The dramatic dimming of Betelgeuse that began in 2019 restored attention to the nearest red supergiant to the Earth. A new study now confirms this fading was caused by something outside the giant star, rather than being an indicator of an impending supernova explosion. Indeed, the research confirms we shouldn't expect a fireworks display anytime soon. In the process, the immense star was found to be smaller and closer than most previous estimates.
Betelgeuse's strange behavior created excitement because we know someday it will become a supernova. For Earthlings, it has almost perfect placement; it is distant enough not to pose a threat but also close enough to provide an astonishing view that's visible worldwide.
Dr Meridith Joyce of the Australian National University took part in an effort to learn Betelgeuse's secrets using seismic modeling. Joyce told IFLScience that to her knowledge the technique has never been applied to a variable supergiant before. If successful, it opens up opportunities to conduct similar studies of other members of this rare but immensely important class of stars.
In The Astrophysical Journal, Joyce confirms that Betelgeuse pulses because of pressure waves. She and her co-authors also verify previous conclusions the giant star is still in the helium-burning stage of its life, meaning there are around 100,000 years before it runs out of fuel entirely and explodes.
Studying the period with which pressure waves move across the star allowed the authors to calculate its size. Although Betelgeuse certainly deserves its status as a supergiant, lots of illustrations that show it having a diameter larger than the orbit of Jupiter will need to be rewritten. Instead, its radius is about 760 times that of the Sun – very much at the smaller of previous estimates.
A smaller Betelgeuse must be closer than previously thought to appear the size it does – about 530 light-years away.
Joyce told IFLScience that Betelgeuse's distance had previously been estimated through different means, and her team's result fits well with those taken from the Hipparcos satellite's measurements of its parallax, while conflicting with radio measurements.
It might be thought a star of such significance would have had its size and distance measured precisely by now, but Joyce said to IFLScience that “Betelgeuse has an unusually extended and complicated atmosphere.” This made establishing where its surface is, and therefore measuring its size, unusually hard.
One unexpected outcome of this study was the discovery that the giant star's spin and movement relative to the galaxy can't be reconciled with a lifetime as a solitary star. Instead, it must have once had a companion that booted it out of the cluster where it formed, giving it some spin in the process. The study didn't investigate the nature of this star, but if it exceeded Betelgeuse's 16.5 to 19 solar masses, it could have already gone supernova with drastic consequences for nearby stars.
As to the question of Betelgeuse's dimming, Joyce found that its most recent dip in brightness is a consequence of its internal pulsation cycle, but the larger one that began last year cannot be explained that way. Instead, she concurs a dust cloud obscured the giant star.