Betelgeuse is recovering after blasting its 2019 peak

Betelgeuse is recovering after blasting its 2019 peak

Enlarge / The 2021 artist’s conception provided a close-up of Betelgeuse’s irregular surface and its giant, dynamic gas bubbles, with distant stars in the background.

European Southern Observatory

Astronomers are still making new discoveries about the red supergiant star Betelgeuse, which experienced a mysterious “dimming” a few years ago. This dimming was eventually attributed to a cold spot and a stellar “burp” that blanketed 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 blackout.

It appears that Betelgeuse suffered a massive surface mass injection (SME) event in 2019, blasting off 400 times the mass of our Sun during coronal mass ejections (CMEs). The sheer magnitude 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 of Orion – one of the closest massive stars to Earth, about 700 light-years away. It is an old star that has reached the stage where it glows dull red and is expanding, with the hot core having only a weak gravitational grip on its outer layers. The star has something like a heartbeat, albeit an extremely slow and irregular one. Over time, the star goes through periods of expanding and then contracting its surface.

One of these cycles is fairly regular, lasting just over five years. Layered on top of this is a shorter, more irregular cycle, lasting anywhere from less than a year to 1.5 years. While the cycles are easy to follow with ground-based telescopes, the shifts do not cause the kind of radical changes in the star’s light that would account for the changes observed during the dimming event.

As we previously reported, in December 2019, astronomers first noticed the strange, dramatic dimming of Betelgeuse’s light. The star dimmed so much that the difference was visible to the naked eye. The dimming continued, decreasing 35 percent in brightness in mid-February before brightening again in April 2020.

Astronomers puzzled over the phenomenon, wondering if it was a sign the star was on the verge of a supernova. A few months later, they had narrowed the most likely explanations down to two: a short-lived cold spot on the star’s southern surface (similar to a sunspot), or a clump of dust that makes the star appear dimmer to observers on Earth. 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 being ejected and pushed further outward by the star’s outward pulsation – something like a stellar “belch”. When a convection-induced cold spot appeared on the surface, the local temperature drop was enough to condense the heavier elements (like silicon) into solid dust, forming a veil that eclipsed the star’s brightness in its southern hemisphere.

This image shows changes in the brightness of the red supergiant star Betelgeuse after the titanic mass ejection of much of its visible surface.
Enlarge / This image shows changes in the brightness of the red supergiant star Betelgeuse after the titanic mass ejection of much of its visible surface.

NASA/ESA/Elizabeth Wheatley (STScI)

According to the authors of this latest publication, the event was significantly more than just a stellar belch. A large convective cloud, over 1 million miles in diameter, bubbled up from deep within the red giant. The resulting shaking and pulsations were strong enough to create an SME that ejected a massive portion of the star’s photosphere into space. This created the dust cloud-covered cold spot that explains the blackout.

The red giant has only just begun to recover from this cataclysmic event. “Betelgeuse continues to do some very unusual things at the moment; the inside kind of bounces,” said co-author Andrea Dupree of the Harvard-Smithsonian Center for Astrophysics, comparing the activity to a bowl of jelly. Its characteristic pulsing has also stopped – hopefully temporarily – perhaps because the inner convection cells are “sloshing around like an unbalanced washing machine tub” as the photosphere begins the slow process of rebuilding.

“We’ve never seen a giant mass ejection from the surface of a star before,” Dupree said. “We’re left with something we don’t fully understand. It’s a completely new phenomenon that we can observe directly and resolve surface details with Hubble. We’re watching stellar evolution in real time.” The Webb Space Telescope may be able to detect the ejected material in infrared light as it moves farther from the star, potentially giving astronomers even more insight into what happened and its impact on other similar stars reveals.

DOI: arXiv, 2022.10.48550/arXiv.2208.01676 (About DOIs).

Listing image of ESO/P. Kervella/M. Montarges et al.

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