  The bright supergiant, Betelgeuse (Alpha Orionis, HD 39801), underwent a
historic optical dimming during 2020 January 27 $-$ February 13. Many imaging
and spectroscopic observations across the electromagnetic spectrum were
obtained prior to, during, and subsequent to this dimming event. These
observations of Betelgeuse reveal that a substantial surface mass ejection
(SME) occurred and moved out through the extended atmosphere of the supergiant.
A photospheric shock occurred in 2019 January - March, progressed through the
extended atmosphere of the star during the following 11 months and led to dust
production in the atmosphere. Resulting from the substantial mass outflow, the
stellar photosphere was left with lower temperatures and the chromosphere with
a lower density. The mass ejected could represent a significant fraction of the
total annual mass loss rate from the star suggesting that episodic mass loss
events can contribute an amount comparable to that of the stellar wind.
Following the SME, Betelgeuse was left with a cooler average photosphere, an
unusual short photometric oscillation, reduced velocity excursions, and the
disappearance of the $\sim$400-day pulsation in the optical and radial velocity
for more than two years following the Great Dimming.
