Betelgeuse, Betelgeuse, Betelgeuse! One of the most well-known stars in the night sky has a hidden companion, according to recent observations made by the ‘Alopeke instrument with the Gemini North Observatory. This companion may explain some of Betelgeuse’s irregular dimming events, which cannot be fully understood by the star’s natural pulsing. Astronomers used the Gemini North Observatory’s telescopes to take clearer images of Betelgeuse during the highly unusual 2020 dimming event and again in 2024. In 2020, during Betelgeuse’s "Great Dimming," the companion wasn’t visible because it was likely hidden behind Betelgeuse. In 2024, just after the companion was predicted to be farthest from Betelgeuse, they spotted a faint object about 6 times dimmer than Betelgeuse, located at the expected distance and angle. While the detection isn’t 100% certain, it matches predictions about the companion’s position, brightness, and mass, supporting the idea that Betelgeuse has a stellar partner.
The Great Dimming Event of 2020
Betelgeuse’s brightness is known to vary and is what is known as a “variable” star, but in late 2019 and early 2020, it underwent an event known as the “Great Dimming,” when its brightness dropped by about 1 magnitude, or roughly 60%, making Betelgeuse appear much fainter to the naked eye. This resulted in a massive wave of excitement and was arguably one of the most talked-about celestial events of the 2020s thus far. Betelgeuse is typically one of the brightest stars in the night sky, making up the “armpit” of Orion, and is noticeably reddish-orange. During the Great Dimming, Betelgeuse’s apparent magnitude dropped a full magnitude, or a 60% reduction in luminosity, as seen by observers here on Earth. This transformed the appearance of the constellation of Orion, and it looked less striking without its famous orange star.
Follow-Up Investigations
Later, NASA’s Hubble Space Telescope observed Betelgeuse and detected a massive ejection of hot plasma from a large convective cell on Betelgeuse’s surface. Importantly, observations during this period, including the 2020 Gemini North data, showed no companion star, consistent with it being hidden behind Betelgeuse. One hypothesis was that Betelgeuse’s unusual and dramatic event was also partially caused by an as-of-yet unknown companion star. Evidence for this goes back to 1968, placing a potential companion within the chromosphere of Betelgeuse. However, no concrete evidence was found due to the low resolution at the time.
Astronomers utilized high-resolution optical speckle imaging using the 8.1-meter Gemini North telescope in 2020 and 2024 to investigate this further. Speckle imaging is a technique that mitigates atmospheric distortion to achieve diffraction-limited resolution, allowing for the detection of faint objects close to bright stars like Betelgeuse. The 2020 observations coincided with Betelgeuse’s “Great Dimming” event. In contrast, the 2024 observations were taken three days after the predicted maximum elongation (quadrature), when the companion was expected to be at its greatest angular separation from Betelgeuse.
The 2020 data showed no evidence of a companion, consistent with the prediction that it was obscured by Betelgeuse’s brightness. However, the 2024 data revealed a faint object with an angular separation of 52 milliarcseconds (mas) from Betelgeuse and a position angle of 115° east of north. This matched closely with dynamical predictions based on the long-hypothesized orbit. The companion appears approximately 6 magnitudes fainter than Betelgeuse at a wavelength of 466 nanometers (blue light), meaning it is roughly 250 times dimmer. The detection has a statistical significance of 1.5σ, indicating it is not conclusive, but still the strongest evidence yet for this unseen companion. Further observations will be required to confirm this exciting discovery beyond a shadow of a doubt!
So why exactly does this matter to us? The potential discovery of a stellar companion orbiting Betelgeuse is important because it could reshape our understanding of this iconic red supergiant, one of the closest and best-studied massive stars to Earth. Betelgeuse has long been used as a reference star when it comes to understanding how stars end their lives. Confirming a companion would provide new and exciting insights into the gravitational dynamics affecting the dying primary star. This finding could refine models of Betelgeuse’s evolution, as the presence of a companion might influence its mass loss, rotation, or eventual path toward a supernova. Additionally, studying such a companion offers a rare opportunity to explore binary star interactions in the context of a supergiant, a phase of stellar evolution that is critical for understanding the life cycles of massive stars!
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