Mystery Solved: Astronomers Unravel the Puzzle of Betelgeuse’s “Great Dimming”

These images show the surface of the red supergiant star Betelgeuse during its unprecedented dimming, which happened in late 2019 and early 2020. The image on the far left, taken in January 2019, shows the star at its normal brightness, while the remaining images, from December 2019, January 2020, and March 2020, were all taken when the star’s brightness had noticeably dropped, especially in its southern region.
Credit: ESO/M. Montargès et al.

The star Betelgeuse became visibly darker in 2019 and 2020, puzzling astronomers; new images show that the star was partially concealed by a cloud of dust, solving the mystery of the ‘Great Dimming’ of Betelgeuse.

When Betelgeuse, a bright orange star in the constellation of Orion, lost more than two-thirds of its brightness in late 2019 and early 2020, astronomers were puzzled.

What could cause such an abrupt dimming?

Now, in a new paper published in Nature, an international team of astronomers reveal two never-before-seen images of the mysterious darkening — and an explanation. The dimming was caused by a dusty veil shading the star, which resulted from a drop in temperature on Betelgeuse’s stellar surface.

This animation combines four real images of the red supergiant 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.
Credit: ESO/M. Montargès et al./L. Calçada

Led by Miguel Montargès at the Observatoire de Paris, the new images were taken in January and March of 2020 using the European Southern Observatory’s Very Large Telescope. Combined with images previously taken in January and December 2019, the astronomers clearly capture how the stellar surface changed and darkened over time, especially in the southern region.

“For once, we were seeing the appearance of a star changing in real-time on a scale of weeks,” Montargès says.

According to the astronomers, this abrupt dimming was caused by the formation of stardust.

Betelgeuse’s surface regularly changes as giant bubbles 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, aided by the star’s outward pulsation. When a patch of the surface cooled down shortly after, that temperature decrease was enough for the heavier elements (e.g. silicon) in the gas to condense into solid dust.

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