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There is something peculiar about the dying star Betelgeuse.
Yes, there was the whole sneezing thing. That’s pretty much resolved for now. But before the Great Dimming Debacle of 2019, scientists detected something even more peculiar about the giant star. Radio measurements of its changing light suggested that it was spinning at 5 kilometers (3.1 miles) per second.
The big problem with this is that stars from the Betelgeuse epoch should, theoretically, have a maximum rotation speed at least two orders of magnitude lower. So astronomers wonder: what the hell is going on?
Well, according to new research, it may have been one big complicated doodle. A team led by astrophysicist Jing-Ze Ma of the Max Planck Institute for Astrophysics in Germany has discovered that the boiling surface of Betelgeuse could be so choppy that it creates an illusion of rapid rotation.
The rotation of stars is measured by carefully analyzing the differences in light coming from opposite sides of a star. Light from the side of the star moving toward view (that’s us) gets a small boost that squishes it toward the blue end of the spectrum, while light from the side moving away from us is stretched toward the red end.
Scientists can measure the amplitude of this blueshift and redshift to determine how fast a star is spinning. Well, at least a typical quiet star. The problem is that Betelgeuse doesn’t sit still. It is in the red supergiant stage of its life; It is running out of nuclear fuel and has swelled to a tremendous size, with a surface churned by convection.
The hot material bubbles, cools and falls. This also happens on the Sun, with convection cells the size of Texas, but the process is much more violent on Betelgeuse, with convection cells that can be as large as Earth’s orbit around the Sun (Betelgeuse is large enough as for extend to the orbit of Jupiter).
Ma and his colleagues wanted to know if this convection could provide an alternative explanation for what we previously interpreted as ultrafast rotation in observations from the Atacama Large Millimeter/submillimeter Array (ALMA), so they turned to 3D simulations.
They created models of red supergiant stars like Betelgeuse, governed by giant convection, which they are not rotating and then processed the results as synthesized ALMA observations.
Their simulations showed huge convective cells rising on one side of the red supergiant, while another group collapsed and fell inward on the other. ALMA does not have the resolution to identify them as convective cells; Instead, telescope data may appear very similar to rotation.
In fact, the researchers showed that in 90 percent of simulations, ALMA observations would look like rotation at a speed of several kilometers per second.
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It’s not compelling evidence that Betelgeuse isn’t spinning at super-fast speed, although it does show that we can’t draw a conclusion based on the data we currently have on hand. But more high-resolution observations have been made and are being processed and analyzed. This should give us more clues as to what the hell the star is up to.
Either way, the results will be interesting. If Betelgeuse is spinning like a deranged whirlpool, it could mean that the red supergiant gained spin by devouring a smaller companion star. If it spins more slowly, we have learned to proceed more cautiously when interpreting the data we collect on unstable stars.
“There are a lot of things we still don’t understand about giant boiling stars like Betelgeuse.” says astronomer Andrea Chiavassa of the French National Center for Scientific Research.
“How do they really work? How do they lose mass? What molecules can form in their flows? Why did Betelgeuse suddenly become dimmer? We are working very hard to make our computer simulations better and better, but really “We need incredible data from telescopes like ALMA.”
The team’s research has been published in The letters from the astrophysical diary.