Zero Genie

On February 15, 2020, a team of U.S. astronomers observed Betelgeuse, a dimming red supergiant located approximately 650 light-years away from Earth, using the DeVeny optical spectrograph on the 4.3-m Lowell Discovery Telescope. They found that the star 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.

This image, taken by ALMA in 2017, shows the supergiant star Betelgeuse. Image credit: ALMA / ESO / NAOJ / NRAO / E. O’Gorman / P. Kervella.

“We see this all the time in red supergiants, and it’s a normal part of their life cycle,” said Dr. Emily Levesque, an astronomer in the Department of Astronomy at the University of Washington, Seattle.

“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 2019, wasn’t necessarily a sign of an imminent supernova.

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 Dr. Philip Massey, an astronomer with Lowell Observatory.

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.

The light from bright stars is often too strong for a detailed spectrum, but the team 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,” Dr. Levesque said.

“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.”

This comparison image shows Betelgeuse, which is also known as Alpha Orionis, before and after its unprecedented dimming. The observations, taken with the SPHERE instrument on ESO’s Very Large Telescope in January and December 2019, show how much the star has faded and how its apparent shape has changed. Image credit: ESO / M. Montargès et al.

By their calculations, Betelgeuse’s average surface temperature in Febaruary 2020 was about 3,325 degrees Celsius (6,017 degrees Fahrenheit). That’s only 50-100 degrees Celsius cooler than the temperature that astronomers had calculated as Betelgeuse’s surface temperature in 2004, years before its dramatic dimming began.

These findings cast doubt that Betelgeuse is dimming because one of the star’s massive convection cells had brought hot gas from the interior to the surface, where it had cooled.

“Many stars have these convection cells, including our own Sun. They resemble the surface of a pot of boiling water,” Dr. Levesque said.

But whereas the convection cells on our Sun are numerous and relatively small — roughly the size of Texas or Mexico — red supergiants like Betelgeuse, which are larger, cooler and have weaker gravity, sport just three or four massive convection cells that stretch over much of their surfaces.

If one of these massive cells had risen to Betelgeuse’s surface, the study authors would have registered a substantially greater decrease in temperature than what they see between 2004 and 2020.

“A comparison with our 2004 spectrum showed immediately that the temperature hadn’t changed significantly. We knew the answer had to be dust,” Dr. Massey said.

Astronomers have observed clouds of dust around other red supergiants, and additional observations may reveal similar clutter around Betelgeuse.

Over the past few weeks, Betelgeuse has actually started to brighten again, albeit slightly. Even if the recent dimming wasn’t an indication that the star would soon explode, that’s no reason to stop looking.

“Red supergiants are very dynamic stars,” Dr. Levesque said.

“The more we can learn about their normal behavior — temperature fluctuations, dust, convection cells — the better we can understand them and recognize when something truly unique, like a supernova, might happen.”

The findings will be published in the Astrophysical Journal Letters.

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Emily M. Levesque & Philip Massey. 2020. Betelgeuse Just Isn’t That Cool: Effective Temperature Alone Cannot Explain the Recent Dimming of Betelgeuse. ApJL, in press; arXiv: 2002.10463