In late 2019 and early 2020 Betelgeuse, a red supergiant in the constellation of Orion, made headlines when it underwent a period of extreme dimming. This dip in brightness for the star, which is usually around the tenth brightest in the night sky over Earth, was so extreme it could even be seen with the naked eye.

Some scientists even speculated that the orange-hued supergiant may be about to go supernova, an event which would have been visible in daylight over Earth for months thanks to its power and relative proximity–700 light-years from Earth. Yet, that supernova didn’t happen and Betelgeuse returned to its normal brightness.

This left the ‘great dimming’ of Betelgeuse–something never seen in 150 years of studying the star–an open mystery for astronomers to investigate.

Now, a team of astronomers led by Miguel Montargès, Observatoire de Paris, France, and KU Leuven, Belgium, and including Emily Cannon, KU Leuven, have found the cause of this dimming, thus finally solving this cosmic mystery. The researchers have discovered that the darkening of Betelgeuse was caused by a cloud of dust partially concealing the red supergiant.

“Our observations show that the Southern part of the star was hidden and that the whole disk of the star was fainter. The modelling is compatible with both a cool spot of the photosphere and a dusty clump in front of the star,” Montargès tells ZME Science. “Since both signatures have been detected by other observers, we conclude that the Great Dimming was caused by a cool patch of material that, due to its lower temperature, caused dust to form in gas cloud ejected by the star months to years before.”

The ‘great dimming’ of this massive star lasted a few months presented a unique opportunity for researchers to study the dimming of stars in real-time.

“The dimming of Betelgeuse was interesting to professional and amateur astronomers because not only was the appearance of the star changing in real time we could also see this change with the naked eye. Being able to resolve the surface of a star during an event like this is unprecedented.”

Emily Cannon, KU Leuven

The team’s research is published in the latest edition of the journal Nature.

A Unique Opportunity to Capture a Dimming Star

Montargès and his team first trained the Very Large Telescope (VLT)–an ESO operated telescope based in the Atacama Desert, Chile–on Betelgeuse when it began to dim in late 2019. The astronomers took advantage of the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument at the VLT as well as data from the telescope’s GRAVITY instrument. This allowed them to create stunning images tracking the great dimming event allowing them to distinguish it from regular dips in brightness demonstrated by the supergiant stars.

Betelgeuse has been seen to decrease in brightness before as a result of its convection cycle, which causes material to rise and fall throughout the star’s layers based on its temperature. This convection cycle results in a semi-regular dimming cycle that lasts around 400 days.

When the ‘great dimming’ was first observed in October 2019 astronomers had assumed this was due to its natural dimming cycle. That assumption was dismissed by December that same year when the star became the darkest that it had been in a century. The star had returned to its normal brightness by April 2020.

“No other red supergiant star has been seen dimming that way, particularly to the naked eye. Even Betelgeuse that has been closely monitored for 150 years has not shown such behaviour.”

Miguel Montargès, Observatoire de Paris, France

Not only does this finding solve the mystery of this star’s dimming, but it also provides evidence of the cooling of a star causing the creation of stardust which goes on to obscure the star.

Even though Betelgeuse is much younger than the Sun–10 million years old compared to our star’s age of 4.6 billion years–it is much closer to the supernova explosion that will signal the end of its lifecycle. Astronomers had first assumed that dimming was a sign that the red supergiant was exhibiting its death throes ahead of schedule.

Thanks to the work of Montargès and his team, we now know this isn’t the case. The dimming is the result of a veil of stardust obscuring the star’s southern region.

“We have observed dust around red supergiant stars in the past,” Cannon explains. “However, this is the first time we have witnessed the formation of dust in real-time in the line of sight of a red supergiant star,”

This stardust will go on to form the building blocks of the next generation of stars and planets, and the observations made by Montargès, Cannon and the team represent the first time we have seen an ancient supergiant star ‘burping’ this precious material into the cosmos.

The Giant that Burped Stardust

The surface of Betelgeuse–which with its diameter of around 100 times that of the Sun would consume the orbits of the inner planets including Earth were it to sit in our solar system–is subject to regular changes as bubbles of gas move around it, change in size, and swell beneath it. Montargès, Cannon and their colleagues believe that sometime before the great dimming began the red supergiant ‘burped’ out a large bubble of gas.

This bubble moved away from the star leaving a cool patch on its surface. It was within this cool patch that material was able to solidify, creating a cloud of solid stardust. The team’s observations show for the first time that stardust can rapidly form on the surface of a star.

“We have directly witnessed the formation of so-called stardust,” says Montargès. “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.”

With regards to the future, the researchers point to the Extremely Large Telescope (ELT), currently under construction in the Atacama Desert as the ideal instrument to conduct further observations of Betelgeuse. “With the ability to reach unparalleled spatial resolutions, the ELT will enable us to directly image Betelgeuse in remarkable detail,” says Cannon. “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.”

For Montargès solving this mystery and observing a phenomenon for the first time, solidifies a lifetime of fascination with Betelgeuse and points towards a deeper understanding of the stardust that is the building blocks of stars, planets, and us. “We have seen the production of star dust, materials we are ourselves made of. We have even seen a star temporarily change its behavior on a human time scale.”