The star Betelgeuse has always been a diva.
Astronomers from ancient times to the present day have watched the red supergiant pulsate on the shoulder of the Orion constellation, and the star has continually put on a show, two new studies suggest. Betelgeuse may still be recovering from a deep dimming episode a few years ago, a team reports. And the star appears to have put on her reddish stage makeup only 2,000 years ago, before which she was wearing yellow, another team says.
Together, these studies could inform researchers about how stars shed their entrails into space and give clues as to how long it will be before Betelgeuse explodes in a supernova.
“This star always fools you,” says astronomer Edward Guinan of Villanova University in Pennsylvania, who studied Betelgeuse for decades and was not involved in the new work. “You think you have it and all of a sudden it changes.”
The “Great Darkening”
In late 2019, Betelgeuse caught astronomers’ attention when it suddenly went dark for several months, an event astronomers now call the Great Dimming. Months of observations later led the researchers to an explanation: The star had coughed up a large bubble of plasma. That stuff got cold, condensed into dust and blocked the face of the star from Earth’s perspective months later (Serial Number: 11/29/20). The surface of the star also cooled, contributing to attenuation (Serial number: 06/16/21).
But what happened next was just as surprising, astrophysicist Andrea Dupree and colleagues report in a paper posted August 2 to arXiv.org. The regular pulsating brightness of the star, it seems, is gone completely out of control.
In its lifetime without the Great Dimming, Betelgeuse’s brightness was on a near-periodic dimmer switch. As the star inhaled and exhaled, puffing up before shrinking again, its brightness rose and fell. “Over 200 years, it had a nice 400-day oscillation in brightness,” says Dupree, of the Harvard & Smithsonian Center for Astrophysics in Cambridge, Massachusetts. “But that doesn’t exist anymore.”
Since then, that regular rhythm has become erratic. Instead of a regular rattle, the oscillations are “like an unbalanced washing machine, going ‘wonka wonka wonka,'” says Dupree.
The instability is a sign that the star is struggling to recover from material loss in 2019, says Dupree. She calculates that Betelgeuse ejected several times the mass of the moon from its surface, leaving behind a large cold spot. Plasma on the star’s surface churns as it returns to equilibrium.
If this image is correct, it means that red supergiants like Betelgeuse can spray material into interstellar space in discrete bursts, rather than a continuous stream. This is important to know because many of the elements that make up planets and people were formed in stars that went through what Betelgeuse is going through right now. Studying the growing pains and death rattles of Betelgeuse can tell us about our own origins.
But while this image of Betelgeuse holds together, it remains speculative, Guinan cautions.
One confounding factor is a new set of observations of Betelgeuse during the four-month period when it’s usually out of sight. From May to August each year, Betelgeuse is too close to the sun from Earth’s perspective to be seen at night. That usually leaves a hole in the data sets of astronomers tracking its periodic behavior.
But amateur observer Otmar Nickel from Mainz, Germany, developed a technique to measure Betelgeuse’s brightness using multiple images taken during the day. Dupree’s paper is the first to include such daytime data.
“That’s great,” says Guinan. “You can follow the star throughout the year.”
Those additional observations may reveal recurring changes that have always been there, rather than detecting something truly new. “Those little variations that you’re seeing…could easily be present just before the Great Dimming,” says Guinan.
Dupree’s team predicts that the dust Betelgeuse lost could become visible to some telescopes on Earth by 2023. “That would be proof” that the brightness changes were due to a single outburst, Guinan says.
The Great Dimming isn’t the first time humans have recorded a major change in Betelgeuse’s personality. Two millennia ago, the star it was a completely different colorastrophysicist Ralph Neuhäuser and his colleagues report in an article in press in Royal Astronomical Society Monthly Notices.
The team analyzed ancient descriptions of more than 200 stars whose colors should have been visible to the naked eye for the last few thousand years. Most stars observed throughout human history had the same recorded color in the past as they do today, the team found. But not Bethelgeuse.
The ancient Roman astronomer Gaius Julius Hyginus, who lived between 64 B.C. C. and 17 d. C., and it is believed that he wrote the work in Latin astronomy, described that the star on the right shoulder of Orion has a color similar to that of Saturn, which is yellow. The astrologer and archivist Sima Qian, who worked during the Chinese Han dynasty around 100 B.C. C., independently described the star as yellow. Observers from other ancient cultures conspicuously left Betelgeuse off their red star lists.
“I thought, ‘Oh, how can this be?’” says Neuhäuser, of AIU Jena in Germany. “I did not expect such a result… to find a star that changed color in historical time.”
The color of a star is a sign of its evolutionary stage (Serial number: 7/23/21). When stars burn the hydrogen fuel in their cores, they swell and expel gases into space. That expansion causes their surface temperatures to drop and change color from blue to red in a fairly short time: about 10,000 years for a giant star like Betelgeuse, which is about 14 times more massive than the sun.
That relatively recent color change suggests that Betelgeuse has just reached the end of its hydrogen-burning life and turned into the red supergiant we know today as human observers watched.
“It is fully consistent with astrophysical knowledge,” says Neuhäuser. “It might have been expected, but no one really checked.”
That result means that anyone expecting Betelgeuse to go supernova will have a long wait. If the star became a supergiant in the last few millennia, it has more than 1 million years left before it booms.