Red supergiant variability, Observations and theory

Betelgeuse's recent dimming event is just one of many intriguing phenomena displayed by this exquisite exemplar of red supergiants. In this invited review I will discuss what is known and thought about variability of red supergiants. This will focus on variations on dynamical and thermal timescales, such as convection, pulsation, and possible eruptions shortly before core collapse or mantle depletion; as well as hydrodynamic, electro-magnetic, and chemo-thermal instabilities from the photosphere out through the wind. The aim is to arrive at a holistic picture of how and why red supergiants vary and shed mass, making their own contribution to the entropy increase of the Univers