that variance in size = a variance in brightness. stars like this get brighter and dimmer as they expand and shrink! so we can *see* Betelgeuse pulsing by measuring its brightness over time.

below graph shows Betelgeuse's brightness vs time. notice the periodicity! (from aamvso) pic.twitter.com/UoUT9aFbtw

— Sarafina El-Badry Nance (@starstrickenSF) December 23, 2019