Artist’s impression of Betelgesue produced using a late 2019 image taken with the Sphere instrument on the European Southern Observatory’s Very Large Telescope. Credits: Eso, Esa / Hubble, M. Kornmesser
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In the years close to 1300, it was not only the Supreme Poet who found himself in a dark forest: a similar misadventure also happened to the red supergiant Betelgeuse. Being just over 700 light-years away from us, we were able to appreciate the effects only between the end of 2019 and the beginning of 2020, when the brightness of the imposing scarlet queen of the Orion constellation gradually faded – until to touch the low around mid-February, raising expectations for one possible explosion – to then return to the usual levels already in April. And, as for Dante, also for Betelgeuse the darkness that enveloped her originated first and foremost within herself.
To understand what happened to it, the Hubble Space Telescope relied on the guide of the ionized magnesium line, following its traces in ultraviolet. Here is a brief account of the winter of Betelgeuse. From September to November 2019, a huge mass of ultra-hot plasma rose from the surface of the star to head towards its outer atmosphere. There it continued to travel, for millions of kilometers. As it moved away, the plasma cooled, and as it cooled it turned to dust. And it was precisely this immense cloud of dust that obscured the red supergiant for months – up to a third of its normal brightness, making it almost unrecognizable. This, at least, is the reconstruction proposed by a study driven by Andrea Dupree, astrophysics of the Center for Astrophysics at Harvard and Smithsonian, forthcoming on The Astrophysical Journal.
“With Hubble, we watched the material as it left the visible surface of the star and receded through its atmosphere before the dust formed that obscured it,” Dupree explains. “We have thus been able to see the effect of a dense and hot region moving outwards from the southeastern edge of the star. It was material two to four times brighter than the star’s normal brightness. Then, about a month later, Betelgeuse’s southern hemisphere darkened noticeably as the star became fainter. We think it is possible that the emission detected by Hubble produced a dark cloud. Only Hubble gives us proof of what led to the blackout. ‘
It should be remembered that Dupree and colleagues had been keeping an eye on Betelgeuse long before the dramatic drop in brightness occurred: the first observations, made as part of a three-year study to monitor variations in the star’s outer atmosphere, date back to early 2019. .
Crucial was the telescope’s sensitivity to ultraviolet light, which allowed researchers to probe the layers above the star’s surface, too hot to be detected in the optical band. Layers heated in part by the star’s convection cells bubbling to the surface, probably causing the massive plasma emission.
“The spatial resolution of a stellar surface is possible only in favorable cases and only with the best available equipment,” he points out Klaus Strassmeier of the Leibniz Institute for Astrophysics Potsdam, Germany, referring both to Hubble’s ability to reconstruct plasma displacements in detail and to the fact that the red supergiant proved to be an ideal subject for this type of observation. “From this point of view, Betelgeuse and Hubble are made for each other.”
Meanwhile, the observation campaign goes on. Or rather: for now it is on a forced pause, as Betelgeuse is still too close to the Sun, but it will resume at the latest in early September – when Hubble returns to see the star again.
To know more:
- Read the preprint of the forthcoming article on The Astrophysical Journal “Spatially Resolved Ultraviolet Spectroscopy of the Great Dimming of Betelgeuse», di Andrea K. Dupree, Klaus G. Strassmeier, Lynn D. Matthews, Han Uitenbroek, Thomas Calderwood, Thomas Granzer, Edward F Guinan, Reimar Leike, Miguel Montargès, Anita M. S. Richards, Richard Wasatonic e Michael Weber
Look at the live of MediaInaf Tv dedicated to the blackout of Betelgeuse:
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