Betelgeuse bounces back after blowing its peak in 2019

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

European Southern Observatory

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

It appears that Betelgeuse underwent a massive surface mass injection (SME) in 2019, spewing 400 times more mass than our sun during coronal mass ejections (CMEs). The sheer size of the event is unprecedented and suggests that CMEs and SMBs are distinctly different types of events, according to a new article posted to 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, located about 700 light-years away. It is an old star that has reached the stage where it glows dull red and expands, with the hot core having only a weak gravitational hold on the outer layers. The star has something akin to a heartbeat, albeit an extremely slow and irregular one. Over time, the star goes through periods where the surface expands and then contracts.

One of these cycles is fairly regular and takes a little over five years to complete. Layered on it is a shorter, more irregular cycle that lasts from less than a year to 1.5 years. While the cycles are easy to track with ground-based telescopes, the shifts don’t cause the kind of radical changes in the star’s light that would explain the changes observed as it dims.

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

Astronomers marveled at the phenomenon and wondered if it was a sign that the star was about to go supernova. A few months later, they’d 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 made the star appear fainter to Earth observers. Last year, astronomers determined that dust was the main culprit, associated with the brief appearance of a cold spot.

The ESO team concluded that a gas bubble was ejected and pushed further out by the star’s outward pulsation – a sort of stellar “burp”. When a convection-driven cold spot appeared on the surface, the local drop in temperature was enough to condense the heavier elements (such as silicon) into solid matter, forming a veil that obscured the star’s brightness in the Southern Hemisphere.

This illustration shows changes in the brightness of the red supergiant Betelgeuse after the ejection of a gigantic mass from much of its visible surface.
enlarge / This illustration shows changes in the brightness of the red supergiant Betelgeuse after the ejection of a gigantic mass from much of its visible surface.

NASA/ESA/Elizabeth Wheatley (STScI)

According to the authors of this latest article, the event was significantly more than a stellar farmer. A large convective plume more than 1 million miles in diameter bubbled up from deep within the red giant. The resulting shocks and pulsations were powerful enough to spawn an SME, which blasted a huge portion of the star’s photosphere into space. That produced the cold spot covered by the dust cloud, which explains the dimming.

The red giant has only just begun to heal from that catastrophic event. “Betelgeuse continues to do some very unusual things at the moment; the interior bounces a bit,” said study co-author Andrea Dupree of the Harvard-Smithsonian Center for Astrophysics, who likens the activity to a plate of Jell-O. Its signature pulsation has also stopped — hopefully temporarily — perhaps because the internal convection cells “slosh around like an unbalanced washing machine tub” as the photosphere begins its slow process of rebuilding.

“We’ve never seen a massive mass ejection from the surface of a star,” Dupree said. “There’s something going on that we don’t fully understand. It’s a totally new phenomenon that we can directly observe and resolve surface details with Hubble. We’re looking at the evolution of stars in real time.” The Webb Space Telescope can detect the ejecta in infrared light as it moves farther from the star, perhaps telling astronomers more about what happened — and its implications for other similar stars.

DOI: arXiv, 2022. 10.48550/arXiv.2208.01676 (About DOIs).

List image by ESO/P. Kervela/M. Montarges et al.

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