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Betelgeuse, the nearest red supergiant, dimmed to an unprecedented level inearly 2020. The star emerged from this Great Dimming episode with its typical,roughly 400-day pulsation cycle halved, and a new dominant period of around 200days. The dimming event has been attributed to a surface mass ejection, inwhich rising material drove shocks through the stellar atmosphere and expelledsome material, partially obscuring the star as it formed molecules and dust. Inthis paper, we use hydrodynamic simulations to reveal the connections betweenBetelgeuse's vigorously convective envelope, the surface mass ejection, and thepulsation mode switching that ensued. An anomalously hot convective plume,generated rarely but naturally in the star's turbulent envelope, can rise andbreak free from the surface, powering an upwelling that becomes the surfacemass ejection. The rising plume also breaks the phase coherence of the star'spulsation, causing the surface to keep expanding even as the deeper layerscontract. This drives a switch from the 400-day fundamental mode of pulsation,in which the whole star expands and contracts synchronously, to the 200-dayfirst overtone, where a radial node separates the interior and exterior of theenvelope moving in opposite phase. We predict that the star's convectivemotions will damp the overtone oscillation and Betelgeuse will return to itsprevious, 400-day fundamental mode pulsation in the next 5-10 years. With itsresolved surface and unprecedentedly detailed characterization, Betelgeuseopens a window to episodic surface mass ejection in the late-stage evolution ofmassive stars.

Left Ringing: Betelgeuse Illuminates the Connection Between Convective outbursts, Mode switching, and Mass Ejection in Red Supergiants