The bright star Betelgeuse has been puzzling astronomers for months. Because at the beginning of the year this red supergiant suddenly darkened dramatically – it lost two thirds of its luminosity. Despite intensive observation, the cause initially remained unclear. Now the Hubble Space Telescope provides a plausible explanation. The data from his UV spectrograph shows that Betelgeuse was experiencing an unusually strong plasma eruption in late 2019. According to astronomers, the material thrown out into space could have become a cloud of dust through cooling and condensation, which obscured the star from our point of view. That would explain the darkening.
Betelgeuse is one of the brightest stars in our night sky – it shines prominently as one of the shoulder stars of the constellation Orion. The star, about 700 light-years away, has about 700 times the diameter of the sun and a luminosity ten thousand times greater in visible light. Its temperature and reddish color reveal that Betelgeuse is a red supergiant – a heavily bloated star at the end of its life cycle. In the end, the star will throw out its outer shells and end in a supernova. However, when this star explosion will take place, astronomers cannot exactly predict either, because so far only a few stars have been observed shortly before their supernova. In theory, it could happen in a few months or a hundred thousand years from now. But when Betelgeuse explodes, it will glow so brightly that it can be seen in the sky even during the day.
Why has Betelgeuse darkened?
A possible sign of a supernova is a strong expansion combined with a clear darkening of the star. The attention of astronomers worldwide was correspondingly great when Betelgeuse began to darken considerably from December 2019. In the course of a few weeks, its brightness dropped by two thirds – this dimming of the star was even visible to the naked eye in the night sky. However, the cause of this darkening remained unclear. Observations with the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile showed that the star was dimmed unevenly. This could indicate, for example, a cloud of dust covering it. Recordings with the Atacama Pathfinder Experiment (APEX) also indicated that the temperature distribution on the star’s surface is also uneven. That also made astronomers consider star spots as a possible cause.
Now, images from the Hubble Space Telescope provide new, crucial information. The telescope regularly targets Betelgeuse with its UV spectrograph. The spectral analysis of this high-energy radiation makes it possible to visualize processes that take place in the hot atmosphere above the star’s surface. In the recordings from September to November 2019, there were actually abnormalities, as astronomers working with Andrea Dupree from the Harvard & Smithsonian Center for Astrophysics report. The UV spectra indicated an outward pressure wave and the ejection of a bright and dense plasma mass from the star’s interior. “With Hubble, we saw the material leave the visible surface of the star and move through the atmosphere,” says Dupree. “We could see the effect of a dense, hot region in the southeastern part of the star moving outward. This material was two to four times brighter than the normal brightness of the star. “
Plasma burst produced darkening cloud of dust
According to the astronomers, Betelgeuse experienced a violent plasma eruption in autumn / winter 2019, in which two to three times more material was ejected than is usual in such eruptions. The Red Supergiant may have lost large amounts of material in this event. The researchers suspect that this eruption was so strong because an upflow of plasma from the interior of the star coincided with a phase of the pulsating expansion of Betelgeuse. Additional observations with the STELLA telescope of the Leibniz Institute for Astrophysics (AIP) in Potsdam on Tenerife provided evidence of this. These recordings confirmed that Betelgeuse was in an expansion phase when the hot material rose. “If a large and very cool star spot had caused the darkening, the speed of the plasma would not have followed the pulsation, but the rotation of the star,” explains co-author Klaus Strassmeier from the AIP.
The resulting plasma eruption now explains the mysterious darkening of the star a few months later. As this hot plasma raced out into space, it cooled and condensed into a huge cloud of dust. This moved in the line of sight between the earth and Betelgeuse and thus obscured the view of the star, as the researchers explain. As a result, the red giant seemed to darken more and more from December 2019. “We believe it is possible that the dark cloud resulted from the ejection discovered by Hubble,” says Dupree. In the meantime the dust cloud has moved further away from the star. You can still see a dimming when observing with optical telescopes. However, Hubble recordings in the UV range show that the surface of Betelgeuse had reached its normal brightness again in April 2020.
Source: Andrea Dupree (Center for Astrophysics, Harvard & Smithsonian) et al., The Astrophysical Journal, doi: 10.3847 / 1538-4357 / aba516