This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: For the first time since its dramatic dimming in 2020, a team of astronomers has observed the inner atmosphere of Betelgeuse using the Atacama Large Millimeter Array (ALMA). Led by Bill Dent at the University of Manchester, the team found that the giant star's inner atmosphere has become increasingly uneven and asymmetric, in a way that could be connected to a far smaller companion star.
Soon to be published in Astronomy & Astrophysics, their results could ultimately reveal deeper insights into the dramatic changes taking place at the sky's 10th-brightest star as it approaches the end of its life. For astronomers, Betelgeuse is one of the most familiar objects in the night sky. Today, its light has been intensively studied across the electromagnetic spectrum—but in the past few years, the aging star has been changing rapidly.
"Betelgeuse is one of the closest red supergiants—stars which will inevitably explode as supernovae but, in the meantime, are polluting interstellar space with strong stellar winds," Dent explains. One particularly dramatic change occurred in 2020, when the star suddenly became around 2.5 times fainter, an event since dubbed the "Great Dimming." In its aftermath, many questions have remained about the changes now rapidly unfolding at Betelgeuse's surface and inner atmosphere. In all red supergiants, astronomers have found that these layers become increasingly uneven as the stars age, producing bright spots and patches that contrast heavily with the rest of the star.
Dent's team suspected that these changes could be connected to the Great Dimming—but they needed clearer observations to confirm their theories. The astronomers observed Betelgeuse using ALMA, a facility made up of 66 radio telescopes in the Atacama Desert in Chile that produces images with extraordinarily high resolution. The last time ALMA had been pointed specifically at Betelgeuse was in 2015, before the Great Dimming.
Crucially, ALMA can detect wavelengths in the range of 0.6–1.4 mm, originating from a layer just above Betelgeuse's visible surface—tracing the innermost part of its extended atmosphere. To maximize the resolution of their images, Dent's team used the longest possible baseline at the facility, taking simultaneous measurements from pairs of antennas with the greatest possible separation distances within the array. Also taking advantage of favorable weather conditions and cutting-edge data reduction techniques, "we were able to image the ionized and molecular gas with resolutions as small as 7 milliarcseconds—equivalent to resolving features 13 meters (43 feet) across on the surface of the moon," Dent describes.
As earlier studies had predicted, the observations showed that Betelgeuse's inner atmosphere has become spottier and more asymmetric since 2015, with some hot spots around 800 degrees hotter than their surroundings. ALMA also revealed peaks and troughs on the star's surface, indicating the presence of immense convective cells. Stars like the sun are covered in these cells, which emerge as heated, upwelling gas expands into the atmosphere before cooling and sinking back beneath the surface.
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