Mystery solved: Astronomers discover what’s up with Betelgeuse

Something strange is up with one of the brightest stars in the sky. Betelgeuse, also known as Alpha Orionis, is usually the 10th-brightest star seen from Earth. But in late 2019, the star dimmed suddenly and dramatically. Although it’s common for stars to dim over time, the speed and degree of dimming were unprecedented, with its brightness dropping to just one-third of its usual levels.

Researchers rushed to theorize about what might be causing this strange dimming, coming up with ideas including that the star was about to go supernova or that it could be covered in sunspots. But now the mystery has been solved, thanks to observations from the European Southern Observatory’s Very Large Telescope (VLT) which show that the star was obscured by a cloud of dust.

These images, taken with the SPHERE instrument on ESO’s Very Large Telescope, 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. The brightness returned to normal in April 2020.
These images, taken with the SPHERE instrument on ESO’s Very Large Telescope, 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. The brightness returned to normal in April 2020. ESO/M. Montargès et al.

The star returned to its normal brightness in April last year, but before that happened a team of researchers was able to make a series of observations showing its changing brightness and shape. The light from the star was being blocked by a “dusty veil” that passed between it and Earth, which was released when the star’s surface temperature dropped.

The researchers theorize that before the dimming, the star released a large bubble of gas. This gas moved away from the star, and later a cooler patch on the surface developed. This cooler patch made the gas condense into dust, which was how the veil formed.

“We have directly witnessed the formation of so-called stardust,” said lead author Miguel Montargès. These observations could have implications beyond just this star, according to co-author Emily Cannon: “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.”

This event is unusual not only because of its dramatic effect on the night sky, with a change in brightness to a significant star that was clearly visible even with the naked eye, but also because it happened on such a relatively short time scale. Stars normally change over periods of thousands and millions of years. “For once, we were seeing the appearance of a star changing in real time on a scale of weeks,” said Montargès.

The findings will be published in the journal Nature.

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