Betelgeuse, one of the most familiar stars in the sky, may have a hidden companion star orbiting it
One of the best known and most luminous stars in the night sky, Betelgeuse has long intrigued anyone who has gazed up and seen its reddish tint, which is visible to the naked eye. What has most fascinated astronomers, however, is that its brightness has been known to change over time. Now, they think the newly detected celestial object may hold the key to understanding Betelgeuse's varying brightness.
From late 2019 to the beginning of 2020, Betelgeuse dimmed so sharply that astronomers 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.
The Great Dimming led to an increased interest in solving longstanding mysteries about one of the cosmos' most observed stars — such as why its brightness appears to fluctuate regularly over a six-year cycle and has for decades.
A team of astronomers has now discovered an explanation. Using an instrument on the Gemini North telescope in Hawaii, they employed an unusual imaging technique to get a glimpse of a suspected companion star, colloquially called 'Betelbuddy,' that builds on a previous theory.
They suggest calling the star 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.)
Understanding more about the dynamic between Betelgeuse and its companion star, also referred to as Ori B in a new study published Thursday in The Astrophysical Journal Letters, could shed light on the entwined fate of both stars.
As a supergiant star, Betelgeuse is immense. Compared with our sun, it's about 700 times the radius and contains 18 times as much mass, said lead study author Steve Howell, a senior research scientist at NASA Ames Research Center in California. If our sun were replaced with Betelgeuse, the star would not only engulf Earth and all the inner planets but reach past the orbit of Jupiter, according to NASA. It also shines 7,500 to 14,000 times as bright as the sun.
At 10 million years old, Betelgeuse is a fraction of the age of our sun, which is estimated to be 4.5 billion years old. However, Betelgeuse's enormousness means it has already burned through all the hydrogen at its core, causing it to expand as it nears the end of its life.
Years of observations have shown that its luminosity varies periodically about every 416 days, growing fainter and then brighter. This pulsation is typical of red supergiant stars.
But Betelgeuse displays an unusual pattern on top of that. 'It has been noted for decades that Betelgeuse also shows a much longer period (of variation) of about 2,170 days (about six years) which remained unexplained,' Howell wrote in an email.
Two independent groups of astronomers published papers in 2024 suggesting that an unseen companion star could cause the variability. The Hubble Space Telescope and NASA's Chandra X-Ray Observatory, however, could see no evidence of such a star. Betelgeuse's size and brightness have posed challenges to attempts to spot a companion.
To see both Betelgeuse and its companion, an image has to be both high-resolution and high-contrast, said Jared Goldberg, a research fellow at the Flatiron Institute's Center for Computational Astrophysics. Goldberg authored a November study suggesting Betelgeuse may have a companion star, but he was not involved with the new research.
'Normally, the Earth's atmosphere makes it hard to do this for the same reason that stars twinkle — the moving gas in the atmosphere scatters the starlight around,' Goldberg said.
Howell's team decided to use a speckle imager called 'Alopeke, which means 'fox' in Hawaiian, to search for the companion.
'Speckle imaging is a technique that obtained many thousands of very short exposures of an astronomical object,' Howell said. 'These images are so short that they do not look like stars or galaxies at all, but a blob of 'speckles.''
The speckles are due to distortions from Earth's atmosphere. The thousands of brief images are processed in a way that removes the atmospheric blurring, resulting in a high-resolution telescope image, Howell said.
When members of Howell's team observed Betelgeuse during the Great Dimming in 2020, they didn't see anything; the companion was likely obscured behind Betelgeuse, according to Goldberg. But in December, they spied a faint blue glow exactly where Goldberg's research — as well as another study authored by Morgan MacLeod at the Harvard-Smithsonian Center for Astrophysics — predicted the companion would be.
The speckle imaging revealed a young, bluish star that isn't burning hydrogen at its core yet and only has a mass of 1.5 times that of the sun.
The companion star's faintness — four-tenths of one percent as bright as Betelgeuse — is just one reason it's been hard to spot, Howell said. The other is the stars' proximity to one another — only about four times the distance between Earth and the sun separates them. On average, the Earth is about 93 million miles (150 million kilometers) away from the sun. The thing that allows the companion star to be seen, said Goldberg, is that it is a different hue than Betelgeuse.
