The mystery of what happens to Betelgeuse, usually one of the brightest stars in the sky, continues. Since late last year, astronomers have noticed that the star was darkening dramatically. Some brightness fluctuations are normal for stars, but Betelgeuse dropped to only 36% of its brightness, which is very unusual.
Now astronomers at the Australian National University (ANU) believe they have solved the puzzle – they believe 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 believed that Betelgeuse ejected plasma, which created a cloud of dust around the star and blocked some of its light from view. Another theory suggested that the star was covered in sunspots caused by temperature fluctuations, which would explain the fluctuations in brightness.
An artistic impression of the Red Supergiant Betelgeuse. Its surface is covered with large star spots that reduce its brightness. MPIA graphics department
However, the ANU researchers have their own data to challenge these theories. There appear to have been two different dimming events that affected the star. They agree that the first event was due to a cloud of dust. But the second event, they believe, was related to the star's pulsations.
Using computer models to study the star's hydrodynamic and seismic properties, the researchers found that pressure waves – "essentially sound waves," as the researchers described them – caused the star to pulsate.
This research also raised questions about the star's size and its estimated distance from Earth. "The actual physical size of Betelgeuse has been a mystery – previous studies suggested it could be larger than Jupiter's orbit," said co-author Dr. László Molnár from the Konkoly Observatory in Budapest in a statement. "Our results say Betelgeuse is only two-thirds of that, with a radius 750 times that of the Sun."
And from this estimate of the star's size, the researchers were able to find the distance, as Molnár explained: “Once we had the physical size of the star, we could determine the distance from Earth. Our results show that we are only 530 light years away – 25% closer than previously thought. "
Another implication of this research is whether the star becomes a supernova. The dramatic blackout had led some researchers to believe that Betelgeuse was nearing the end of its life and could explode in an epic supernova event. The ANU researchers believe this is unlikely, however. "There is currently helium burning in its core, which means it is nowhere near exploding," said lead researcher Dr. Meridith Joyce in the statement. "We could look at about 100,000 years before an explosion happens."