Betelgeuse, a supergiant star in the constellation Orion, has been recognized and documented by civilizations across millennia: from prehistoric paintings in the Lascaux Caves to records of its color evolution in Mediterranean and Chinese Antiquity to current Hubble Space Telescope observations. This is a star that has charmed and beguiled humanity: bright enough to observe with the naked eye and consistent enough to rely on its presence. But in 2020, something dramatically changed.

Since 1850, documented observations of Betelgeuse have revealed a 400-day cyclical pattern of luminosity variation. However, in late 2019, the bright star began to fade considerably. By earlyFebruary 2020, it was the faintest it had ever been in nearly two centuries of recorded observations. This unprecedented behavior—the Great Dimming—prompted both alarm and excitement among the press, social media, and scientists alike. Telescopes worldwide, both on the ground and in space turned to focus on this remarkable event. We discuss observations including images and plasma diagnostics across the spectrum to assess what happened to the star.

It appears that a unique Surface Mass Ejection occurred . This happens when a vigorous convective cell creates shock waves and ejects gas from the stellar surface into the star’s vast outer atmosphere. As the gas passes through the atmosphere, it cools and turns to dust and other material. Although small events from a corona are not uncommon, Betelgeuse’s SME was gigantic: producing a cloud of material more than a hundred million miles wide with a mass comparable to a year’s worth of the typical stellar wind. This astronomical dust cloud obscured the star’s light, resulting in the historic dimming. The ejection event also left the star’s surface cooler and less dense. Even today—more than four years after the SME—Betelgeuse remains in an unstable state. Hydrodynamic simulations reveal the connections between Betelgeuse’s vigorously convective envelope, the Surface Mass Ejection and the pulsation mode switching that ensued.

Betelgeuse is unique in our neighborhood because it is intrinsically large (about 1000 times the size of our Sun), and relatively close. These characteristics allow detailed surface observations that are impossible on any other star, except the Sun. Consequently, Betelgeuse holds a valuable place in stellar astrophysics as it provides myriad research opportunities applicable to more distant, less-observable stars in their later stages of evolution.