The artist's vision for 2021 offers a close-up of Betelgeuse's irregular surface and its giant dynamic bubbles, dotted with distant stars in the background.

Betelgeuse is bouncing back after peaking in 2019

enlarge / The artist’s vision for 2021 offers a close-up of Betelgeuse’s irregular surface and its giant dynamic bubbles, dotted 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 ultimately attributed to a cold spot and a stellar “burp,” which enveloped the star in interstellar dust. Now, new observations from the Hubble Space Telescope and other observatories reveal more about the events that preceded the dimming.

Betelgeuse appears to have suffered a massive surface mass ejection (SME) event in 2019, blasting 400 times the mass of our sun during a coronal mass ejection (CME). The scale of the event is unprecedented, showing that the CME and SME are distinctly different types of events, according to a new paper published last week on Physics arXiv. (It has been accepted for publication in The Astrophysical Journal.)

Betelgeuse, a bright red star in the constellation Orion, is one of the closest massive stars to Earth, about 700 light-years away. This is an ancient star that has reached a stage where it glows dim red and expands, while the hot core has only a weak gravitational pull in its outer layers. The star has something like a heartbeat, albeit very slowly and irregularly. Over time, stars cycle through cycles of expanding and then contracting their surfaces.

One of the cycles was fairly regular and took more than five years to complete. Stratification is a shorter, more irregular cycle that takes less than a year to 1.5 years to complete. While these cycles are easy to track with ground-based telescopes, the changes do not cause the drastic changes in the star’s light that would cause the changes seen during dimming events.

As we previously reported, astronomers first noticed a strange, dramatic dimming from Betelgeuse in December 2019. The star dimmed to the point that it was visible to the naked eye. The dimming persisted, with a 35% drop in brightness in mid-February before brightening again in April 2020.

Astronomers were baffled by the phenomenon and wondered if it was a sign that the star was about to go supernova. After a few months, they narrowed down the most likely explanations to two: a brief cold spot on the star’s southern surface (similar to a sunspot) or a cloud of dust making it appear dimmer to observers on Earth. Last year, astronomers determined that dust was the culprit, linked to the brief appearance of cold spots.

The ESO team concluded that the star’s outward pulsation ejects a bubble and pushes it further away — sort of like a star’s “hiccup.” When convection-driven cold spots appear on the surface, the local temperature drop is enough to condense heavier elements, such as silicon, into solid dust, forming a veil that obscures the star’s southern hemisphere brightness.

This illustration depicts the change in brightness of the red supergiant star Betelgeuse after a large mass ejection from its visible surface.
enlarge / This illustration depicts the change in brightness of the red supergiant star Betelgeuse after a large mass ejection from its visible surface.

NASA/ESA/Elizabeth Whitley (STScI)

According to the authors of the latest paper, the event was more than a stellar hiccup. A large convective plume more than a million miles across emerges from deep within the red giant star. The resulting shocks and pulsations are enough to create an SME that blasts a chunk of the star’s photosphere into space. This creates cold spots covered in dust clouds, which explain the dimming.

The red giant is just beginning to recover from that catastrophic event. “Betelgeuse is now going on to do some pretty unusual things; it’s kind of bouncing inside,” said co-author Andrea Dupri of the Harvard-Smithsonian Center for Astrophysics, who likened the activity to a plate of jelly. Its signature pulsation also ceased—hopefully temporarily—perhaps because the internal convective cells “swinged around like an unbalanced washing machine tub” as the photosphere began a slow process of rebuilding itself.

“We’ve never seen a massive mass ejection from the surface of a star before,” Dupri said. “We don’t yet fully understand what’s going on. It’s an entirely new phenomenon, and we can directly observe and resolve surface details with Hubble. We’re watching stellar evolution in real time.” Webb Space Telescope may be able to detect ejections in infrared light of matter as it continues to move away from the star, which may tell astronomers more about what’s going on — and its effects on other similar stars.

DOI: arXiv, 2022. 10.48550/arXiv.2208.01676 (on DOI).

List image for ESO/P. Kervera/M. Montagis et al.

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