  Despite being the best studied red supergiant star in our Galaxy, the physics
behind the photometric variability and mass loss of Betelgeuse is poorly
understood. Moreover, recently the star has experienced an unusual fading with
its visual magnitude reaching a historical minimum. We investigate the nature
of this event with the help of a recently developed tomographic technique.
Tomography allows us to probe different depths in the stellar atmosphere and to
recover the corresponding disk-averaged velocity field. We apply the
tomographic method to a few-year time-series high-resolution spectroscopic
observations of Betelgeuse in order to relate its atmospheric dynamics to the
photometric variability. Our results show that a sudden increase of the
molecular opacity in the cooler upper atmosphere of Betelgeuse is likely the
reason of the observed unusual decrease of the star's brightness.
