Betelgeuse, the giant red star that forms the shoulder of the constellation Orion, is being shaped and sculpted by a companion star.
And this smaller companion star may even explain why Betelgeuse has been acting so strangely in recent years.
More on Betelgeuse
Discovering Betelgeuse's companion
The news of Betelgeuse's companion star was announced in July 2025.
It was long thought that Betelgeuse may have had a companion star in orbit around it, but astronomers were finally able to confirm this using the Hubble Space Telescope and ground-based observatories.
Observations have now revealed how the companion star – named 'Siwarha' – churns up gas surrounding the red supergiant, leaving behind a swirling trail that helps explain one of Betelgeuse’s long-standing mysteries.
A wake through Betelgeuse's atmosphere
The team detected Betelgeuse's companion’s influence by monitoring subtle changes in the red star's light over nearly eight years.
These revealed a dense wake of gas moving through the star’s vast outer atmosphere: direct evidence the companion star is having an observable effect on Betelgeuse.
"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," says Andrea Dupree, an astronomer at the Center for Astrophysics, Harvard & Smithsonian and the lead study author.
"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 behaviour.”
Why Betelgeuse is an astronomer's dream
Betelgeuse is about 650 lightyears from Earth, which makes it just a stone's throw away, in cosmic terms.
It can be seen from Earth in the constellation Orion and is easily visible with the naked eye, appearing as the left shoulder of the hunter, from our perspective on Earth.
Betelegeuse is a red supergiant star, so big you could fit over 400 million Suns inside it.
Because it's so enormous and so close to Earth, it's one of the few stars whose surface and surrounding atmosphere can be directly observed.
That makes Betelgeuse a crucial laboratory for studying how massive stars age, lose material and eventually explode as supernovae.
Decoding decades of data
In recent years, Betelgeuse has undergone a series of dimming events, during which it has periodically changed in brightness.
In 2020, the star suddenly dimmed in an event that became known as its stellar 'sneeze.'
Scientists identified two key cycles in Betelgeuse’s variability:
- A 400-day cycle, now attributed to pulsations within the star
- A longer 2,100-day cycle, which remained unexplained
This new study shows the longer cycle of variability matches the orbital period of Siwarha as it passes through Betelgeuse’s atmosphere roughly every six years.
Revealing the companion's signature
Using the Hubble Space Telescope, along with telescopes at the Fred Lawrence Whipple Observatory and Roque de Los Muchachos Observatory, the team detected changes in Betelgeuse’s spectrum, which refers to the different colours of light given off by different elements in the star.
They also found changes in the speed and direction of gas in Betelgeuses's outer atmosphere, which occur just after the companion crosses in front of Betelgeuse from Earth’s perspective.
This is evidence, the team say, that there's a wake – a trail of gas created by the companion star’s motion.
Solving the Betelgeuse mystery
Astronomers had previously proposed many explanations for Betelgeuse’s behaviour, including giant convection cells, dust clouds, magnetic activity and the possibility of a hidden companion.
While earlier studies suggested a low-mass star embedded deep within the supergiant’s atmosphere, proof had remained elusive.
"With this new direct evidence, Betelgeuse gives us a front-row seat to watch how a giant star changes over time," says Dupree.
"Finding the wake from its companion means we can now understand how stars like this evolve, shed material, and eventually explode as supernovae."
From Earth’s perspective, Betelgeuse's companion is currently behind it, and therefore not visible.
But astronomers say they're already planning new observations to coincide with Siwarha's re-emergence in 2027, which will give them another chance to observe how it shapes the Betelgeuse's atmosphere.
