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Betelgeuse is still acting strange. As the star recovers from blasting out a huge amount of its innards in late 2019 in an event that became known as the Great Dimming, its surface is now bouncing like gelatin on a plate, and it has lost the natural 400-day heartbeat that has been present for at least two centuries.

at the Harvard-Smithsonian Center for Astrophysics in Massachusetts and her colleagues analysed observations from a slew of telescopes to unravel what happened in the Great Dimming and figure out what is going on now. They found that the most likely cause of the dimming was a confluence of a hotspot on the star鈥檚 surface and a peak in the 400-day cycle in which the surface used to pulsate in and out.

When these two events happened at once, the researchers think that it caused a huge plume of material to blast off of Betelgeuse, leaving a cool spot and a cloud of dust in its wake. 鈥淚t鈥檚 as if you put a hand in and took a big piece 鈥� it鈥檚 about a quarter of the surface coming off and just being blasted into space,鈥� says Dupree. The plume was billions of times as massive as any ejection we have seen from the sun. 鈥淲hen you take out a big piece of the surface like that, it all kind of sloshes around like an unbalanced washing machine.鈥�

This sloshing, which manifests as a sort of bouncing of Betelgeuse鈥檚 surface, is most likely caused by the plasma within the star rushing into the hole that the plume left. While the bouncing is currently much faster than the natural pulsation, it is gradually slowing down. 鈥淚 would assume the pulsation is going to come back 鈥� but I just got some new measurements a few days ago, and it鈥檚 not there yet,鈥� says Dupree. 鈥淩ight now, it鈥檚 still doing the wrong thing.鈥�

None of this behaviour, from the plume to the dimming to the sloshing about, has ever been seen before in any other star. That is most likely because very few other stars have been observed in the detail that Betelgeuse has, and it makes this a unique opportunity to learn about a strange phenomenon that probably affects other large stars as well, Dupree says.

While Betelgeuse is near the end of its life, the researchers don鈥檛 think that these weird events presage its death in a supernova anytime soon. 鈥淭he theorists tell me it鈥檒l blow up in 10,000 years, if then,鈥� says Dupree. However, because of an additional six-year cycle in the star鈥檚 atmosphere, they will be looking out for another huge flare in 2026.

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