Extreme Velocity Gradients in the Atmosphere of Betelgeuse: Is Another Dimming Event Imminent?

ATel #18097; Daniel Jadlovsky (Masaryk University), Thomas Granzer (Leibniz Institute for Astrophysics Potsdam), Michael Weber (Leibniz Institute for Astrophysics Potsdam), Kateryna Kravchenko (Max Planck Institute for Extraterrestrial Physics), Andrea Dupree (Center for Astrophysics | Harvard & Smithsonian), Miguel Montarges (Observatoire de Paris), Laszlo Molnar (Konkoly Observatory)
on 8 Oct 2026; 18:50 UT
Credential Certification: Daniel Jadlovský (jadlovsky@mail.muni.cz)

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Red supergiant Betelgeuse (Alpha Ori; HD 39801; MI1-2 Iab) underwent a historical Great Dimming event during 2019/2020, reaching a minimum of V ~ 1.6 mag in February 2020 (see ATel #13341, #13365, #13410, #13439, #13512, #13518, #13601, #13901, #13982, #15240, #16001, and #16501), close to the brightness minimum of the long secondary period (LSP). Since then, the brightness has returned to similar values as prior to the Great Dimming, but with its primary photometric period shifting from ~ 400 d to ~ 200 d and showing weaker oscillations. Following the expected LSP minimum in June 2026 (Goldberg et al., 2024, ApJ, 977, 35), Betelgeuse has been steeply declining in brightness, going from V ~ 0.5 mag to V ~ 0.95 mag between August and October 2026 (see CTOA and NOT measurements at AAVSO). We report on continued high-resolution optical spectroscopy monitoring of Betelgeuse with STELLA. Betelgeuse has been regularly monitored with STELLA since 2008, and, as of 20 September 2026, it showed a radial velocity of +23.7 km/s roughly consistent with the LSP. A more detailed analysis of the spectra using the tomographic method (Kravchenko et al., 2018, A&A, 610, A29; Jadlovsky et al., 2024, A&A, 685, A124) allowed us to determine radial velocities across different atmospheric layers, revealing a more dramatic picture. The atmospheric configuration is now similar to that of Betelgeuse at the beginning of 2020, during the Great Dimming, when the atmosphere of Betelgeuse showed extreme velocity gradients. Namely, the inner atmospheric layers began their infall in October 2019, followed by a less steep infall by the outermost layer in December 2019, producing a velocity gradient of ~ -7 km/s between the outermost layer (C5) and the innermost layer (C1) by February 2020. We report that since August 2026, the inner layers have been steeply infalling, and, as of 20 September (the last spectrum), there was a velocity gradient of ~ -5.5 km/s. The only other date with a larger gradient value since 2008 was the Great Dimming itself. Based on the last spectrum, the outermost layer appeared to slowly begin an infall, suggesting that the gradient will further increase. The time delay between the layers is similar to that during the Great Dimming, i.e., ~ 2 months. Therefore, together with the reported decline in brightness, we propose that Betelgeuse may be on route to another dimming event. If similar to the Great Dimming, there are only a few weeks remaining until the minimum brightness (likely around November 2026). In any case, a further drop in brightness below V ~ 1 mag would exceed the previous minima recorded in the AAVSO V-band light curve (apart from the Great Dimming).

Figure: light curve and radial velocity of Betelgeuse