The companion star Betelgeuse kept hidden for a century has finally been photographed

Q: What caused the Great Dimming of Betelgeuse in 2019–2020?

A dust cloud ejected from Betelgeuse's own surface. The companion was not identified as a cause, and the connection between the two — if any — remains an open question.

Every great star has its secrets. Betelgeuse, the red supergiant at the shoulder of Orion and one of the most observed stars in history, has been keeping one for about 100 years: astronomers expected it to have a companion. They predicted its existence. They modeled its mass and its orbit. What they had not done, until this year, was see it directly.

The image is not dramatic. It is a single bright point, about 30,000 times dimmer than Betelgeuse itself, captured in December 2024 by the SPHERE instrument on ESO’s Very Large Telescope in Chile’s Atacama Desert. Behind that point lies a century’s worth of physics and suspicion — and, in the words of lead researcher Miguel Montargès of the Observatoire de Paris, the kind of moment that makes scientists jump from their chairs.

How they made the detection

SPHERE was not built for this. The instrument was designed to detect exoplanets by suppressing the overwhelming glare of their host stars. Betelgeuse’s companion presented a specific version of that problem: Betelgeuse is roughly 700 times the diameter of the Sun, and at 30,000-to-one in brightness, any companion hides in its light.

To isolate the companion’s signal, the team first needed the right geometry. The companion had to be at its maximum angular separation from Betelgeuse as seen from Earth — the moment when the two objects sit furthest apart in the sky. That window opened in December 2024. The team then spent months processing the data, subtracting Betelgeuse’s contribution pixel by pixel until a faint secondary source emerged. That source — informally named Siwarha — is the object that has been missing from 100 years of stellar models.

What Betelgeuse B actually is

The companion turns out to be more massive than most models predicted. Earlier estimates placed it near the Sun’s mass. The new data points to a star 2 to 3 times heavier — a hot, blue B-type main-sequence star with an estimated orbital period of roughly six years around Betelgeuse.

That mass changes the significance of the discovery. A companion this size is not a passive bystander. It influences how Betelgeuse sheds mass through its stellar wind, how the outer layers of the red supergiant move, and potentially how the eventual supernova will unfold. “The question is truly opened whether this companion is going to have an impact on the evolution of the red supergiant,” Montargès said.

A century of suspicion

The idea that Betelgeuse was not alone dates back roughly a century, when astronomers noticed that its periodic brightness variations were too complex to explain by the star’s own pulsations alone. A gravitational companion could account for some of that variability — pulling on Betelgeuse’s outer layers and adding irregularities to its light curve.

Betelgeuse is already among the most watched stars in the sky for a separate reason: it will go supernova within the next 100,000 years. When it does, the explosion will be briefly visible in daylight from Earth. The 2019–2020 “Great Dimming” event, in which Betelgeuse faded dramatically, drew global attention before Montargès’s team showed it was caused by a massive dust ejection from Betelgeuse’s own surface — not by impending collapse.

A companion adds a new variable to that history. Binary star systems evolve differently from single stars. The presence of Siwarha introduces unknowns into models of Betelgeuse’s mass-loss history, its rotation, and the geometry of whatever explosion eventually comes.

What this does not settle

“This is the conclusion of a century-long quest,” Montargès said. The search, however, is not over. The image is the strongest evidence yet of the companion — it is not a final confirmation. To be certain that Betelgeuse B is physically orbiting the red supergiant, rather than a background star that happens to appear nearby, the team needs a second observation. With an estimated orbital period of six years, the companion should appear on the opposite side of Betelgeuse in approximately one year. If it does, the interpretation becomes unambiguous.

Gravitational binding is also unresolved. The companion’s position matches where models predicted it should be at this phase of the orbit — strong circumstantial evidence. But confirming that prediction requires the follow-up measurement.

The implications for Betelgeuse’s supernova remain incalculable. Orbital distance, the exact mass ratio, and the degree of tidal interaction between the two stars must all be established before models can incorporate Siwarha into realistic explosion scenarios. The discovery opens the question; it does not answer it.

Common questions about Betelgeuse and its companion

When will Betelgeuse go supernova? Within the next 100,000 years, though the estimate carries large uncertainties. Predicting stellar death timing is genuinely hard even with complete models, and the companion’s existence introduces new variables that existing predictions did not include.

Could Betelgeuse B affect the supernova? Possibly. Binary companions can alter a star’s rotation rate, mass-loss rate, and the geometry of its outer envelope — factors that shape the explosion’s energy and shape. Researchers cannot quantify that influence yet because the orbital parameters are not fully established.

What caused the Great Dimming of 2019–2020? A dust cloud that Betelgeuse ejected from its own surface. The companion was not identified as a cause, and the connection between the two — if any — remains an open question.

How far is Betelgeuse? Approximately 700 light-years. Its eventual supernova will be visible from Earth in daylight but poses no danger at that distance. The nearest stars that could produce a dangerous gamma-ray burst are much closer and differently oriented.

Montargès’s team will observe Betelgeuse again in late 2025, watching for Siwarha on the far side of the orbit. If the companion appears where the six-year orbital model places it, a century of hypothesis becomes a confirmed fact.

Montargès et al., “VLT/SPHERE imaging of the candidate companion of Betelgeuse,” Astronomy & Astrophysics, 2026. DOI: 10.1051/0004-6361/202661023

Tags: astronomy, ESO, Supernova, Betelgeuse, Miguel Montargès, SPHERE instrument