Study of the super giant star Betelgeuse reveals the cause of its pulsations

Betelgeuse is normally one of the brightest and most recognizable stars in the winter sky, marking the left shoulder of the constellation Orion. But lately, it’s behaving strangely: an unprecedented drop in its brightness was observed in early 2020, which sparked speculation that Betelgeuse was about to explode.

To find out more, an international team of scientists, including Ken’ichi Nomoto from the Kavli Institute for Physics and Mathematics of the Universe (Kavli IPMU), conducted a rigorous review of Betelgeuse. They concluded that the star was at the start of the helium combustion phase (over 100,000 years before an explosion occurred) and had a smaller mass and radius – and closer Earth – than previously thought. They also showed that smaller variations in Betelgeuse’s brightness were caused by stellar pulses, and suggested that the recent large dimming event involved a cloud of dust.

The research team is led by Dr Meridith Joyce from the Australian National University (ANU), who was a guest speaker at Kavli IPMU in January 2020, and includes Dr Shing-Chi Leung, a former Kavli IPMU project researcher and currently postdoctoral. researcher at the California Institute of Technology, and Dr. Chiaki Kobayashi, associate professor at the University of Hertfordshire, who was an affiliate member of Kavli IPMU.

The team analyzed the variation in luminosity of Betelgeuse using evolutionary, hydrodynamic and seismic modeling. They came to a clearer idea than before that Betelgeuse is currently burning helium in its nucleus. They also showed that the stellar pulses driven by the so-called kappa mechanism cause the star to continuously brighten or fade with two periods of 185 (+/- 13.5) days and around 400 days. But the sharp drop in brightness at the start of 2020 is unprecedented and is likely due to a cloud of dust in front of Betelgeuse, as shown in the image.

Their analysis showed a current mass of 16.5 to 19 solar masses, which is slightly lower than the most recent estimates. The study also revealed the size of Betelgeuse, as well as its distance from Earth. The actual size of the star has been a bit of a mystery: previous studies, for example, suggested that it could be larger than Jupiter’s orbit. However, the team’s results showed Betelgeuse only extends two-thirds of that, with a radius 750 times the sun’s radius. Once the physical size of the star is known, it will be possible to determine its distance from the Earth. So far, the team’s results show it’s only 530 light-years away from us, 25% closer than previously thought.

Their results imply that Betelgeuse is not at all close to exploding, and that it is too far from Earth for the eventual explosion to have a significant impact here, although it is still a very big problem when a supernova is triggered. And since Betelgeuse is the closest candidate for such an explosion, it gives us a rare opportunity to study what happens to stars like this before they explode.

Provided by the Kavli Institute for the Physics and Mathematics of the Universe