Recent observations of Betelgeuse, a star in the constellation Orion, have created a mystery about the red supergiant star. They suggest that it spins much faster than a star of its size should be able to do.
Now a team from the Max Planck Institute for Astrophysics, led by Ph.D. student Jing-Ze Ma, may have an explanation for why Betelgeuse appears to be spinning so incredibly fast. Perhaps, the researchers say, it’s actually an illusion created by the star’s violently boiling surface.
Ma and colleagues think the star’s bubbling surface can be mistaken for rotation, even by the most advanced telescopes. This error could lead observers to believe that Betelgeuse, located between 500 and 600 light-years from Earth, appears to be rotating faster than should be possible for a star of such enormous size.
‘For most people, stars are just glowing dots in the sky. Our results once again emphasize that stars like Betelgeuse have such drastic surface boiling motions that we can see those motions in action in the telescopes,” Ma told Space.com. Theorists are very excited that we can actually make predictions based on our simulations that will be tested against observations in the coming years.”
Related: Betelgeuse could be the result of a ‘silent’ stellar merger
Betelgeuse is a notorious red supergiant star that recently made headlines when its dimming led scientists to speculate that it was about to explode.
“Most stars are just tiny points of light in the night sky. Betelgeuse is so incredibly large and close that, with the very best telescopes, it is one of the few stars where we actually observe and study the boiling surface,” says Selma de Nerts,
co-author and director of the Max Planck Institute for Astrophysics said in a statement. “It still feels a bit like a science fiction movie, as if we’ve traveled there to see it up close. And the results are so exciting.”
Betelgeuse, Betelgeuse, Betelgeuse!
Betelgeuse is one of the brightest stars in the Northern Hemisphere above Earth, meaning it is well studied for research. But, as observations of his dimming show, that doesn’t mean he can’t spring surprises.
With a diameter of more than 1 billion kilometers, Betelgeuse is more than a thousand times larger than the Sun, making it one of the largest stars in the known universe. If the Sun and Betelgeuse were swapped, and the red supergiant were placed at the heart of the solar system, it would engulf Mercury, Venus, Earth and Mars, with its atmosphere all the way to the orbit of Jupiter.
Such massive stars are expected to have relatively gentle rotation rates. This is because when stars “blow out” and expand during their red giant phase, the conservation of angular momentum suggests that their rotation should slow down. This is similar to an ice skater on Earth lowering and spreading his arms to slow his rotations.
Yet recent observations of Betelgeuse, especially those made with the Atacama Large Millimeter/submillimeter Array (ALMA) in northern Chile, have shown that Betelgeuse spins at a speed of about 11,200 miles per hour (5 kilometers per second). That’s about eight times as fast as a fighter jet.
Research with ALMA, an observatory consisting of 66 radio antennas that together form one telescope, has shown that while half of Betelgeuse appears to be approaching Earth, the other appears to be retreating. It was this so-called “dipolar radial velocity map” on Betelgeuse’s outer layer that scientists interpreted as rapid rotation.
However, this interpretation depends on considering Betelgeuse as a perfectly round sphere – and this is not the case, the researchers of the new study point out. The surface of the red supergiant star is turbulent with boiling bubbles. Some of those bubbles are actually the size of the Earth’s entire orbit around the sun. These bubbles, powered by a heat transfer mechanism called convection, can rise and fall at speeds of up to 30 km/s, about three times as fast as the Orion spacecraft, the Artemis mission’s crew vehicle.
To analyze this precise, sparkling image of Betelgeuse, the team developed a new post-processing computer package to simulate synthetic ALMA images and compare them to 3D radiative hydrodynamic simulations of non-rotating red supergiant stars. This showed that a cluster of boiling bubbles rising on one side of Betelgeuse while another cluster falls on the other side would create a dipolar radial velocity map. This convection would be faint in actual ALMA observations, the team says, making it indistinguishable from fast rotation.
In fact, the crew found that in about 90% of their simulations, Betelgeuse would be interpreted as rotating at tens of thousands of miles per hour due to large-scale boiling on the red supergiant’s surface. However, if the team’s modeling is wrong, there may be other explanations. For example, it could indicate that the red supergiant was involved in stellar cannibalism long ago.
“If Betelgeuse does spin quickly, we think it must have spun after eating a small companion star that was orbiting it,” De Mink added.
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The team will now use further observations of Betelgeuse to assess its rotation speed and better understand how the boiling surface affects such measurements, putting their model to the test.
“There’s so much we still don’t understand about giant boiling stars like Betelgeuse. How do they really work? How do they lose mass? What molecules can form in their outflow? Why did Betelgeuse suddenly become less bright?” Andrea Chiavassa, co-author of the study and CNRS astronomer, said. “We work very hard to keep making our computer simulations better, but we really need the incredible data from telescopes like ALMA.”
The team’s research was published in February in the Astrophysical Journal Letters.