Betelgeuse’s elusive binary companion finally comes into view

One long-running puzzle about the red giant Betelgeuse can, at last, be settled. Astronomers have now directly spotted a faint, low-mass binary companion racing around the enormous star.

What they saw aligns closely with earlier forecasts of the companion’s characteristics inferred from Betelgeuse’s observed behaviour. As Betelgeuse’s name is commonly rendered as “Hand of the Giantess” in Arabic, the research team suggests the object orbiting this “hand” should be called Siwarha - “Her Bracelet”.

It is the sort of observation many researchers suspected might be out of reach altogether, which makes the result all the more remarkable.

"This detection was at the very extremes of what can be accomplished with Gemini in terms of high-angular resolution imaging, and it worked," says Steve Howell, an astrophysicist at NASA Ames Research Centre. "This now opens the door for other observational pursuits of a similar nature."

Betelgeuse in Orion: a nearby giant nearing the end

Betelgeuse sits in Orion at an imprecise but comparatively nearby distance of about 548 light-years. It is among the largest and brightest stars visible from Earth, a hugely swollen object in a late-life phase, with a mass estimated at between 16.5 and 19 times that of the Sun and a radius of roughly 764 times the Sun’s.

Although it is only around 10 million years old - which sounds youthful next to stars approaching the 13.8 billion-year age of the Universe - its great mass explains the apparent contradiction. High-mass stars run hotter and brighter and consume their fuel far more quickly than smaller stars, and they finish in dramatic supernova explosions that leave behind either a neutron star or a black hole.

That combination makes Betelgeuse a particularly valuable target: a rare, short-lived star caught in the brief final stretch of its lifespan, close enough for detailed study.

The “great dimming” and an unexplained six-year rhythm

Those close observations have revealed that Betelgeuse behaves oddly. Most famously, during the “great dimming” its brightness fell by 35 percent - a mystery later linked to a major dust ejection that cooled and temporarily blocked some of the star’s light.

Its brightness varies in other patterns as well. Two strong cycles dominate: one repeating about every 400 days, and another on a timescale of roughly six years. The 400-day variation is the primary one, understood as the result of pulsations within the star.

The secondary, six-year cycle has been harder to pin down, largely because it did not match what internal stellar activity alone would predict.

Why astronomers expected a companion - and why it was hard to see

In several recent papers, researchers argued that a nearby, low-mass binary companion could explain the six-year signal, suggesting an object with between about one and two times the Sun’s mass. Those studies also indicated December 2024 would be the most favourable window to try to spot it.

The challenge is straightforward: Betelgeuse is so bright and so large that teasing out a much dimmer neighbour beside it is extremely difficult. To tackle this, Howell and colleagues used speckle imaging, which relies on extremely short exposure bursts to counteract the blurring introduced by Earth’s atmosphere.

Siwarha: a faint companion at 4 astronomical units

The team’s reported detection carries a confidence level of only 1.5 sigma, but it matches key expectations: a companion star appears in the predicted location and at the predicted time.

Howell and his collaborators estimate the companion’s mass at about 1.6 times that of the Sun, with an orbital separation of 4 astronomical units - approximately four times the distance between Earth and the Sun. That would place Siwarha nearer to Betelgeuse than Jupiter is to the Sun, and it implies an orbital period of 5.94 years.

"The results presented here are not definitive, as the detection is at the limit of the instrument capabilities," the researchers write in their paper. "However, the results do present the most direct and substantive evidence for the existence of a stellar companion to Betelgeuse, as well as the properties of that companion."

Boom. Time for the mic drop.

An F-type star that may never reach the main sequence

From the observations, Siwarha appears to be an F-type star that has not yet reached the main sequence - it is still insufficiently developed for hydrogen fusion to begin in its core.

This also fits with how stellar lifetimes scale with mass: massive stars evolve quickly, while lower-mass stars take longer to settle into stable fusion. Betelgeuse and Siwarha were likely born together, yet Betelgeuse is now close to the end, whereas Siwarha’s long-term, “adult” phase has barely begun.

In fact, it may never properly begin at all. Astronomers expect Betelgeuse to explode as a supernova at some point within the next 100,000 years, most likely leaving a neutron star. When that happens, Siwarha will be uncomfortably close to the blast.

The next good chance to observe Siwarha is expected around November 2027. That leaves astronomers only a short couple of years to refine their methods in the hope of getting a clearer look at this hard-to-catch - and possibly doomed - star.

The research has been published in The Astrophysical Journal Letters.