A “Stellar Sneeze” May Have Caused The Dramatic Dimming Of Betelgeuse Last Winter

This artist's impression shows the starlight being blocked by a cloud of dust. It was generated using an image of Betelgeuse from late 2019 taken with the SPHERE instrument on the European Southern Observatory's Very Large Telescope. ESO, ESA/Hubble, M. Kornmesser

Red supergiant star Betelgeuse surged in popularity last winter during its incredible dimming event, which began in December and concluded in February. The dip was so dramatic the star dropped out of the Top 20 brightest stars, puzzling scientists as to its cause.

Betelgeuse is known to be variable in its brightness, changing on a more or less regular cycle. Even though the latest event was within this cycle, the magnitude requires an explanation. 

A few hypotheses took center stage to describe what happened. Some envisioned an intrinsic explanation: the star dimmed due to something happening on the inside, such as dust in its atmosphere or a giant cold spot. Others suggested the light was blocked by a cloud of material, erupted by the star itself, cooling down and blocking some of the light. This view now has new evidence to back it up thanks to the Hubble Space Telescope. 

UV observations of Betelgeuse by Hubble. Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA

Researchers used the orbital observatory to study the outer layers of the star in ultraviolet light. Between September and November 2019, they observed hot material leaving the stellar surface and moving through its atmosphere. As this material moved through space, it expanded, cooled, and eventually blocked some of the starlight. This produced the enhanced dimming we witnessed a few months ago. The study is published in The Astrophysical Journal.

“With Hubble, we see the material as it left the star’s visible surface and moved out through the atmosphere, before the dust formed that caused the star appear to dim,” lead researcher Andrea Dupree, an associate director at The Center for Astrophysics, Harvard & Smithsonian, said in a statement. “We could see the effect of a dense, hot region in the southeast part of the star moving outward.”

“This material was two to four times more luminous than the star’s normal brightness,” Dupree continued. “And then, about a month later, the southern hemisphere of Betelgeuse dimmed conspicuously as the star grew fainter. We think it is possible that a dark cloud resulted from the outflow that Hubble detected. Only Hubble gives us this evidence of what led up to the dimming.”

It's possible the release of the material is linked to the stellar cycle, which means we could observe the phenomenon again. The team will continue to study the star with the Hubble Space Telescope as soon as possible.

Although the mystery of the dimming star is getting clearer, many questions remain. Perhaps, however, it is easier to just see Betelgeuse’s dramatic changes as an omen we failed to grasp for 2020.

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