Free Astronomy Magazine September-October 2020

4 STELLAR EVOLUTION The alpha star of the constellation Orion is perhaps the closest red supergiant to Earth and one of the most ob- served and studied variable stars in history. It still contin- ues to surprise us and to hide valuable information that would be fundamental to our understanding of how such massive dying stars, whose diameter can exceed a billion kilometers before exploding as supernovae, evolve. Betelgeuse − 100 years of uncertainties by Michele Ferrara revised by Damian G. Allis NASA Solar System Ambassador SEPTEMBER-OCTOBER 2020 E xactly 100 years ago, the physicist Albert Abraham Michelson and the astronomer Francis Gladheim Pease were preparing the equipment for and set- ting up, on top of the Hooker Telescope at Mount Wilson Observatory, an experiment of interferometry. On 13 December 1920, they made the first measurement of Betel- geuse’s angular diameter. Before that day, the only star whose real size was known was the Sun, because both its angular di- ameter and its distance could be calculated with relative ease. All other stars appeared point-like even in the most powerful tele- scopes and, although the distances to many were known, it was impossible to deter- T he unmistak- able constella- tion Orion rises above the Swedish- ESO Submillimeter Telescope at La Silla Observatory. [Y. Beletsky (LCO)/ESO]