NASA, ESA, Elizabeth Wheatley (STScI) 

Back in 2019, the bright red supergiant star Betelgeuse was observed by the Hubble Space Telescope, essentially blowing its top in 2019. Known as a surface mass ejection (SME), this was an unprecedented observation.

In a phenomenon known as a coronal mass ejection (CME), our Sun periodically throws out portions of its flimsy outer atmosphere, the corona. However, the Betelgeuse SME ejected 400 billion times more mass than an average CME.

The monstrous star is still regaining its strength after this explosive event, with the interior of Betelgeuse currently experiencing a sort of bouncing, according to Andrea Dupree of the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Massachusetts.

"Betelgeuse continues doing some very unusual things right now; the interior is sort of bouncing," Dupree added.

These new discoveries provide information on how red stars lose mass toward the end of their lives when their nuclear fusion furnaces exhaust, leading to their eventual explosion as supernovae. Their fate is highly impacted by the degree of mass loss. Therefore, a large-scale loss event is not always an indication of an impending explosion.

Dupree is now pulling together all the pieces of the star's recent odd behavior before, after, and during the eruption into a coherent story of a never-before-seen titanic convulsion in an aging star.

The STELLA robotic observatory, the Tillinghast Reflector Echelle Spectrograph (TRES) at the Fred L. Whipple Observatory, the Solar Terrestrial Relations Observatory spacecraft (STEREO-A), the Hubble Space Telescope, and the American Association of Variable Star Observers have all contributed new spectroscopic and imaging data to this (AAVSO). Dupree highlights how important the Hubble data was in helping to solve the puzzle.

“We've never before seen a huge mass ejection of the surface of a star. We are left with something going on that we don't completely understand. It's a totally new phenomenon that we can observe directly and resolve surface details with Hubble. We're watching stellar evolution in real-time.”

A convective plume that was more than a million miles across and was boiling up from the star's interior may have been the origin of the colossal outburst in 2019. The cooling portion of the photosphere was blasted off by shocks and pulsations, leaving the star with a sizable cold surface area beneath the dust cloud it had created.

Currently, Dupree explains, Betelgeuse is apparently having difficulty healing from this wound.

The fragmented photosphere, which was roughly several times as heavy as our Moon, flew off into space, cooled, and formed a dust cloud that obscured the star from Earth-based observers. Even backyard watchers who watched the star's brightness change were able to easily detect the fading, which started in late 2019 and continued for a few months. Betelgeuse, one of the brightest stars in the sky, may be easily located on Orion's right shoulder.

The supergiant's 400-day pulsation rate has disappeared, which is even more amazing. Astronomers have been measuring this rhythm for about 200 years, observing changes in Betelgeuse's brightness variations and surface motions. Its disruption demonstrates the blowout's intensity.

Dupree hypothesizes that the convection cells inside the star, which are responsible for the regular pulsation, may be sloshing around like an unbalanced washing machine tub. The surface is still bouncing like a dish of gelatin dessert as the photosphere rebuilds itself, according to TRES and Hubble spectra, even if the outer layers appear to be returning to normal.

Astronomers have never seen such a significant portion of a star's visible surface get blown into space, despite the fact that our Sun occasionally experiences coronal mass ejections that blast off small chunks of its outer atmosphere. As a result, coronal mass ejections and surface mass ejections can occur at separate times.

Betelgeuse has grown to such a size that if it were to take the place of the Sun as the primary star of our solar system, its surface would stretch beyond Jupiter's orbit. In 1996, Dupree utilized Hubble to distinguish between hot patches on the star's surface. It was the first time a star other than the Sun had been directly imaged.

The ejected material may be visible to NASA's Webb Space Telescope as it continues to move away from the star in infrared light.

You can view Dupree's full study in the journal Arxiv.org.

"The bright supergiant, Betelgeuse (Alpha Orionis, HD 39801), underwent a historic optical dimming during 2020 January 27 − February 13. Many imaging and spectroscopic observations across the electromagnetic spectrum were obtained prior to, during, and subsequent to this dimming event. These observations of Betelgeuse reveal that a substantial surface mass ejection (SME) occurred and moved out through the extended atmosphere of the supergiant. A photospheric shock occurred in 2019 January - March, progressed through the extended atmosphere of the star during the following 11 months and led to dust production in the atmosphere. Resulting from the substantial mass outflow, the stellar photosphere was left with lower temperatures and the chromosphere with a lower density. The mass ejected could represent a significant fraction of the total annual mass loss rate from the star suggesting that episodic mass loss events can contribute an amount comparable to that of the stellar wind. Following the SME, Betelgeuse was left with a cooler average photosphere, an unusual short photometric oscillation, reduced velocity excursions, and the disappearance of the ∼400-day pulsation in the optical and radial velocity for more than two years following the Great Dimming.The bright supergiant, Betelgeuse (Alpha Orionis, HD 39801), underwent a historic optical dimming during 2020 January 27 − February 13. Many imaging and spectroscopic observations across the electromagnetic spectrum were obtained prior to, during, and subsequent to this dimming event. These observations of Betelgeuse reveal that a substantial surface mass ejection (SME) occurred and moved out through the extended atmosphere of the supergiant. A photospheric shock occurred in 2019 January - March, progressed through the extended atmosphere of the star during the following 11 months and led to dust production in the atmosphere. Resulting from the substantial mass outflow, the stellar photosphere was left with lower temperatures and the chromosphere with a lower density. The mass ejected could represent a significant fraction of the total annual mass loss rate from the star suggesting that episodic mass loss events can contribute an amount comparable to that of the stellar wind. Following the SME, Betelgeuse was left with a cooler average photosphere, an unusual short photometric oscillation, reduced velocity excursions, and the disappearance of the ∼400-day pulsation in the optical and radial velocity for more than two years following the Great Dimming."

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