Expand / The artist’s conception in 2021 provided a close-up of Betelgeuse’s jagged surface and its giant, dynamic gas bubbles, with distant stars dotting the background.
Southern European Observatory
Astronomers are still making new discoveries about the red supergiant star Betelgeuse, which experienced a mysterious “darkening” a few years ago. This decrease was eventually attributed to a cold spot and a stellar “donkey” that surrounded the star in interstellar dust. Now, new observations from the Hubble Space Telescope and other observatories have revealed more about the event that preceded the eclipse.
Betelgeuse appears to have undergone a massive surface mass injection event (SME) in 2019, ejecting 400 times more mass than our Sun during coronal mass ejections (CMEs). The sheer scale of the event is unprecedented and suggests that CMEs and SMEs are distinctly different types of events, according to a new paper published in the physics arXiv last week. (It has been accepted for publication in The Astrophysical Journal.)
Betelgeuse is a bright red star in the constellation Orion, one of the closest massive stars to Earth, about 700 light-years away. This is an old star that has reached the stage where it glows dull red and is expanding, with the hot core having only a tenuous gravitational grip on its outer layers. The star has something like a heartbeat, although it is extremely slow and irregular. Over time, the star goes through periods in which its surface expands and then contracts.
One such cycle is fairly regular, taking a little over five years to complete. In layers there is a shorter and irregular cycle that takes less than a year to 1.5 years to complete. Although the cycles are easy to follow with ground-based telescopes, the changes do not cause the kind of radical changes in starlight that would explain the changes observed during the dimming event.
As we previously reported, astronomers first noticed Betelgeuse’s strange and dramatic dimming in December 2019. The star dimmed so much that the difference was visible to the naked eye. The dimming persisted, dimming by 35 percent in mid-February before brightening again in April 2020.
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Astronomers were puzzled by the phenomenon and wondered if it was a sign that the star was about to go supernova. Several months later, they had narrowed down the most likely explanations to two: a short-lived cold patch on the star’s southern surface (similar to a sunspot) or a cluster of dust that makes the star appear fainter to Earth observers. Last year, astronomers determined that dust was the main culprit, linked to the brief appearance of a cold spot.
The ESO team concluded that a bubble of gas was ejected and pushed further away by the star’s outer pulsation, like a stellar “donkey”. When a convectively driven cold patch appeared on the surface, the local temperature drop was sufficient to condense the heavier elements (such as silicon) into solid dust, forming a veil that dimmed the star’s brightness in the his southern hemisphere.
Enlarge / This illustration depicts changes in the brightness of the red supergiant star Betelgeuse following the titanic mass ejection of a large portion of its visible surface.
NASA/ESA/Elizabeth Wheatley (STScI)
According to the authors of this latest paper, the event was much more than just a stellar belch. A large convective plume more than 1 million miles in diameter rose from deep inside the red giant. The resulting shocks and pulsations were powerful enough to produce an SME, which threw a large portion of the star’s photosphere into space. This produced the cold patch covered by the dust cloud, which explains the dimming.
The red giant is just beginning to heal from that catastrophic event. “Betelgeuse is still doing very unusual things right now; the interior is bouncing around,” said co-author Andrea Dupree of the Harvard-Smithsonian Center for Astrophysics, comparing the activity to a plate of jello. Its trademark pulsation has also stalled, hopefully temporarily, perhaps because the inner convection cells “are playing like an unbalanced washing tub” as the photosphere begins the slow rebuilding process.
“We’ve never seen a large mass ejection from the surface of a star before,” Dupree said. “We’re left with something that we don’t fully understand. It’s a completely new phenomenon that we can observe directly and resolve the surface details with Hubble. We’re seeing stellar evolution in real time.” The Webb Space Telescope may be able to detect the ejected material in infrared light as it continues to move away from the star, which could tell astronomers even more about what happened and its implications for other similar stars.
DOI: arXiv, 2022. 10.48550/arXiv.2208.01676 (About DOI).
Listing image by ESO/P. Kervella/M. Montargès et al.