At long last, astronomers have firm evidence that Betelgeuse, one of the most famous stars in the night sky, is not alone. Using the European Southern Observatory's Very Large Telescope (ESO's VLT), a team led by French researcher Miguel Montargès obtained the clearest image ever of what likely is Betelgeuse B, the star orbiting Betelgeuse. "This is the conclusion of a century-long quest," says Montargès.
"We have shown that Betelgeuse is not single, it is accompanied by a faint stellar companion," says Montargès, astronomer at the Observatoire de Paris - PSL, France, and lead author of the study published today in Astronomy & Astrophysics. Betelgeuse — a reddish star in the Orion constellation that is easily visible with the naked eye and is known to change in brightness — has been observed for millennia. But it still surprises astronomers.
"I jumped from my chair when I saw the processed images," recalls Montargès. Astronomers have looked for the potential companion, originally proposed to explain Betelgeuse's brightness changes, for about a century, but without success. Two studies published in 2024 robustly predicted that in December that year, the companion would be furthest from Betelgeuse and hence easier to spot. Montargès and his team got to work, observing the star with ESO's VLT in December 2024 and spending several months processing the data.
"Honestly, I thought we did not have the sensitivity to detect Betelgeuse B as it was predicted," explains Montargès. "Because it is more massive than predicted, we see it!" Originally thought to be about as massive as the Sun, the new observations reveal that Betelgeuse B has instead around two to three times the mass of the Sun. "The fact that we can still discover a nearby companion, more massive and brighter than the Sun, around such a well-studied star is remarkable," says Montargès. "These are among the best moments in science: seeing something new, unexpected."
Betelgeuse B was directly imaged, meaning the light from the star itself was detected, using the SPHERE instrument on ESO's VLT in Chile's Atacama Desert. While there was evidence for the existence of this companion star, including a possible direct detection with the Gemini North Telescope in Hawaiʻi, USA, this is the strongest evidence for, and clearest image yet of, Betelgeuse B. "It is remarkable to see how SPHERE and advanced post-processing techniques, originally developed to find exoplanets, also excel at detecting a companion around a massive, evolved star like Betelgeuse," says co-author Anthony Boccaletti, also an astronomer at the Observatoire de Paris.
"To be certain that the companion is really there, we still need to observe it in one year on the other side of the star, but there is very little space left for doubt," adds Montargès.
A few years back, Betelgeuse was the subject of attention from astronomers and non-astronomers alike when it visibly started dimming. As an evolved supergiant star, Betelgeuse is expected to die in a supernova explosion, and the dimming led some to speculate it could be about to explode. A group of astronomers led by Montargès studied the star with ESO's VLT , finding it was instead obscured by a cloud of dust.
The potential detection of Betelgeuse B will prompt astronomers to investigate how the companion could affect Betelgeuse's anticipated supernova explosion. "The question is truly opened whether this companion is going to have an impact on the evolution of the red supergiant," concludes Montargès.
More information
This research was presented in a paper titled "VLT/SPHERE images the candidate companion of Betelgeuse" to appear in Astronomy & Astrophysics.
The team is composed of M. Montargès (Laboratoire d'Instrumentation et de Recherche en Astrophysique, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, France [LIRA]), A. Boccaletti (LIRA), O. Flasseur (Universite Claude Bernard Lyon 1, Centre de Recherche Astrophysique de Lyon UMR5574, ENS de Lyon, CNRS, France), A. de Koter (University of Amsterdam, Anton Pannekoek Institute for Astronomy, The Netherlands), J. Milli (Univ. Grenoble Alpes, CNRS, IPAG, France), P. Kervella (French-Chilean Laboratory for Astronomy, IRL 3386, CNRS and U. de Chile, Chile and LIRA), S. Ridgway (National Optical Astronomy Observatory, USA), E. Bordier (I. Physikalisches Institut der Universität zu Köln, Germany), E. Lagadec (Université Côte dAzur, Observatoire de la Côte dAzur, CNRS, Laboratoire Lagrange, France), A. K. Dupree (Center for Astrophysics-Harvard & Smithsonian, USA), F. Backs (Institute of Astronomy, KU Leuven, Belgium), T. Calderwood (American Association of Variable Star Observers, USA [AAVSO]), and P. Morgan (AAVSO).
The European Southern Observatory (ESO) enables scientists worldwide to discover the secrets of the Universe for the benefit of all. We design, build and operate world-class observatories on the ground — which astronomers use to tackle exciting questions and spread the fascination of astronomy — and promote international collaboration for astronomy. Established as an intergovernmental organisation in 1962, today ESO is supported by 16 Member States (Austria, Belgium, Czechia, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom), along with the host state of Chile and with Australia as a Strategic Partner. ESO's headquarters and its visitor centre and planetarium, the ESO Supernova, are located close to Munich in Germany, while the Chilean Atacama Desert, a marvellous place with unique conditions to observe the sky, hosts our telescopes. ESO operates three observing sites: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its Very Large Telescope Interferometer, as well as survey telescopes such as VISTA. Also at Paranal, ESO will host and operate the south array of the Cherenkov Telescope Array Observatory, the world's largest and most sensitive gamma-ray observatory. Together with international partners, ESO operates ALMA on Chajnantor, a facility that observes the skies in the millimetre and submillimetre range. At Cerro Armazones, near Paranal, we are building "the world's biggest eye on the sky" — ESO's Extremely Large Telescope. From our offices in Santiago, Chile we support our operations in the country and engage with Chilean partners and society.
Links
- Research paper
- Photos of the VLT: http://www.eso.org/public/images/archive/category/paranal/
