The brightest red star in the night sky periodically becomes slightly elongated before growing rounder again. Radio astronomers have linked these changes to the position of a companion orbiting within its diffuse atmosphere. This is the strongest evidence yet that the supergiant Betelgeuse, in the constellation Orion, has a partner. The same hypothesis could explain why its brightness varies in two distinct cycles.
The Companion
The companion star, named Siwarha, or Betelgeuse B, orbits at a distance of about 2.3 times Betelgeuse’s radius. This means it travels through the supergiant’s extended atmosphere rather than outside it. If Betelgeuse replaced the Sun, its surface would reach roughly as far as the asteroid belt, while Siwarha would orbit somewhere between Jupiter and Saturn.
According to Universe Today, Betelgeuse’s outer layers are so diffuse that gas of this density would be considered a vacuum in a laboratory on Earth. Yet even this tenuous material is enough for the companion to disturb as it moves through it.
Siwarha belongs to a class of young yellow stars whose cores have not yet begun sustained hydrogen fusion. Observing it directly is extremely difficult, which is why some astronomers have doubted that the companion exists.
Changes in Shape and the Companion’s Motion
The new data were obtained using the Very Large Array (VLA) in the United States and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. From 2021 to 2026, astronomers observed Betelgeuse at seven frequencies and were able to resolve its disk rather than seeing it as a point. The results have been published in the peer-reviewed journal The Astronomical Journal.
The disk turned out to be slightly elongated, although its shortest diameter is no more than a tenth smaller than its longest. In a press release from the U.S. National Radio Astronomy Observatory, Lynn Matthews of MIT’s Haystack Observatory explains that combining data from both radio telescopes makes the result particularly convincing.
The shape is not constant. Betelgeuse is most elongated when Siwarha is calculated to be at its greatest apparent separation from the star, and roundest when the companion passes in front of or behind it.
The simplest explanation is that Siwarha’s gravity and motion through the gas stretch the supergiant’s atmosphere along the line connecting the two stars. When the companion appears to one side, this bulge extends across our line of sight and is visible in the images. When Siwarha is in front or behind, the bulge points toward or away from Earth, making the disk appear round.
The same observations showed that Betelgeuse’s atmosphere has warmed again. In 2019, ahead of the Great Dimming, when the star faded noticeably, the temperature of its outer layers fell to record lows. Now, at a distance of about 2.7 stellar radii, the gas has a temperature of approximately 3,500 kelvins, falling to 2,200 closer to the surface.
Two Brightness Cycles
Betelgeuse pulsates with a period of about 425 days, with a longer cycle of approximately 5.9 years superimposed on it. This longer cycle matches both the changes in the atmosphere’s shape and Siwarha’s presumed orbital period. Some of the variations in brightness and temperature may therefore be caused by the companion rather than the supergiant itself.
Siwarha’s existence has yet to be conclusively confirmed, as it is too faint at radio wavelengths to be observed directly. Testing the connection between the supergiant’s shape and the companion’s motion will require further observations with the same radio telescopes, covering a larger portion of the orbit.
