Free Astronomy Magazine September-October 2020

SEPTEMBER-OCTOBER 2020 sources we consult, we find distances be- tween less than 500 and over 700 light- years. Not even the uncertainty about the crucial datum of the angular diameter has been eliminated: its value undoubtedly dropped in the last century, but the last sixteen years of measurements still pro- duce results that oscillate between 42.3 and 44.3 mas. Note that these measures are also affected by the wavelengths chosen for the obser- vations. It has been difficult to establish to what extent the brightness variations of Betelgeuse are due to surface phe- nomena, rather than to the expulsion of material which, once cooled, would ob- scure part of the photosphere. All these uncertain- ties have made Betelgeuse one of the most observed stars by astronomers and amateur as- tronomers. This made it possible to grasp from the very beginning the extraordinary drop in brightness that began in the Au- tumn of 2019 and which once again highlighted how approximate our knowl- edge of this star is. It all started in October, with a first dim- ming of light that still could have returned to normality − Betelgeuse has recorded os- cillations between magnitudes +0.5 and +1.25, the latter value recorded between late 1926 and early 1927. When, on December 19 of last year, the magnitude of Betelgeuse dropped to +1.29, it soon became clear that something B etelgeuse’s V-band light curve, which compares the normal minimum at the beginning of 2019 with the extraordinary minimum at the beginning of 2020. [AAVSO] To the side, an image of Betel- geuse's lower chromosphere, taken in 2017 by the Atacama Large Millimeter/ submillimeter Array (ALMA). The asymmetry in the distribution of temperatures is evident and is due to huge con- vection cells. [ALMA (ESO/ NAOJ NRAO)/ E. O’Gorman/ P. Kervella]