Today’s top story brings back our favorite red giant star, Betelgeuse. A new paper published in The Astrophysical Journal (ApJ) with lead author Meridith Joyce looks at Betelgeuse as a pulsating variable star and uses atmospheric physics to bring us new insights on what the red giant is up to.

This paper combines new observations with new computer models to derive the star’s mass and size from its outward behavior in much the same way that one might derive the size of a jug based on what notes can be created by blowing across its opening. Different sized objects naturally resonate in specific ways. By understanding how Betelgeuse pulsates in the fundamental mode and first overtone mode – those first two pitches you can make with your bottle – they were able to work backward to figure out Betelgeuse’s physical size. This method let them then figure out more accurately the star’s distance because we know how big it appears on the sky, and there is a simple mathematical relationship between how big something is and how big it appears. 

Our current estimates had been based on Hipparcos measurements of how Betelgeuse appears to move relative to background objects when the Earth moves around the Sun. This technique, called parallax, can be super useful but trying to measure how much an object shifts, when that object is a blobby star like Betelgeuse, can be a challenge.

Betelgeuse appears to be about 42 microarcseconds (mas) across and seems to shift just 5.95+0.58 or -0.85 mas. This is a really hard measurement, and Betelgeuse hasn’t been looked at yet with Gaia. 

By working from the theoretical size they got from its pulsation modes, and calculating a new distance, this team found that the lower edge of that error in the distance appears to be accurate, putting Betelgeuse just 530 lightyears away. This is 25% of the distance from the prior estimate, but as I said, it’s within error. This closer distance makes Betelgeuse a bit smaller; it appears to be 750 times the radius of the Sun. Nevertheless, it still probably started with 20+ solar masses of material, and it will still explode in about 100,000 years. At this point, they believe, it is still burning helium in its core and is nowhere near going boom.

It’s always cool when completely different techniques can narrow in on more precise values. This shows us once again that there is no one way to do science.

More Information

Australian National University press release 

“Standing on the Shoulders of Giants: New Mass and Distance Estimates for Betelgeuse Through Combined Evolutionary, Asteroseismic, and Hydrodynamic Simulations with MESA,” Meridith Joyce et al., 2020 Oct. 13, Astrophysical Journal (preprint on arxiv.org)