northeastern hotspot was almost in the same location in 2023 and showed a similar intensity. The structure has apparently existed for at least seven years—longer than current models predict for large convective structures.

What comes next? Astronomers now have a detailed view of the atmosphere of a nearby red supergiant. ALMA measured variations of up to about six percent in the visible radius; weaker emissions extended several stellar radii into the atmosphere. Further high-resolution observations should show whether the hotspots remain fixed and how they are connected to convection, mass loss and the extended atmosphere.

The star became significantly fainter in late 2019 and early 2020, prompting speculation about an imminent supernova. That has not happened so far; researchers now consider an explosion in the immediate future less likely. The new data provide a detailed picture of Betelgeuse's surface. The team's findings have been accepted for publication in “Astronomy & Astrophysics” and are already available on arXiv.