Betelgeuse is brightening up again after a period of extreme dimming. The star reached its minimum measured brightness between 7–13 February 2020, its magnitude dropping lower than mag. +1.6 – a third of its normal brightness.
The star is a variable so its brightness does fluctuate according to a 430-day pattern, but in December 2019 the star began dipping to an unprecedented low.
As it’s a red supergiant, Betelgeuse is expected to go supernova in the future. While some astronomers hoped the fainting could be a sign of imminent explosion, the star is not expected to die for another 100,000 years or so.
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Instead it appears that two cycles of dimming and brightening lined up to create the unusual low.
Using these patterns, astronomers from Villanova University predicted the star would bounce back within a week of the 21 February, matching up with the observed recovery.
“The star has stopped dimming and has started to slowly brighten,” says Edward Guinan of Villanova University. “Thus, this ‘fainting’ episode is over.”
Not all astronomers are as convinced. A paper by Emily Levesque from the University of Washington looked at the temperature of the star.
This suggests a fundamental change to Betelgeuse. And this isn’t the only evidence of a shift in the star.
In December 2019, ESO’s Very Large Telescope in Chile observed its surrounding dust was different to observations taken in January 2019.
“The two scenarios we are working on are a cooling of the surface due to exceptional stellar activity or dust ejection towards us,” says Miguel Montargès, from KU Leuven, who conducted the observation.
“Of course, our knowledge of red supergiants remains incomplete and this is still a work in progress, so a surprise can still happen.”
“The world’s largest scopes rarely look at something as bright as Betelgeuse!” says Prof Chris Lintott, astronomer and co-presenter of The Sky at Night.
“The results were both stunning and intriguing, but they may not have happened without the contributions of thousands of amateurs, using the naked eye.
“As the star faded, there was debate about how unusual its behaviour was. The answer came from the American Association of Variable Star Observers (AAVSO). Its Betelgeuse observations stretch back to the 19th century.
“Red stars like Betelgeuse aren’t easy to monitor, and it’s so bright that there aren’t good, nearby comparison stars. But, thanks to the work of AAVSO, astronomers have moved quickly to investigate this dip.
“A triumph, perhaps, for the old-fashioned art of looking at the sky.”