Invisible companion leaves evidence trail around supergiant star Betelgeuse

By Ashley Strickland, CNN

(CNN) — Astronomers have long searched for clues that a hidden companion star sits out of view near the red supergiant star Betelgeuse. Now, they’ve uncovered a new piece of evidence: a trail like the wake behind a boat, cutting through Betelgeuse’s upper atmosphere, likely forged by the unseen companion.

Betelgeuse’s reddish tint can be spotted gleaming from the Orion constellation, situated about 650 light-years away from Earth.

The bright star is so large that more than 400 million suns could fit inside of it. Its relative proximity and luminosity have made it a favorite among astronomers who observe and study the giant star’s evolution.

Yet, despite being so well known, Betelgeuse has its share of secrets — one of the biggest being why it appears to vary in brightness over a six-year cycle, and if the unseen stellar companion, nicknamed “Betelbuddy,” is responsible for this variability.

Hints about the possible companion star were shared in research published last year, in which scientists suggested formally naming the object Siwarha, or “her bracelet,” an Arabic name befitting the companion to Betelgeuse, which means “Hand of the Giant.” (“Elgeuse” is also the historic Arabic name of the Orion constellation.)

Siwarha would likely be too small and faint to be seen given its proximity to Betelgeuse, which has expanded as it has burned through all the hydrogen at its core, bringing it near the end of its life.

Now, observations taken over the last eight years have revealed the effects of Siwarha on Betelgeuse: a never-before-seen dense trail of gas spotted moving through the larger star’s outer atmosphere, where Siwarha closely orbits.

Siwarha’s trail appeared just after the star crossed in front of Betelgeuse from Earth’s perspective. Astronomers believe Siwarha completes one orbit around Betelgeuse every six years — hence the larger star’s change in brightness every six years. The observations are included in a new study that has been accepted for publication in the Astrophysical Journal.

“It’s a bit like a boat moving through water. The companion star creates a ripple effect in Betelgeuse’s atmosphere that we can actually see in the data,” said lead study author Andrea Dupree, an astronomer at the Harvard-Smithsonian Center for Astrophysics, in a statement.

“For the first time, we’re seeing direct signs of this wake, or trail of gas, confirming that Betelgeuse really does have a hidden companion shaping its appearance and behavior.”

Spotting the wake

Betelgeuse is about 15 times the mass of our sun and 1,400 times bigger in diameter, Dupree said. Meanwhile, Siwarha is tiny and might be smaller than our sun.

“If you placed Betelgeuse at the center of our solar system, the surface would extend to Jupiter and the hot atmosphere above it would extend at least 6 times further out,” Dupree said. “So the companion is really plowing through a dense atmosphere of the supergiant star.”

Dupree’s team has been tracking changes in Betelgeuse’s light for years, using the Hubble Space Telescope as well as ground-based observatories like the Fred Lawrence Whipple Observatory and Roque de Los Muchachos Observatory.

Patterns emerged in the observations, captured both close to the star and further away from it, that suggested a companion star was whirling through Betelgeuse’s extended atmosphere. The team recorded shifts in the speed and direction of gases in the large star’s outer atmosphere due to a disturbance.

Hubble enabled the team to see how Betelgeuse’s deep atmosphere, or chromosphere, reacted to Siwarha’s motion, while the ground-based observations revealed changes in the extended atmosphere.

“What we learn from these latest results is that Siwarha seems to ‘stir up’ Betelgeuse’s extended atmosphere as it orbits, leaving a wake that impacts what we see from Betelgeuse itself,” said study coauthor Morgan MacLeod, a postdoctoral fellow in theoretical astrophysics and member of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics.

“This wake is evidence of Siwarha’s presence and it’s also a trace of how such a small companion is able to affect what we see of Betelgeuse, by modulating the gas and dust around it,” he added.

The results add support to findings about Siwarha made by Steve Howell and his team in 2025. Howell is a senior research scientist at NASA Ames Research Center in California.

His team’s observations suggested that the detected size of Siwarha also included a larger region around it as it plowed through Betelgeuse’s atmosphere.

“The idea of some expanding plasma wake surrounding Siwarha would seem to agree with our finding of a larger, blobby source and not just a naked stellar photosphere,” Howell said. He was not involved in the new study.

Windows into giant stars

Tracking Betelgeuse’s behavior and variations can help astronomers better understand other, faint and distant giant stars that are difficult to study, MacLeod said.

But in recent years, Betelgeuse has thrown astronomers a few puzzling curveballs that have required serious detective work to solve.

From late 2019 to the beginning of 2020, Betelgeuse dimmed so sharply that experts thought the star was on the brink of exploding in a supernova. Since the event, called the “Great Dimming,” teams of astronomers have determined that the star ejected a large dust cloud, which temporarily blocked some of its light from Earth’s perspective.

Additionally, the star has exhibited two regular variations in luminosity: the six-year period, as well as a cycle that lasts a little over a year.

Data collected over years have shown that Betelgeuse’s luminosity varies about every 416 days, growing fainter and then brighter. This pulsation, which occurs within Betelgeuse’s core, was found to be typical of red supergiant stars.

The longer 2,100-day period was thought to be due to dust clouds, large convection cells on the star, magnetic activity or an elusive companion star always out of the optical reach of telescopes.

Within the last year, evidence from multiple research teams has pointed to the reason being the presence of Siwarha.

“With this new direct evidence, Betelgeuse gives us a front-row seat to watch how a giant star changes over time,” Dupree said. “Finding the wake from its companion means we can now understand how stars like this evolve, shed material, and eventually explode as supernovae.”

Other supergiant stars that also display similar patterns of long and short variability may also have invisible companions, Dupree added.

Jared Goldberg, a research fellow at the Flatiron Institute’s Center for Computational Astrophysics, believes the new observations are an important piece of the puzzle in understanding what causes Betelgeuse’s six-year variability. Goldberg previously authored research about the companion star, but was not involved in this study.

If astronomers can conclude that periodic variability is due to the presence of a second star, “we can make direct connections to the physics behind formation and evolution of both stars and planets,” Goldberg said.

The team is also working on hydrodynamic models to determine how the wake forms behind Siwarha and reconstruct how that effects Betelgeuse’s brightness, MacLeod said.

A hydrodynamic analysis would provide the vital information needed to interpret the new observations, such as what to expect from the motion of a solar-size companion moving through the outer atmosphere of a huge red supergiant, said Edward Guinan, professor of astronomy and astrophysics at Villanova University in Pennsylvania. Guinan has studied Betelgeuse but was not involved in the new research.

The uncertain future of Siwarha

With Siwarha in such close proximity to Betelgeuse, what is the smaller star’s fate?

MacLeod believes Siwarha is being drawn increasingly closer to its massive companion due to gravitational forces, which could result in a merger within 9,000 years.

“We think the stars might merge even before Betelgeuse goes supernova,” MacLeod said. “This would splash out some gas, spin up Betelgeuse and potentially affect the properties of the eventual supernova.”

Based on astronomers’ calculations, Siwarha is now on the far side of Betelgeuse relative to Earth’s telescopes, but it is expected to emerge into view in 2027.

“Further detections will allow refinements to its orbit and thus give a better mass of Siwarha, helping to explain the evolution of the binary system, Betelgeuse’s rapid rotation, and the future of the two stars,” Howell said.

Multiple teams are hoping to find a way to detect Siwarha optically.

“Seeing the companion will be the ultimate proof of its implied existence,” Guinan said. “I can’t wait.”

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