What's going on with giant star Betelgeuse? Scientists play down supernova fears

These features are similar to sunspots, which appear as dark regions on our Sun's surface

                            What's going on with giant star Betelgeuse? Scientists play down supernova fears
( MPIA graphics department)

Betelgeuse, a red giant star — 20 times more massive than our Sun — dramatically dimmed for a period, leaving scientists puzzled. Some thought the act signaled an impending death. Others believed that cosmic dust was behind the fading.

Now, a new study thinks something else is at play. According to experts from Max Planck Society, Betelgeuse may have sported large starspots covering 50 to 70% of its surface, leading to its subsequent dimming. These features are similar to sunspots, which appear as dark regions on the solar surface. The findings rule out dust as a culprit.  And, the star might not be nearing its end either, as scientists say that it has regained its former sheen.

The red giant could go supernova in the next hundred, thousand, or million years from now. The explosion could light up the skies on Earth. The star is over 500 light-years away, which means it could take that many years to reach our planet. In other words, people could witness an event that happened centuries ago.

It is not uncommon for monsters like Betelgeuse to show a periodic increase and decrease in brightness. Stars transform into red giants when they reach their end. "As their fuel supply runs out, the processes change by which the stars release energy." said Thavisha Dharmawardena from the Max Planck Institute for Astronomy in Heidelberg. They begin bloating, while also showing instability and fluctuations in brightness. 

Changes in brightness before (left) and during the dimming (right) (ESO / M Montargès et al)

But what caught the attention of experts was the star's odd behavior between October 2019 and April 2020,  when its brightness dipped by 40%.  Since then, scientists, including Dharmawardena and her colleagues, set out to look for a likely explanation. They searched for evidence that could implicate cosmic dust, which is often released when stars change in area and temperature, otherwise called pulsations.

So they looked at data both old and new. Using the Atacama Pathfinder Experiment in Chile and the James Clerk Maxwell Telescope in Hawaii, the scientists measured radiation in submillimeter waves. Their wavelength, which is a thousand times greater than that of visible light, is suited to study, the otherwise invisible, interstellar dust. "What surprised us was that Betelgeuse turned 20% darker even in the submillimetre wave range," says Steve Mairs, study co-author and researcher at the East Asian Observatory in Hawaii.

This darkening did not support the idea that dust was behind the star's dimming, according to the study. Instead, Betelgeuse was responsible for its change in brightness, it adds. Further, the findings also suggest a drop in the star's mean surface temperature.

"Corresponding high-resolution images of Betelgeuse from December 2019 show areas of varying brightness,"  Pieter Scicluna, study co-author and researcher at the European Southern Observatory, explains. "Together with our result, this is a clear indication of huge starspots covering between 50 and 70% of the visible surface and having a lower temperature than the brighter photosphere [or luminous surface of the star]."

 "Observations in the coming years will tell us whether the sharp decrease in Betelgeuse's brightness is related to a spot cycle. In any case, Betelgeuse will remain an exciting object for future studies," Dharmawardena adds. The study is published in The Astrophysical Journal Letters.

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