Betelgeuse: The Mysterious ‘Great Dimming’ Of Orion’s Red Supergiant Star Explained At Last

New images from a massive telescope in Chile have helped solve the mystery of why red supergiant star Betelgeuse suddenly dimmed in late 2019. 

At the time it was thought that the “great dimming” of Betelgeuse—which was visible to the naked eye—could be a sign that the massive star was about to explode as a bright supernova—something that it definitely will do sometime in the next 100,000 years. 

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A supernova hasn’t been seen in our galaxy since the 17th centruy.

However, a team of astronomers have now published new images of the star’s surface—taken using the European Southern Observatory’s Very Large Telescope—that show exactly how Betelgeuse’s brightness changed. 

The photos—taken in December 2019 through March 2020—reveal for the first time that Betelgeuse was partially hidden by a massive cloud of dust. Dropping 1.6 magnitude to just a third of its brightness, by April 2020 the star had returned to its normal brightness.

The cause? A massive bubble of gas.

Published today in Nature, the team’s study reveals uses new images taken in January 2020 and March 2020 that show that the southern region of the star’s surface was darker. 

That, they argue, was the result of a drop in temperature on Betelgeuse’s stellar surface after the star ejected a gas bubble.

The temperature decrease was enough for the gas to condense into solid dust.

“For once, we were seeing the appearance of a star changing in real time on a scale of weeks … and we directly witnessed the formation of so-called stardust,” said Miguel Montargès, from the Observatoire de Paris, France. 

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It was thought that Betelgeuse was bigger than the orbit of Jupiter in our Solar System, but recent studies suggest that the red suppliant star is about a third less than that at about 750 the radius of our Sun.

Betelgeuse is about 530 light-years away, which is well beyond the 50 light-year “danger zone” for Earth if a nearby star when supernova. 

The team’s discovering that dust can occur very quickly and close to a star’s surface could have wider implications for life. “The dust expelled from cool evolved stars, such as the ejection we’ve just witnessed, could go on to become the building blocks of terrestrial planets and life,” said co-author Emily Cannon, Institute of Astronomy, Katholieke Universiteit Leuven, Belgium.

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The team, who used the SPHERE instrument on ESO’s VLT in Chile’s Atacama Desert to image the surface of Betelgeuse, hope to use ESO’s Extremely Large Telescope (ELT)—scheduled to go live in 2024—to resolve the star’s surface in more detail.

“It will also significantly expand the sample of red supergiants for which we can resolve the surface through direct imaging, further helping us to unravel the mysteries behind the winds of these massive stars,” said Montargès.

The ELT will have a colossal 39.3-meter mirror. 

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In a keynote delivered virtually last week at the 238th meeting of the American Astronomical Society, Emily Levesque, a professor in the University of Washington’s astronomy department and expert on red supergiant stars, gave three possible reasons for the “great dimming” of Betelgeuse: 

  • Convection caused a cold spot in its southern hemisphere, making it appear dimmer. 
  • It’s a normal variation in brightness caused by pulsations or interactions with other stars. 
  • It’s dust, something that red supergiant stars eject a lot of.

Wishing you clear skies and wide eyes. 


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