Is Betelgeuse About to Explode, or is it Just Dusty?

James Maynard
Mar 6 · 4 min read

For months, Betelgeuse has been seen dimming quite a bit, even to the naked eye. Is this red giant star about to explode, or is it just dusty?

The star Betelgeuse is well-known as a red star seen as one of the shoulders of the constellation Orion. Since late autumn 2019, both amateur and professional astronomers have noted significant dimming from this tawny star.

Betelgeuse dimmed by around 60 percent over the course of just a few months beginning in October 2019, leading astronomers to speculate on the causes of it errant behavior. One idea held that this star was on the verge of exploding as a powerful supernova. However, a new study suggests this dimming may be the result of dust blocking light from the massive star.

We interview Dr. Emily Levesque of the University of Washington, lead researcher on this story, on Astronomy News with The Cosmic Companion Podcast March 17, 2020

Betelgeuse dimmed to less than half its original brightness in the closing months of 2019, as seen in this pair of images taken by the ESO’s Very Large Telescope in January and December 2019. Image credit: ESO/M. Montargès et al.

Cool, Daddy-O, but Still Really Hot!

New images of Betelgeuse created by astronomers from the University of Washington (UW) and Lowell Observatory in Arizona accurately measured the temperature of the surface of that star. The researchers found the stellar surface is significantly warmer than expected if the star were cooling in preparation for a supernova.

The observations suggest that Betelgeuse may be simply sloughing off some material from its outer layers into space.

“We see this all the time in red supergiants, and it’s a normal part of their life cycle. Red supergiants will occasionally shed material from their surfaces, which will condense around the star as dust. As it cools and dissipates, the dust grains will absorb some of the light heading toward us and block our view,” explains Emily Levesque, assistant professor in the Department of Astronomy at the University of Washington.

By measuring the peak wavelength of light put out by a star, astronomers are able to determine the surface temperature of that body.

Calculations suggest that stars which are about to undergo supernova eruptions will cool significantly. Therefore, measuring the surface temperature of a star should provide evidence of an impending explosion.

However, it is difficult to obtain spectra from stars as bright as Betelgeuse (even taking dimming into account). To take these measurements, Philip Massey, an astronomer with Lowell Observatory, utilized a filter restricting light to only those frequencies associated with the absorption of light by titanium oxide. This material can accumulate in the upper layers of large, cool stars like Betelgeuse. By measuring dips in the amount of light seen at various frequencies in the starlight, it is possible to measure the temperatures of the star and determine if a supernova explosion is imminent.

Zooming on Betelgeuse, from our location in space to the surface of the star. Video credit: European Southern Observatory (ESO)

“Let’s turn on the juice and see what shakes loose!” — Beetlejuice (Michael Keaton), Beetlejuice

The team found Betelgeuse currently has a surface temperature around 3,325 degrees Celsius (a little over 6,000 Fahrenheit). That is just 50–100 degrees Celsius cooler than Levesque and Massey measured in 2004, suggesting little cooling is taking place within the red giant star.

“While this is slightly cooler than previous measurements taken prior to Betelgeuse’s recent lightcurve evolution, this drop in effective temperature is insufficient to explain Betelgeuse’s recent optical dimming. We propose that episodic mass loss and an increase in the amount of large-grain circumstellar dust along our sightline to Betelgeuse is the most likely explanation for its recent photometric evolution,” researchers write in a letter published in The Astrophysical Journal.

Time to Hit the Dusty Trail

Many stars (including our own Sun) exhibit convection cells, where hot plasma rises from the interior of the star before striking the surface and cooling. Some researchers suggest these convection cells may be responsible for the dimming of Betelgeuse, but such activity would have resulted in significantly cooler temperatures on the stellar surface than what the team recorded.

Clouds of dust like those which may envelope Betelgeuse have been seen surrounding other stars, and, this red giant star has even become slightly brighter over the last few weeks.

Betelgeuse is still very likely to explode as a supernova some time in the next 100,000 years or so. However, it now seems unlikely that the dimming seen over the last five months is a sign that this star is ready to explode.

James Maynard is the founder and publisher of The Cosmic Companion. He is a New England native turned desert rat in Tucson, where he lives with his lovely wife, Nicole, and Max the Cat.

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