The Mystery of Betelgeuse’s Weird Dimming Is Likely Solved

The Mystery of Betelgeuse’s Weird Dimming Is Likely Solved

Astronomers are attributing the unexpected dimming of Betelgeuse to a gigantic gassy belch, which formed a dust cloud that partially obscured our view of the red supergiant star.

The Great Dimming, as it’s known, began in late 2019 and ended in March 2020. During this time, Betelgeuse — the tenth brightest star in the night sky, visible in the constellation Orion — became noticeably darker, even to the unaided eye. At its peak, astronomers observed as much as a 40% drop in the star’s normal luminosity.

This strange, unexpected episode was not immediately understood, leading to all sorts of speculation. A theory emerged that Betelgeuse had developed a bad case of the sunspots, or that the aged star, just 548 light-years from Earth, was on the cusp of going supernova, in which it would explode and die in a huge burst of light and energy.

At the same time, early evidence suggested a huge cloud of dust was responsible for the dimming, but other work pointed to a measurable cooling of the star, which was likewise cited as a possible cause. Turns out, these two seemingly unrelated observations have a lot to do with each other, as new research in Nature reveals.

Betelgeuse, it now appears, experienced a mass-loss episode about a year before the Great Dimming, in which the star released an enormous bubble of gas. The authors of the new paper argue that an emerging cold patch in the star’s southern hemisphere was responsible for the observed drop in temperature, which turned the expelled gas into dust. The resulting dust cloud temporarily clouded our view of the gigantic star, resulting in the dimming episode. The new study was led by Miguel Montargès, an astronomer at the Paris Observatory.

Data gathered by the European Southern Observatory’s Very Large Telescope provided new views of the star’s surface, which allowed the team to track changes in the object’s luminosity over time. Montargès, who co-authored a 2020 paper claiming that dust was responsible for the Great Dimming, said the initial spectral signature captured by the Hubble Space Telescope could only be used to infer the presence of a passing gas cloud in the star’s chromosphere (the warm layer above the star). Using the SPHERE instrument on VLT, the astronomers saw more than just a point source — they saw the actual geometry of the star, which offered an enhanced view of the dust cloud and the subsequent dimming, along with a view of the process over shorter timescales.

SPHERE images showing Betelgeuse's surface before and during its 2019-2020 Great Dimming. (Image: ESO/M. Montargès et al.)
SPHERE images showing Betelgeuse’s surface before and during its 2019-2020 Great Dimming. (Image: ESO/M. Montargès et al.)

An image from December 2019 was compared to one taken in January 2019, confirming that the surface had become darker, particularly in the southern hemisphere. Two new views, one taken in March 2020 and the other in April 2020, showed the star returning to its normal luminosity.

“Being able to see the event evolve over time was key to identifying it,” explained Montargès in an email. “Using thousands of models we are able to report that it is a conjunction of a localised cooling of the stellar surface and a dust cloud in front of the star.”

Indeed, the new paper is bolstering the prevailing burp theory — that Betelgeuse ejected a large gas bubble, which travelled millions of miles away. Then, when a patch of the star’s surface cooled down shortly afterward, the resulting drop in temperature caused the gas to condense into dust. The researchers were literally watching the formation of stardust, some of which might eventually go on to form the building blocks of asteroids, rocky planets, and even life.

The new evidence is confirmation that mass loss episodes among red supergiant stars are more intense than previously thought. This will help astronomers to better “understand the mechanisms behind the mass loss and to better predict the fate of red supergiant stars,” such as when they might explode as supernovae, and what will remain afterwards, such as a black hole or neutron star, said Montargès.

That the Great Dimming is not immediately linked to a supernova explosion may seem like a downer, but as University of Washington astronomer Emily Levesque explained in an accompanying News & Views article, similar episodes might be detected on other red supergiants.

“Next-generation facilities focused on monitoring stellar brightness over time, or on studying the signatures of dust in the infrared spectra of stars, could prove invaluable for expanding the lessons learnt here,” wrote Levesque, who wasn’t involved with the new research. “This exquisitely detailed study of Betelgeuse’s unexpected behaviour lays the groundwork for unravelling the properties of an entire population of stars.”

Indeed, “blinking stars,” or variable stars, are now documented with surprising regularity (we covered one earlier this week!). The challenge now will be for scientists to further pinpoint the causes and determine how they might be related. The science of supergiants is finally getting sharper.

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