Astronomers Capture Clearest-Ever Image of Betelgeuse's Elusive Companion Star
ESO's Very Large Telescope captured the clearest-ever image of Betelgeuse B, showing the long-sought companion is 2.6 to 3.1 times the sun's mass.
Overview
Astronomers using the European Southern Observatory’s Very Large Telescope (VLT) have obtained the clearest image yet of Betelgeuse B, a companion star long suspected to orbit the red supergiant Betelgeuse, according to ESO. Betelgeuse, a reddish star in the constellation Orion, is easily visible to the naked eye and has been observed for millennia, and astronomers have searched for a companion to explain its brightness changes for approximately a century without success, according to ESO.
“This is the conclusion of a century-long quest,” said team leader Miguel Montargès, an astronomer at the Observatoire de Paris - PSL, France, according to Space.com. “We have shown that Betelgeuse is not single, it is accompanied by a faint stellar companion,” Montargès said, according to ESO.
What We Know
Two studies published in 2024 predicted that a hypothetical companion would reach its furthest point from Betelgeuse — and be easiest to detect — in December of that year, according to ESO. Montargès and his team observed Betelgeuse with the VLT’s SPHERE instrument on the nights of December 3 and 6, 2024, using its Zurich Imaging Polarimeter in a technique called angular plus spectral differential imaging, according to the research paper published in the journal Astronomy & Astrophysics. The team then spent several months processing the data, according to ESO.
Using two independent processing methods that produced consistent results, the researchers detected the candidate companion at significances of 6.1 sigma and 5.1 sigma respectively in the telescope’s CntHa filter — a continuum band centered at 644.9 nanometers near the hydrogen-alpha line — while no secondary source appeared in the narrower NHa filter centered at 656.34 nanometers, according to the research paper. The detected object sits at a projected separation of 52.32 milliarcseconds from Betelgeuse at a position angle of 117.12 degrees, the paper states.
Assuming the two stars are the same age, the team estimates Betelgeuse B has a mass of 2.6 to 3.1 times that of the sun and matches a young main-sequence star of spectral type B8.5V to B9.5V, according to the research paper. That is well above earlier theoretical predictions that the companion was only about as massive as the sun, according to ESO. “Because it is more massive than predicted, we see it!” Montargès said, according to ESO.
The new detection is far stronger than a previous attempt: researchers using the Gemini North telescope’s ‘Alopeke speckle imager in Hawai’i searched for the companion at the same predicted maximum elongation but reported only a tentative signal at 1.5 sigma, according to the research paper.
The separation also matches earlier orbital-period estimates: for a Betelgeuse mass of 16.5 to 19 times the sun, the newly measured separation implies a minimum orbital period of roughly 5.5 to 5.9 years, in line with the 5.94-year and 5.78-year periods separately derived by two research teams in 2024 and 2025, and consistent with a roughly 2,200-day, six-year brightness cycle some astronomers have linked to the hypothetical companion, according to the research paper.
“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,” said Anthony Boccaletti, a co-author and astronomer at the Observatoire de Paris, according to ESO.
What We Don’t Know
Researchers still need to observe Betelgeuse B again on the opposite side of the star to confirm it is truly a gravitationally bound companion rather than a chance alignment, according to the research paper. “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,” Montargès said, according to ESO. “The question is truly opened whether this companion is going to have an impact on the evolution of the red supergiant,” he added, according to ESO. Additional observations are planned to assess how the companion could affect Betelgeuse’s anticipated supernova explosion, according to Phys.org.
Background
Betelgeuse gained wide attention a few years ago when it visibly began dimming, according to ESO. As an evolved supergiant star, Betelgeuse is expected to eventually die in a supernova explosion, according to ESO, though a team led by Montargès later found the dimming was caused by a cloud of dust rather than an imminent explosion, according to ESO.