Stars

Astronomers Caught Betelgeuse Just Before it Started Dimming and Might Have Seen a Pressure Wave Rippling Through its Atmosphere

A couple of years ago, Betelgeuse generated much interest when it started dimming. That caught the attention of astronomers worldwide, who tried to understand what was happening. Was it about to go supernova?

Evidence showed that dust was the most likely culprit for the red supergiant’s dimming, though there are still questions. A new study shows that the star was behaving strangely just before the dimming.

The optical dimming that Betelgeuse—or Alpha Orionis—exhibited in late 2019 and early 2020 was unprecedented. Betelgeuse often undergoes periodic optical dimming, which happens in timescales of ~300-500 days and ~2000 days. But the Great Dimming, as it came to be known—when the star dimmed to about two-thirds of its normal brightness— was the faintest that the star had become in almost 200 years of observations.

Researchers got to work generating possible explanations for the dimming. Some hypothesized that dust caused the dimming, others hypothesized that a reduction in photosphere temperature caused it, and some thought both played a role.

Astronomers continued studying it, and evidence showed that temperature decrease alone couldn’t be the culprit. Episodic mass loss was proposed as the cause, paired with a rise in large grain dust in the line of sight. Others argued that large inhomogeneities in the photosphere caused the dimming. Another suggested a critical shift in Betelgeuse’s pulsation dynamics caused the Great Dimming. If feedback from UT readers is any indication, there’s still confusion around the cause of the dimming event.

Betelgeuse, as seen by the Hubble Space Telescope. Credit: NASA

But there were really two dimming events, and that’s helped create some of the confusion. The scientific community has settled on dust as the cause of the first dimming. “We know the first dimming event involved a dust cloud,” said Dr. Meridith Joyce from The Australian National University in 2020.

According to Dr. Joyce, the second dimming event had a different cause: “We found the second smaller event was likely due to the pulsations of the star.”

Now a new paper presents observations of Betelgeuse just before the Great Dimming. Its title is “SPATIALLY RESOLVED OBSERVATIONS OF BETELGEUSE AT ?7 MM AND ?1.3 CM JUST PRIOR TO THE
GREAT DIMMING.” The Astrophysical Journal will publish it, but for now, it’s available on the pre-press site arxiv.org. The authors are Dr. Lynn D. Matthews of MIT’s Haystack Observatory and Andrea Dupree from the Harvard and Smithsonian Center for Astrophysics. The paper is based on observations of Betelgeuse with the Karl G. Jansky Very Large Array.

“Our measurements suggest recent changes in the temperature and density structure of the
atmosphere,” the authors write. The star’s photosphere “… is ~20% dimmer than in previously published
observing epochs between 1996–2004.” This is … lower than previously reported temperatures at comparable radii and >1200 K lower than predicted by previous semi-empirical models of the atmosphere.”

Betelgeuse is known for pulsating, as it swells and shrinks symmetrically. These are Hubble images of the star from 1998 and 1999. Image Credit: NASA/ESA/Hubble

The researchers also found that the measured brightness temperature was cooler than expected and that there were “… no obvious signatures of giant convective cells or other surface features.” The star’s brightness profile was also more complex than a uniform elliptical disk (A uniform elliptical disk is a tool astrophysicists use to characterize the mean properties of a star.) Their observations were from about six weeks before ultraviolet measurements found increases in electron density in Betelgeuse’s southern hemisphere, coupled with a large-scale outflow.

So what does all that mean?

“We discuss possible scenarios linking these events with the observed radio properties of the star, including the passage of a strong shock wave.”

Researchers in the astrophysical community have postulated that a shock wave could’ve caused the Great Dimming. A 2021 paper found that Betelgeuse’s photosphere experienced successive shock waves in February 2018 and January 2019, with the initial shock amplifying the second one. Other research showed that a shock wave passed through the southwestern portion of Betelgeuse’s chromosphere between 2019 September and November. Since the photosphere is under the chromosphere, it’s reasonable to think that the two shock waves are related.

This diagram of a star’s layers shows how the photosphere is below the chromosphere. The photosphere is the lowest layer of a star’s atmosphere and the lowest observable layer. Image Credit: ESA

The researchers say they can’t conclude that their observations are directly responsible for the Great Dimming, even though their data suggests “… recent changes in the density and/or temperature structure of the atmosphere…” But a large-amplitude shock or pressure wave passing through Betelgeuse’s atmosphere could cause changes in density and temperature.

The authors are cautious and inconclusive, but they do point out that the pressure wave they observed could’ve caused Betelgeuse’s Great Dimming. “Such an event may be linked to a largescale mass ejection from the star that has been postulated as an explanation for the steep decline in optical magnitude associated with the Great Dimming.”

There’s no question that Betelgeuse will explode as a Type IIP supernova, likely in the next 100,000 years or so. It’s the 10th brightest star in the night sky and is giving astronomers an opportunity to study intensely the behaviour of a star as it approaches its cataclysmic end.

The familiar constellation of Orion. Orion’s Belt can be clearly seen, as well as Betelgeuse (red star in the upper left corner) and Rigel (bright blue star in the lower right corner). Credit: NASA Astronomy Picture of the Day Collection NASA

This study won’t be the final word on Betelgeuse and its dynamic behaviour. The star is still up there, anchoring the Orion the Hunter’s right shoulder. Generations and generations of astronomers are bound to keep watching it.

If humanity lasts long enough, our distant descendants will get to watch it explode.

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Evan Gough

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