Free Astronomy Magazine September-October 2020

9 SEPTEMBER-OCTOBER 2020 STELLAR EVOLUTION ter which, at a given moment, constitutes the photosphere and the chromosphere. For several decades, astronomers had not been able to prove Betelgeuse’s mass loss (also predicted by models of super- nova progenitors). We had to wait for the new millennium and the develop- ment of revolution- ary investigation techniques and in- struments. The first confirmations that the mass-loss process was real came in mid-2009, thanks to two separate studies carried out with the ESO’s Very Large Tel- escope. The first was conducted with the NACO instrument (installed on VLT’s Unit Telescope 1) by a team of researchers led by Pierre Kervella (Observatoire de Paris). The team managed to obtain an image of Betel- geuse with a resolution of 37 mas, close to the theoretical limits of the instrument used. In the image, plumes of gas are visible rising on the surface of Betelgeuse and ex- tending into outer space for at least six times the star’s diameter. The asymmetrical appearance of the dispersed gas showed for the first time that Betelgeuse was not losing mass uniformly in all directions, sug- gesting that the sizes of the convective cells were remarkably large and that the more impressive plumes were the result of large- scale gas movements inside the star. This scenario was confirmed by a team of researchers led by Keiichi Ohnaka (then at the Max Planck Institute for Radio Astron- omy, Bonn), who observed Betelgeuse with the AMBER instrument in combina- tion with the interferometer formed by three auxiliary telescopes of the VLT. This solution made it possible to produce im- ages of the gases surrounding the star with a resolution four times greater than that of NACO. The team revealed vigorous vertical movements of the gas in different areas of the photosphere (the first time for a star other than the Sun). The larger con- vection cells were as large as the star itself, confirming that they were the engines of the huge mass ejections observed. I mage of Betel- geuse obtained in the near-infra- red by the NACO instrument, which shows for the first time the mass ejection from the stellar surface. [ESO/P. Kervella] In the video be- low, a zooming in of the red su- pergiant. [ESO, P. Kervella, Digi- tized Sky Survey 2, and A. Fujii. Music by John Dyson from the CD Darklight]