Betelgeuse, to put it more politely, belched.
In the fall of 2019, the star, a red supergiant on the shoulder of the Orion the Hunters constellation, began to darken dramatically to less than half its usual brightness, and some astronomers were worried – or perhaps hoping – that it would explode in a supernova .
Astronomers now say that dust was the culprit in the Great Blackout and that Betelgeuse was responsible for that dust itself. A giant clump of gas erupted from the star, it is said, then cooled and condensed into solid particles that temporarily obscured their origin.
“We witnessed the formation of so-called stardust directly,” said Miguel Montargès, astrophysicist at the Paris Observatory, in a statement from the European Southern Observatory. He and Emily Cannon from the Catholic University of Leuven in Belgium led an international team that examined Betelgeuse during the Great Dimming with the Very Large Telescope of the European Southern Observatory on Cerro Paranal in Chile.
Parts of the star were only a tenth as bright as normal and significantly cooler than the rest of the surface, allowing the ejected blob to cool and condense into stardust. They reported their results in Nature on Wednesday.
Research, they said, shows that such dust formation can occur very quickly and close to the surface of a star. From there it can land anywhere; As the old saying goes, we’re all made of stardust.
“The dust ejected from cool evolved stars, like the ejection we just saw, could become the building blocks for terrestrial planets and life,” said Dr. Cannon in the statement.
Their new results seem to support the results published by Andrea Dupree of the Harvard-Smithsonian Center for Astrophysics and her colleagues a year ago, who discovered upwelling on Betelgeuse in the summer of 2019.
“We saw the material move through the chromosphere in the south from September to November 2019,” wrote Dr. Dupree in an email. She described the expulsion as “a sneeze”. You and Dr. Montargès were co-authors on each other’s work.
But Edward Guinan of Villanova University, who followed Betelgeuse closely, was more measured by this enthusiasm. Three other studies favor the growth of cool regions on the star’s surface to explain the significant decrease in light.
Betelgeuse is a so-called red supergiant, 887 times the size of our own sun. Its surface, like that of the sun, resembles boiling oatmeal when drops of gas rise and transport heat and energy. Such spots on the sun are often described by American astronomers as being comparable to Texas.
“In France we say that the sun’s convection cells are as big as France,” said Dr. Montargès in an email. “It’s really fun to see every country comparison.”
But on Betelgeuse, he said, these blobs are half the width of the star itself, 350 million miles in diameter. There are always only a few of them.
Betelgeuse also goes through a 400-day cycle of pulsing, darkening, and lightening, though usually not nearly as extreme as has just been shown.
Dr. Montargès and Dr. Cannon began observing Betelgeuse in 2019 with a special instrument called SPHERE on the Very Large Telescope, which they could use to track changes on the surface of the distant star in high resolution.
“For once we saw a star change in real time over several weeks,” said Dr. Montargès in his statement. In late 2019, they observed that part of the star was only a tenth as bright as the year before and about 300 to 500 Kelvin – 80 to 440 degrees Fahrenheit – cooler than the rest of the star.
Dr. Montargès and his colleagues argue that the boiling star emitted a lump of gas months, if not years, before the Great Dimming. The gas cloud was about the size of the star. It hung around Betelgeuse as a gas because the region around the star was still too warm for the cloud to condense into dust by the next shrinkage and cooling cycle.
“Then the photosphere cooled down,” noted Dr. Montargès, “probably in the initially bright region that ejected the lump.” That would have lowered the ambient temperature in the cloud so much that dust could form and cover up his place of birth.
“This adventure with Betelgeuse was really exciting,” said Dr. Montargès.
And so Betelgeuse is back to normal for the time being – which always means “normal” for a star on the verge of doom. It is certain that the star will explode at some point. Betelgeuse, pronounced “beetle juice” and also known as Alpha Orionis, is at least 10 times and maybe 20 times as massive as the sun. If it were placed in our solar system, it would devour everything up to the orbit of Jupiter.
Red supergiants are stars in the final violent stages of their evolution. Betelgeuse has already spent millions of years burning primordial hydrogen and converting it into helium, the next lightest element. The helium burns into more massive elements. Once the star’s core becomes solid iron, the star will collapse sometime within the next 100,000 years and then rebound in a supernova explosion, likely leaving behind a dense nugget called a neutron star.
It’s going to be quite a show. The last bright supernova in our galaxy occurred in 1604 and was as bright as Venus in the sky, said Dr. Guinan.
He said he still glanced at Betelgeuse every day, but had recently come to believe that an even bigger supergiant named VY Canis Majoris would be the first to explode.
“I’ve been observing since 1980,” he said, “and I’m 79 now and don’t have much time to see these supernovae.”
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