One of the brightest stars in the firmament suddenly lost its shine: The collapse in brightness of Betelgeuse in late 2019 to early 2020 excited astronomers: Will the old giant star explode soon? Evaluations of recordings of the red supergiant now clarify the reason for the so-called “Great Blackout”. It was caused by a dusty veil created by a drop in temperature on the star surface of the red supergiant, reports an international team of astronomers. The phenomenon was therefore not a sign of an imminent supernova.
Betelgeuse is one of the superstars in the night sky: With its bright glow, it forms one of the shoulders of the well-known constellation Orion. The celestial body, which is around 700 light years away from us and around 700 times the diameter of the sun, has long been the focus of astronomy. From its features, it can be seen that Betelgeuse is a red supergiant – a bloated star in the final stages of its evolution. In the finale, Betelgeuse will expand even further and then eventually transform into a neutron star or black hole after an explosion. When this supernova will occur, however, is unclear – maybe soon or for many millennia.
Tracking down a famous astro event
A possible sign could be a darkening of the star. This is one of the reasons why Betelgeuse’s observations from December 2019 caused a great stir among scientists and amateur astronomers. “If you look up at the stars at night, these tiny, blinking points of light seem to be eternal. The fading of Betelgeuse breaks this illusion, ”says co-author Emily Cannon from the Catholic University of Leuven. Over the course of a few weeks, the star’s brightness decreased by two thirds, which could be seen with the naked eye. But then the luminosity recovered: In April 2020, the star had reached its normal brightness again. In their current publication, Cannon and her colleagues clarify what the phenomenon was all about.
Their results are based on images and data from the SPHERE instrument and the GRAVITY instrument on the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile, in whose field of view Betelgeuse stood during the darkening. “They were the critical diagnostic tools to uncover the cause of this blackout event,” says Cannon. First author Montargès from the observatory in Paris says: “We were able to observe the star not only as a point, but we were able to resolve details of its surface and examine them throughout the event. The pictures that have now been published clearly show for the first time how Betelgeuse’s surface changed in brightness over time, ”says the astronomer.
Effect of a “stellar flatulence”
The scientists were able to document that the mysterious darkening was caused by a dusty veil that shaded the star’s southern hemisphere. From the data and modeling, it appears that this was the result of a loss of mass and a subsequent drop in temperature on Betelgeuse. As the researchers explain, the surface of the giant star changes regularly as gigantic gas bubbles move, swell and shrink inside the star. The team suspects that Betelgeuse emitted one of these gas bubbles before the blackout.
The star’s surface then partially cooled, which in turn affected the nearby gas bubble: the temperature rise caused it to condense into solid dust, the astronomers suspect. “We apparently observed the formation of so-called stardust directly,” says Montargès. “The dust ejected from cool, evolved stars, like the ejection we just observed, could later become the building blocks of terrestrial planets and life,” adds Cannon.
But do the new results also provide indications of the fate of the aging giant star? According to the researchers, the Great Blackout was no sign that we could soon witness a gigantic sky spectacle: When the supernova will occur remains unclear. But in any case, Orion’s shoulder will continue to be firmly in the sights of astronomers. Montargès and Cannon are looking forward to the sharp gaze of the ESO’s planned Extremely Large Telescope. “With the ability to achieve unparalleled spatial resolutions, this telescope will enable us to image Betelgeuse with great attention to detail,” says Cannon. “It will also significantly increase the number of red supergiants whose surfaces we can resolve through direct imaging and will further help us to unlock the secrets of these massive stars,” said the astronomer.
Source: ESO, technical article: Nature, doi: 10.1038 / s41586-021-03546-8
Video: The animation shows Betelgeuse while the dusty veil of the gas released by the star condenses and covers the southern region of the seething giant star. (Credit: ESO / L. Calçada)