Seething supergiant Betelgeuse –a star so huge it could someday collapse into a black hole or neutron star, which would make it the closest black hole to Earth some 725 light-years distant– has displayed unprecedentedly large drop in its brightness in early 2020, prompting speculation that the pulsing may be a dire prelude. A new study by an international team of scientists concluded that the star is in the early core helium-burning phase (more than 100,000 years before a supernova event) and has smaller mass and radius–and is closer to Earth–than previously thought. If the bright-red object replaced the Sun at the center of our solar system, its outer surface would extend past the orbit of Jupiter.
Recent Dimming Caused by a Dust Cloud?
The researchers, reports the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), also showed that smaller brightness variations of Betelgeuse have been driven by stellar pulsations, and suggested that the recent large dimming event involved a dust cloud.
Evolutionary, Hydrodynamic, Seismic Modelling
The team, led by Meridith Joyce from the Australian National University (ANU), analyzed the brightness variation of Betelgeuse (Figure 2) by using evolutionary, hydrodynamic and seismic modelling. They achieved a clearer idea than before that Betelgeuse is currently burning helium in its core. They also showed that stellar pulsations driven by the so-called kappa-mechanism is causing the star to continuously brighten or fade with two periods of 185 (+-13.5) days and approximately 400 days. But the large dip in brightness in early 2020 is unprecedented, and is likely due to a dust cloud in front of Betelgeuse, as seen in the image (Figure 1).
Star’s Actual Size a Mystery –25 % Closer than Previously Thought
Their analysis reported a present-day mass of 16.5 to 19 solar mass–which is slightly lower than the most-recent estimates. The study also revealed how big Betelgeuse is, as well as its distance from Earth. The star’s actual size has been a bit of a mystery: earlier studies, for instance, suggested it could be bigger than the orbit of Jupiter. However, the team’s results showed Betelgeuse only extends out to two-thirds of that, with a radius 750 times the radius of the sun. Once the physical size of the star is known, it will be possible to determine its distance from Earth. Thus far, the team’s results show it is a mere 530 light years from us, or 25 percent closer than previously thought.
Planet Earth is Safe
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 significant impact, but it may one day join a string of supernovae that went off over the past 10 million years—a blink of an eye in galactic terms–with two of them near enough to seed Earth with radioactive particles and radiation.
Image credit top of page –Betelgeuse imaged in ultraviolet light by the Hubble Space Telescope.
Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA