Free Astronomy Magazine September-October 2021

25 SEPTEMBER-OCTOBER 2021 ASTRO PUBLISHING ming was caused by a dusty veil shading the star, which in turn was the result of a drop in tem- perature on Betelgeuse’s stellar surface. Betelgeuse’s surface regu- larly changes as giant bub- bles of gas move, shrink and swell within the star. The team concludes that some time before the Great Dimming, the star ejected a large gas bubble that moved away from it. When a patch of the sur- face cooled down shortly after, that temperature decrease was enough for the gas to condense into solid dust. “We have directly wit- nessed the formation of so-called stardust,” says Montargès, whose study provides evidence that dust formation can occur very quickly and close to a star’s surface. “The dust expelled from cool evolved stars, such as the ejection we’ve just witnessed, could go on to become the building blocks of ter- restrial planets and life,” adds Emily Cannon, from KU Leuven, who was also involved in the study. Rather than just the result of a dusty outburst, there was some speculation on- line that Betelgeuse’s drop in brightness could signal its imminent death in a spectacular super- nova explosion. A supernova hasn’t been observed in our galaxy since the 17 th century, so present-day astronomers aren’t entirely sure what to expect from a star in the lead-up to such an event. However, this new research confirms that night, these tiny, twin- kling dots of light seem perpetual. The dimming of Betelgeuse breaks this illusion.” The team used the Spec- tro-Polarimetric High-con- trast Exoplanet REsearch (SPHERE) instrument on ESO’s VLT to directly im- age the surface of Betel- geuse, alongside data from the GRAVITY instru- ment on ESO’s Very Large Telescope Interferometer (VLTI), to monitor the star throughout the dimming. The telescopes, located at ESO’s Paranal Observatory in Chile’s Atacama Desert, were a “vital diagnostic tool in uncovering the cause of this dimming event,” says Cannon. “We were able to observe the star not just as a point but could resolve the details of its surface and moni- tor it throughout the event,” Mon- targès adds. Montargès and Cannon are looking forward to what the fu- ture of astronomy, in particular what ESO’s Ex- tremely Large Telescope (ELT), will bring to their study of Betelgeuse, a red supergiant star. “With the ability to reach unparalleled spatial reso- lutions, the ELT will en- able us to directly image Betelgeuse in remarkable detail,” says Cannon. “It will also significantly ex- pand the sample of red supergiants for which we can resolve the surface through direct imaging, further helping us to un- ravel the mysteries be- hind the winds of these massive stars.” T his artist’s animation shows a close-up view of Betel- geuse’s irregular surface — with its giant, dynamic gas bubbles — and distant stars dotting the background. As the “virtual camera” rotates from right to left we see a dusty clump, which condensed from gas released by the star, obscur- ing the southern region of Betelgeuse from different vantage points. [ESO/L. Calçada] Betelgeuse’s Great Dimming was not an early sign that the star was heading towards its dramatic fate. Witnessing the dimming of such a recognisable star was exciting for professional and amateur as- tronomers alike, as summed up by Cannon: “Looking up at the stars at ! T his animation combines four real images of the red super- giant star Betelgeuse, the first taken in January 2019 and the others taken in December 2019, January 2020 and March 2020, during the star’s unprecedented dimming. All images, which allow us to resolve the surface of the star, were taken with the SPHERE instrument on ESO’s Very Large Telescope. [ESO/M. Montargès et al./L. Calçada]