Astronomers may finally have found a companion star orbiting the red giant on Orion's shoulder

Astronomers may finally have found a companion star orbiting the red giant on Orion's shoulder

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This orange blob shows the nearby star Betelgeuse, as seen by the Atacama Large Millimeter/submillimeter Array (ALMA). This is the first time that ALMA has ever observed the surface of a star and this first attempt has resulted in the highest-resolution image of Betelgeuse available. Betelgeuse is one of the largest stars currently known — with a radius around 1400 times larger than the Sun’s in the millimeter continuum. About 600 light-years away in the constellation of Orion (The Hunter), the red supergiant burns brightly, causing it to have only a short life expectancy. The star is just about eight million years old, but is already on the verge of becoming a supernova. When that happens, the resulting explosion will be visible from Earth, even in broad daylight. The star has been observed in many other wavelengths, particularly in the visible, infrared, and ultraviolet. Using ESO’s Very Large Telescope astronomers discovered a vast plume of gas almost as large as our Solar System. Astronomers have also found a gigantic bubble that boils away on Betelgeuse’s surface. These features help to explain how the star is shedding gas and dust at tremendous rates (eso0927, eso1121). In this picture, ALMA observes the hot gas of the photosphere of Betelgeuse at sub-millimeter wavelengths — where localised increased temperatures explain why it is not symmetric. Scientifically, ALMA can help us to understand the extended atmospheres of these hot, blazing stars. Links: Size comparison: Betelgeuse and the Sun

Iain Todd

Find the main facts in our quick 1-minute read:

  • Official discovery: Astronomers using the Gemini North telescope’s ‘Alopeke speckle imager have likely detected a stellar companion to Betelgeuse for the first time, solving a century‑old hypothesis
  • Hypothesis confirmed? The companion aligns with predictions from two 2024 modeling studies that linked Betelgeuse’s roughly 6‑year brightness cycle to a hidden orbiting star
  • Imaging technique: By taking thousands of ultra‑short exposures, the team avoided Betelgeuse’s glare and captured the faint signal of the close companion
  • Companion properties: The star appears to be about 1.5 times the mass of the Sun, an early-stage pre-main-sequence star (likely A‑ or B‑type), and six magnitudes fainter than Betelgeuse
  • Tight orbit: It orbits at roughly four times the Earth–Sun distance, placing it within Betelgeuse’s enormous outer atmosphere
  • Mystery solved? This companion offers a natural explanation for Betelgeuse’s Long Secondary Period (LSP) brightness variations, linked to periodic dust distribution changes
  • Fate uncertain: Gravitational drag and tidal forces suggest the companion may spiral into and be consumed by Betelgeuse within the next 10,000 years
  • Next opportunity: Another observation window in November 2027 should place the companion at its maximum apparent separation—ideal for follow-up study
Betelgeuse and its companion star, the companion discovered using ‘Alopeke instrument on the Gemini North telescope, one half of the International Gemini Observatory. Credit: International Gemini Observatory/NOIRLab/NSF/AURA. Image Processing: M. Zamani (NSF NOIRLab)
Credit: International Gemini Observatory/NOIRLab/NSF/AURA. Image Processing: M. Zamani (NSF NOIRLab)

Get the full facts by reading our report on the Betelgeuse companion star discovery

Iain Todd

Science journalist

Iain Todd is BBC Sky at Night Magazine's Content Editor. He fell in love with the night sky when he caught his first glimpse of Orion, aged 10.
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