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Astro Bob: Betelgeuse still recovering after 'blowing its top'

Astronomers reveal new details about the eruption that rocked the star in 2019.

Betelgeuse eruption
This illustration plots the progress of massive eruption in 2019 on the supergiant star Betelgeuse in Orion that blasted a significant portion of its visible surface into space. The blast, similar to a CME (coronal mass ejection) on the sun, released material that cooled to form a cloud of dust that temporarily dimmed the star.
Contributed / NASA, ESA, Elizabeth Wheatley (STScI)
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Before the pandemic hit, the bright, red star Betelgeuse made news (at least in the astronomy community) when it began to dim in early November 2019. Just about the time COVID-19 overtook the news cycle in February 2020, the star had dipped to a historic "minimum," fading more than a full magnitude in a few short months.

Orion figure
Orion represents a hunter with Betelgeuse at the shoulder of his upraised arm. The constellation is just now returning to view in the morning sky. You can spot the star and Orion's Belt low in the southeastern sky starting around 4 a.m. local time.
Contributed / Stellarium

Some of us wondered if this sudden change meant a supernova explosion was imminent. While that didn't pan out, something big must have happened to this stellar giant 764 times the size of the sun. But what exactly?

Analyzing data from NASA's Hubble Space Telescope and several other observatories, astronomers concluded that Betelgeuse literally blew its top in 2019, losing a substantial part of its visible surface and producing a gigantic surface mass ejection (SME), something never seen before in a normal star.

Our sun regularly blows off parts of its thin, outer atmosphere, the corona, in coronal mass ejections (CMEs). Sometimes that material whacks the Earth's magnetic field and initiates a geomagnetic storm and eye-popping northern lights. A typical CME releases about a billion tons of solar matter (electrons and protons) at speeds of around a million miles an hour. But the Betelgeuse SME blasted off 400 billion times that amount!

Betelgeuse view from Earth
This is an artist's view of the dust cloud partially blocking the view of Betelgeuse from Earth.
Contributed / NASA, ESA and E. Wheatley (STScI)

Weighing roughly several times as much as the moon, the expelled chunk of the star's luminous surface (called the photosphere), sped off into space and cooled to form a dust cloud that blocked the light of Betelgeuse. Anyone who cared to look, even if you lived in a city, saw the star perceptibly dim week after week until it was almost indistinguishable in brightness from its fainter neighbor, Bellatrix.

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"We've never before seen a huge mass ejection of the surface of a star," said Andrea Dupree of the Center for Astrophysics in Cambridge, Mass. "We're watching stellar evolution in real time."

Betelgeuse max and min
Betelgeuse normally shines around magnitude 0.5, shown in the photo at left. But during its unusually deep minimum in February 2020, it faded to about magnitude 1.7, similar to the brightness of neighboring Bellatrix, magnitude 1.6.
Contributed / H. Raab, CC BY-SA 4.0

We know from previous observations of Betelgeuse and stars of its ilk that supergiants lose material to space through powerful stellar winds driven by the physical pressure of radiation emitted from their fiercely hot cores.

During their brief lives, supergiants can eject many solar masses of material before their nuclear fusion "furnaces" shut down. Once that happens, the star collapses, which creates a rebounding shock wave that tears through the star and blows it to smithereens in a supernova explosion.

Astronomers think the 2019 outburst began with a gigantic plume of hot gas called a convective cell that bubbled up from the star's interior. Slamming into the stellar behemoth's upper layers, it set off shocks and convulsions that blasted part of Betelgeuse's photosphere into an expanding cloud of gas that cooled to form a sooty cloud.

Tamer forms of convection are found on Earth. For example, on a sunny day, the air in contact with the ground heats up and forms rising convective cells (or bubbles). As they cool with altitude, the water vapor the cells contain condenses to form puffy cumulus clouds.

Betelgeuse evolution
Shown here are stages of Betelgeuse's 2019 eruption which disrupted the star's 400-day-long cycle (dashed blue curve) of brightening and dimming that astronomers had measured for more than 200 years. Both its interior and exterior may now be jiggling like a plate of gelatin dessert.
Contributed / NASA, ESA, Elizabeth Wheatley (STScI)

Equally mind-bending, the expulsion was so catastrophic that Betelgeuse's 400-day pulsation period disappeared, at least temporarily. For almost 200 years astronomers have measured this slow rhythm as cyclic changes in Betelgeuse's brightness. The fact that it's now gone attests to the magnitude of the explosion.

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

The star's convective cells appear to have been knocked off-kilter by the eruption, causing them to slosh around like an "improperly balanced washing machine tub. Even the surface continues to jiggle like Jello on a plate. At the moment, Betelgeuse has returned to its former brilliance, but more upheavals are practically guaranteed, so it never hurts to give the star a look from time to time to see what's up.

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As the dust cloud disperses, it will seed space with heavy elements forged within the star including familiar carbon. That element and others will be incorporated into future generations of stars, planets and perhaps even new life forms, proving once again that nothing in the universe goes to waste. Nothing is inconsequential.

Related Topics: SCIENCE AND NATURE
"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune. You can reach him at nightsky55@gmail.com.
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