Free Astronomy Magazine September-October 2020

16 SEPTEMBER-OCTOBER 2020 STELLAR EVOLUTION emit light essentially in the ultraviolet, which Hubble manages to reach. Spectro- scopic observations of the plasma in mo- tion in Betelgeuse’s atmosphere, started in January 2019 and continued over the fol- lowing months, have re-proposed the sce- nario of the interposition of relatively cold material with rather convincing evidence. Between September and November 2019, Dupree’s team recorded, in the southern hemisphere of the star, the dynamic evolu- tion of an immense bubble of plasma that rose from the photosphere through the densest layers of the atmosphere. Along its path, the plasma had cooled more and more to form dust which, by block- ing the light emitted by about a quarter of the visible photos- phere, would then have been responsible for the deep minimum observed. Here is how Dupree describes the phenomenon: “With Hubble, we see the material as it left the star’s visible sur- face and moved out through the atmos- phere, before the dust formed that caused the star [to] appear to dim. We could see the ef- fect of a dense, hot region in the southeast part of the star moving outward. This mate- rial was two to four times more luminous than the star’s normal brightness. And then, about a month later, the southern hemisphere of Betelgeuse dimmed conspic- uously as the star grew fainter. We think it is possible that a dark cloud resulted from the outflow that Hubble detected. Only Hubble gives us this evidence of what led up to the dimming.” To follow the evolution of the bubble of plasma, the researchers used T he scenario that emerged from the observa- tions of Dupree's team, represen- ted using an image of Betel- geuse taken at the end of 2019 by ESO's SPHERE instrument. [ESO, ESA/Hubble, M. Kornmesser] singly-ionized magnesium as a tracer, which made it possible to estimate the as- cent rate of the material at about 200,000 miles per hour. The plasma crossed the at- mosphere over many millions of miles be- fore reaching a temperature sufficient to create grains of dust. It is not clear which process produced that bubble of plasma, but according to Dupree its development was facilitated by the con- vective motions linked to the 420-day cycle with which the star appears to pulsate.