A hidden star is orbiting inside Betelgeuse’s own atmosphere

Betelgeuse, the red supergiant marking Orion’s shoulder, is not traveling through space alone. A team led by Steve Howell, a senior research scientist at NASA’s Ames Research Center, directly imaged a companion star orbiting well inside the giant’s own extended atmosphere, roughly four times the distance between Earth and the Sun from Betelgeuse’s surface. The companion, provisionally named Betelgeuse B, is about 1.5 times the mass of the Sun and six magnitudes fainter than Betelgeuse itself in visible light.

The finding closed a debate that had run since astronomers first suspected Betelgeuse might have a hidden partner more than a century ago. Fluctuations in Betelgeuse’s brightness and measured velocity had long hinted at a companion, but the supergiant’s own glow made any direct observation of a fainter neighbor nearly impossible with earlier instruments.

Siwarha, a star hiding in Betelgeuse’s glow

Howell’s team gave the newly confirmed star the name Siwarha, Arabic for “her bracelet,” a nod to the traditional name Betelgeuse, which derives from Arabic for “the hand of al-Jawza’.” The pairing fits: Siwarha orbits close enough to sit almost against Betelgeuse’s outstretched hand, tracing a path that keeps it inside the outermost layers of the supergiant’s puffed-up atmosphere rather than in open space beyond it. Publishing the result in The Astrophysical Journal Letters, Howell’s team described the paper as the “Probable Direct Imaging Discovery of the Stellar Companion to Betelgeuse.”

Two independent teams of astronomers had reignited the search in the years before Howell’s confirmation, drawing on more than a century of accumulated Betelgeuse observations to predict where the companion should sit and how bright it should appear. Their predictions gave Howell’s group a narrow target rather than an open-ended search of the sky around one of the brightest stars visible from Earth.

The two stars likely formed together less than 10 million years ago, but their evolution since then has run on very different clocks. Betelgeuse ballooned into a red supergiant with a radius estimated at roughly 764 times the Sun’s, while Siwarha remains a pre-main-sequence star that has not yet started fusing hydrogen at its core, the ordinary marker of true stellar adulthood. That mismatch is itself a clue: a star of Siwarha’s estimated mass typically needs on the order of 10 million years to reach the main sequence, consistent with an age that lines up with Betelgeuse’s own.

The prediction that told Howell where to look

If the companion star existed, the location predictions suggested observers had only a few months to catch it at its widest separation from Betelgeuse, near the visible edge of the supergiant. Miss that window, and the companion would swing back around to the opposite side and vanish into Betelgeuse’s glare for roughly three more years before its next wide separation. Earlier attempts using space-based telescopes, chosen specifically to avoid the blurring that Earth’s atmosphere causes, still failed to detect anything in that gap.

Howell recognized that the ground-based Gemini North telescope in Hawaii, paired with a specialized NASA-built camera, could succeed where the space-based searches had not. “I hope our discovery excites other astrophysicists about the robust power of ground-based telescopes and speckle imagers,” Howell said, calling the combination “a key to opening new observational windows” that “can help unlock the great mysteries in our universe.”

What ‘Alopeke’s speckle imaging actually did

The instrument responsible, officially called the ‘Alopeke speckle imager, works by capturing many thousands of exposures lasting only milliseconds each. Each individual exposure freezes the atmospheric distortion that normally blurs ground-based images, and stacking thousands of them together lets researchers computationally remove that blur during processing. That technique let Howell’s team pull a companion roughly six magnitudes fainter than Betelgeuse out of the supergiant’s glare, directly imaging it right where the location predictions said it should be.

The Gemini North observations, run through NSF NOIRLab, mark the first time anyone has captured a direct image consistent with Betelgeuse’s predicted companion rather than an indirect signal in its brightness or velocity data. Howell’s team plans to keep observing the pair, and Siwarha is due to swing back out to its widest separation from Betelgeuse again in November 2027, offering the next best opportunity to refine measurements of its orbit and mass.

A companion that could explain Betelgeuse’s slow pulse

Confirming Siwarha does more than settle a century-old argument about one star. Betelgeuse and other red supergiants are known to dim and brighten on cycles lasting years, a pattern separate from their much shorter pulsation periods that has never had a fully satisfying explanation. NASA’s Jet Propulsion Laboratory notes that a close orbiting companion, periodically passing in front of or interacting with the outer layers of its host’s atmosphere, is exactly the kind of mechanism that could produce that longer secondary variation.

If Siwarha’s orbit turns out to be typical of what sits around other red supergiants, astronomers may need to revisit brightness records for stars they had assumed were single. Betelgeuse itself is expected to end its life as a supernova at some point in the astronomically near future, an event that would leave Siwarha’s own fate, whether destroyed, flung free, or otherwise altered, as the next open question in the story Howell’s team has only begun to answer.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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