The dimming, six years on
In late 2019 and early 2020, Betelgeuse, the bright red star on Orion's shoulder, dropped to magnitude 1.614, its faintest level in more than a century of record keeping. The event became known as the Great Dimming. A cloud of dust ejected by the star and partly obscuring it is the accepted explanation for the drop in visible light.
The question since then has been what happened to the atmosphere itself. New radio observations with the US National Science Foundation's Karl G. Jansky Very Large Array and the Atacama Large Millimeter/submillimeter Array give an answer: it has largely bounced back.
Temperature readings, then and now
The researchers measured brightness temperatures at different distances from the star's centre. At about 2.7 times the stellar radius, they found roughly 3,500 kelvin. At the VLA frequencies, that value has risen from the unusually low levels recorded in 2019, before the dimming. The atmosphere has recovered much of the structure it lost.
They also compared the new maps with earlier measurements at millimetre wavelengths. Some of those older readings, taken with ALMA, still sit below the trend defined by more recent data, which may point to a lasting temperature drop in the inner atmosphere. The authors are careful: with limited epochs, they cannot yet prove that link.
A companion leaving a wake
The maps also show Betelgeuse's outline changing. When Siwarha, the recently identified companion, should be farther from the star along our line of sight, the disc appears more elongated. When the companion should be in front of or behind the star, the disc looks rounder. The pattern fits a small hot star ploughing through the outer atmosphere at a distance of only about twice Betelgeuse's radius.
Lead author Lynn Matthews compared it to a jet boat tearing across a lake, leaving a wake. Siwarha is too faint to see directly at radio wavelengths, so its presence is inferred from the shape it carves.
The result, published in the Astronomical Journal, matters beyond one famous star. Red supergiants shed much of their mass through extended, weakly bound atmospheres, and a close companion may shape that outflow in ways models have not included. More observations across the orbit are the next step.