Gas in the outer layer of Betelgeuse now measures around 3,500 Kelvin at a radius of about 2.7 times the star’s size, matching the red supergiant’s own surface temperature according to new radio observations spanning seven frequencies from 2021 to 2026. Researchers attribute the perturbations to a close-in companion star named Siwarha, which orbits deep within the red supergiant’s outer shell.
The red supergiant star marking Orion’s shoulder experienced a dramatic 2019 and early 2020 dimming when it dimmed to its faintest in more than a century of record-keeping. Now, combined data from the U.S. National Science Foundation’s Karl G. Jansky Very Large Array and the Atacama Large Millimeter/submillimeter Array show the star’s extended atmosphere has largely recovered. Observations collected between 2021 and 2026 across seven radio frequencies, ranging from 22 GHz to 485 GHz, reveal that gas in the outer layer now measures around 3,500 Kelvin at a radius of about 2.7 times the star’s size, matching Betelgeuse’s own surface temperature.
Siwarha Pows Through Betelgeuse
The recovery comes paired with evidence that Betelgeuse’s atmosphere is being actively perturbed by a recently identified companion star. Proposed in 2024 to explain a mysterious 2,100-day brightness cycle, the companion was dubbed Siwarha, meaning “her bracelet,” in reference to the star’s Arabic name meaning the hand of al-Jawza.
Led by Dr. Lynn D. Matthews, principal research scientist and assistant director at the MIT Haystack Observatory, researchers calculated that Siwarha orbits deep within Betelgeuse’s extended atmosphere at a distance of only about twice the giant star’s radius.
This companion is orbiting so close to the star that it actually passes through its extended atmosphere.
Dr. Lynn D. Matthews, MIT Haystack Observatory
Like a jet boat tearing through a lake, the smaller star leaves a wake in its path as it rips through the gas. Although Siwarha itself is far too faint to detect directly at radio wavelengths—calculated to be tens to hundreds of times too faint to see—its physical presence leaves a measurable mark. By tracking the subtle egg-like shape, or ellipticity, of Betelgeuse’s radio-emitting atmosphere across multiple epochs, the research team found that the star becomes noticeably more elongated when the companion reaches its greatest apparent separation, and rounder when passing in front of or behind the giant host.
Telescopes Map The MOLsphere
Combining data from the NSF VLA and ALMA allowed Matthews and colleagues from the Center for Astrophysics | Harvard & Smithsonian, the University of Manchester, and the European Southern Observatory to construct a layer-by-layer picture of the stellar atmosphere. The highest-frequency ALMA measurements reached closer to the stellar surface than ever before, confirming a long-suspected temperature minimum where gas temperatures drop to roughly 2,200 Kelvin—over 1,000 degrees cooler than the photosphere itself.