Supergiant star Betelgeuse could be smaller and closer than we thought

The mystery of what’s happening to Betelgeuse, normally one of the brightest stars in the sky, continues. Since late last year, astronomers noticed that the star was dimming dramatically. Some fluctuation in brightness is normal with stars, but Betelgeuse dropped to just 36% of its brightness, which is very unusual.

Now, astronomers from the Australian National University (ANU) think they have solved the puzzle — they believe that Betelgeuse is smaller and closer to us than previously thought.

This interpretation differs from previous theories announced this summer. Astronomers using data from the Hubble Space Telescope thought that Betelgeuse was ejecting plasma which created a dust cloud around the star, blocking some of its light from view. Another theory posited was that the star was covered in sunspots caused by variations in temperature, which would explain the variations in brightness.

Red Supergiant Betelgeuse
An artist’s impression of the Red Supergiant Betelgeuse. Its surface is covered by large starspots, which reduce its brightness. MPIA graphics department

But the ANU researchers have data of their own to challenge these theories. There appear to have been two distinct dimming events which affected the star. The first event, they agree was due to a dust cloud. But the second event, they believe was related to the pulsations of the star.

The researchers used computer modeling to investigate the hydrodynamic and seismic properties of the star, and they found that pressure waves — “essentially, sound waves,” the researchers described them — caused the star’s pulsations.

This research also raised questions about the size of the star and its presumed distance from Earth. “The actual physical size of Betelgeuse has been a bit of a mystery — earlier studies suggested it could be bigger than the orbit of Jupiter,” co-author Dr László Molnár from the Konkoly Observatory in Budapest said in a statement. “Our results say Betelgeuse only extends out to two-thirds of that, with a radius 750 times the radius of the sun.”

And from this estimate of the star’s size, the researchers could figure out is distance, as Molnár explained: “Once we had the physical size of the star, we were able to determine the distance from Earth. Our results show it’s a mere 530 light-years from us — 25% closer than previously thought.”

Another implication of this research is about whether the star will go supernova. The dramatic dimming had led some researchers to posit that Betelgeuse could be approaching the end of its life and could explode in an epic supernova event. But the ANU researchers think that’s unlikely. “It’s burning helium in its core at the moment, which means it’s nowhere near exploding,” lead researcher Dr Meridith Joyce said in the statement. “We could be looking at around 100,000 years before an explosion happens.”

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