'If the two headlights on a car represent the two stars, our view from Earth to Betelgeuse and its companion would be the same as trying to separate the two car headlights with your eye from a distance of 50,000 miles,' Howell said. 'Our observations were aided by the fact that we can directly observe Betelgeuse using very short exposures (14 milliseconds each) so as to not saturate our cameras and the large mirror size of Gemini (8 meters) allows us to obtain very high angular resolutions in images of the sky, enough resolution to separate the two stars.'
It's the first time a stellar companion has been detected orbiting a supergiant star so closely, the study authors said.
'I was surprised that the companion was so obvious immediately after our data was processed,' Howell said. 'I was thinking it'd be hard to find, but boom, it was right there.'
MacLeod, a postdoctoral fellow in theoretical astrophysics and member of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, worked on research published in December that collected historical measurements of Betelgeuse's radial velocity, or motion toward or away from Earth, that began around 1896 on photographic glass plates. The team saw a repeating six-year pattern consistent with the tug of a smaller, orbiting companion star, MacLeod said.
'Putting these lines of evidence, collected from a century of astronomical measurements, together let us predict right where a companion 'should be' if it were real,' he said. 'But we hadn't seen it directly. Howell and his team made a pioneering observation in order to be able to make this initial detection.'
MacLeod, who was not involved with the new study, calls its finding 'an amazing result … that shows that even the best-studied stars in our night sky have mysteries to reveal.'
'Because this was such a challenging detection to make, the observations are on the very edge of detection,' MacLeod said. 'What pushed this over the edge is that the star appeared just where we expected when we pulled together the predictions of a century's worth of astronomers.'
While the discovery of the companion aligns with Goldberg's research predictions, future observations are still needed to confirm the detection. Speckle imaging is a hard measurement to make and isn't always accurate, Goldberg said.
Given that the star was discovered near the limits of the instrument, its presence is probable but 'not yet a slam dunk,' 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.
However, seeing the companion star track along its proposed six-year orbit would represent a definitive detection of the companion, Guinan said.
'Currently, we think the companion is moving away from us, and going behind Betelgeuse. So there is a clear path to confirm the new study's results: Look again when we expect the companion to be fully behind Betelgeuse, and it will be gone. Look once more when it should be coming back around on the other side, and it should be there,' Goldberg said.
A new opportunity to confirm the companion's presence with telescopes will occur in November 2027 when the star would be at its farthest distance from Betelgeuse, making it easier to spot.
Like MacLeod's team, Goldberg and his colleagues also determined that Betelgeuse wobbles toward and away from Earth within the same six-year period due to the presence of a stellar companion. Still, questions remain about how exactly a companion star is contributing to Betelgeuse's six-year variability, which appears to be connected to changes in dust around the star, Goldberg said.
'The dimmer phase happens when the companion is behind Betelgeuse, and the brighter phase is when the companion is in front of Betelgeuse,' Goldberg said by email. 'This means it's the opposite of an eclipse, so it seems most likely that Betelgeuse is producing its own dust and the companion is shaping it, rather than dragging it along.'
About 30% of pulsating red giant and supergiant stars show the same type of variability, and if that means a companion is present, 'then many more stars harbor these little friends,' Goldberg added. 'Understanding this stellar pair can help us understand the population of things like it. And understanding that population will teach us about star and planet formation in systems that are otherwise extremely hard to observe.'
Meanwhile, astronomers still wonder when Betelgeuse will explode, a catastrophic event that has been anticipated since the Great Dimming. While Betelgeuse and its companion star were likely born at the same time, the companion is still forming as a normal star, Howell said. But companion's close orbit, within the outer layers of Betelgeuse's atmosphere, will be its doom, he said.
One of two things will happen. The companion star's orbit may cause it to drift slowly closer and plunge into Betelgeuse in about 10,000 years.
'At that point Betelgeuse and its companion will enter into an eternal hug,' Goldberg said. 'If we can get decades of precise direct observations, we might be able to directly test that prediction by seeing if the orbit is shrinking, and if so how quickly.'
But if Betelgeuse explodes before that — 'maybe tomorrow, maybe in 100 years' — then the companion star will be destroyed in the supernova, Howell said. 'The future is not good for either star.'
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