Free Astronomy Magazine September-October 2020

15 SEPTEMBER-OCTOBER 2020 STELLAR EVOLUTION cycles (100-180 days, 420 days, 5-6 years); the interposition of large quantities of dust and gases re- cently ejected from the star; the forma- tion of a gigantic dark region on the photosphere, some- thing like sunspots. Initially, the most promoted hypothe- sis was that of the interposed material, with Kervella’s team obtaining a new mid-infrared image with VISIR in Decem- ber that clearly revealed the presence of relatively cold clouds most likely made of silicate and alumina dust, as well as gas. Between March and May, Betelgeuse’s light curve moved back towards normal values. This could have been interpreted as the dissolution of the intervening mate- rial, but once again reality was different. At the end of June, a study coordinated by Thavisha Dharmawardena (Max Planck In- stitute for Astronomy, Heidelberg) on sev- eral Betelgeuse observations made with ESO’s Atacama Pathfinder Experiment tel- escope (APEX) definitively confirmed the stellar spot scenario. The reason is quite simple. APEX observes at submillimeter wavelengths, where an increase in the pro- duction of dust by the star would have translated into an increase in magnitude; instead, Betelgeuse had also faded in the APEX images, a fact that is explained by a decrease in the surface temperature on a large scale, i.e. through a conspicuous 50- 70% darkening of the visible photosphere. Problem solved? Not exactly. In mid-Au- gust, the first results of a three-year study conducted on Betelgeuse with the Hubble Space Telescope by Andrea Dupree (Center for Astro- physics − Harvard & Smithsonian, Cambridge) and a dozen of his col- laborators were released. The team moni- tored changes in temperature and plasma move- ments affecting the atmospheric layers overlying the photosphere. These are layers so hot (over 20,000 degrees Fahren- heit) that they V isual repre- sentation of Betelgeuse based on the knowl- edge acquired in the last decades. [MPIA graphics department] T his is the first ultraviolet im- age of Betelgeuse made with the Hubble Space Te- lescope. A huge hot spot is visible that is hundreds of times larger than the Sun, with a temperature ex- ceeding the sur- rounding surface temperature by at least 2000 Kelvin. [Andrea Dupree (Harvard-Smith- sonian CfA), Ro- nald Gilliland (STScI), NASA/ESA]