Betelgeuse first came to attention in late 2019 when the star, which shines like a red gem in Orion’s upper right shoulder, experienced an unexpected dimming. The supergiant continued to decline in 2020.
Some scientists hypothesized that the star would explode as a supernova and have been trying to determine what happened to it ever since.
Astronomers have now analyzed data from the Hubble Space Telescope and other observatories and believe the star has undergone a titanic surface mass ejection, losing a significant portion of its visible surface.
“Never before have we seen a massive ejection of mass from the surface of a star. We’re left with something going on that we don’t fully understand,” said Andrea Dupree, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Massachusetts, in a statement.
“This is a completely new phenomenon that we can observe directly and resolve the surface details with Hubble. We observe the evolution of stars in real time.”
Our sun regularly undergoes coronal mass ejections, where the star sheds parts of its outer atmosphere, known as the corona. If this space weather were to hit Earth, it could affect satellite communications and power grids.
But the surface mass ejection that Betelgeuse experienced released more than 400 billion times more mass than a typical coronal mass ejection from the sun.
The life of a star
Observing Betelgeuse and its unusual behavior allowed astronomers to observe what happens at the end of a star’s life.
As Betelgeuse burns fuel in its core, it has swelled to enormous proportions, becoming a red supergiant. The massive star is 1 billion miles (1.6 billion kilometers) in diameter.
Eventually, the star will explode in a supernova, an event that can be seen briefly during the day on Earth. In the meantime, the star has been experiencing several hot-tempered outbursts.
The amount of mass that stars lose at the end of their lives as they burn through nuclear fusion can affect their survival, but even losing a significant amount of surface mass is not a sign that Betelgeuse is ready to blow up, according to astronomers.
Astronomers like Dupree have studied how the star behaved before, during and after the eruption in an attempt to understand what happened.
Scientists believe that a convective jet stretching more than 1 million miles (1.6 million kilometers) across originates from inside the star. The plume created shocks and ripples that triggered the eruption, peeling off part of the star’s outer shell, called the photosphere.
The piece of Betelgeuse’s photosphere, which weighed several times more than the moon, was released into space. As the mass cooled, it formed a large cloud of dust that blocked the star’s light when viewed through telescopes on Earth.
Betelgeuse is one of the brightest stars in Earth’s night sky, so its eclipse — which lasted several months — was visible through observatories and backyard telescopes alike.
Recovering from the blast
Astronomers have been measuring Betelgeuse’s rhythm for 200 years. The pulse of this star is essentially a dimming and brightening cycle that restarts every 400 days. That pulse has stopped for now – a testament to how serious the eruption was.
Dupree thinks the star’s inner convection cells, which drive the pulsation, are still reverberating from the blast, and compares it to the sloshing of an unbalanced washing machine tub.
Data from the telescope show that the star’s outer layer has returned to normal as Betelgeuse slowly recovers, but its surface remains elastic as the photosphere recovers.
“Betelgeuse continues to do some very unusual things right now,” Dupre said. “The interior is kind of bouncy.”
Astronomers have never seen a star lose so much of its visible surface before, suggesting that surface mass ejection and coronal mass ejection may be two very different things.
Researchers will have more subsequent chances to observe the mass ejected from the star using the James Webb Space Telescope, which can reveal additional clues through otherwise invisible infrared light.
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