Late last year, news broke that the star Betelgeuse was fading significantly, ultimately dropping to around 40% of its usual brightness. The activity fueled popular speculation that the red supergiant would soon explode as a massive supernova.
But astronomers have more benign theories to explain the star’s dimming behavior. And scientists at the University of Washington and Lowell Observatory believe they have support for one of them: Betelgeuse isn’t dimming because it’s about to explode—it’s just dusty.
Emily Levesque, a UW associate professor of astronomy, and Philip Massey, an astronomer with Lowell Observatory, report that observations of Betelgeuse taken Feb. 14 at the Flagstaff, Arizona, observatory allowed them to calculate the average surface temperature of the star. They discovered that Betelgeuse is significantly warmer than expected if the recent dimming were caused by a cooling of the star’s surface.
The new calculations lend support to the theory that Betelgeuse—as many red supergiant stars are prone to do—has likely sloughed off some material from its outer layers.
“We see this all the time in red supergiants, and it’s a normal part of their life cycle,” said Levesque. “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.”
It is still true: Astronomers expect Betelgeuse to explode as a supernova within the next 100,000 years when its core collapses. But the star’s dimming, which began in October, wasn’t necessarily a sign of an imminent supernova, according to Massey.
One theory was that newly formed dust was absorbing some of Betelgeuse’s light. Another posited that huge convection cells within Betelgeuse had drawn hot material up to its surface, where it had cooled before falling back into the interior.
“A simple way to tell between these possibilities is to determine the effective surface temperature of Betelgeuse,” said Massey.
Measuring a star’s temperature is no straightforward task. Scientists can’t just point a thermometer at a star and get a reading. But by looking at the spectrum of light emanating from a star, astronomers can calculate its temperature.
“Emily and I had been in contact about Betelgeuse, and we both agreed that the obvious thing to do was to get a spectrum,” said Massey. “I already had observing time scheduled on the 4.3-meter Lowell Discovery Telescope, and I knew if I played around for a bit I would be able to get a good spectrum despite Betelgeuse still being one of the brightest stars in the sky.”
The light from bright stars is often too strong for a detailed spectrum, but Massey employed a filter that effectively “dampened” the signal so they could mine the spectrum for a particular signature: the absorbance of light by molecules of titanium oxide.
Titanium oxide can form and accumulate in the upper layers of large, relatively cool stars like Betelgeuse, according to Levesque. It absorbs certain wavelengths of light, leaving telltale “scoops” in the spectrum of red supergiants that scientist can use to determine the star’s surface temperature.
By their calculations, Betelgeuse’s average surface temperature on Feb. 14 was about 3,325 degrees Celsius, or 6,017 F. That’s only 50-100 degrees Celsius cooler than the temperature that a team—including Massey and Levesque—had calculated as Betelgeuse’s surface temperature in 2004, years before its dramatic dimming began.
Source: “Dimming Betelgeuse likely isn’t cold, just dusty, new study shows” PhysOrg, 6 March 2020.<https://phys.org/news/2020-03-dimming-betelgeuse-isnt-cold-dusty.html>