Betelgeuse first drew attention in late 2019 when the star, which glitters like a red gem in the upper-right shoulder of Orion, experienced an unexpected darkening. The supergiant continued to grow dim in 2020.
Some scientists speculated that the star would explode as a supernova, and they’ve been trying to determine what happened to it ever since.
Now, astronomers have analyzed data from the Hubble Space Telescope and other observatories, and they believe the star experienced a titanic surface mass ejection, losing a substantial part of its visible surface.
“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,” said Andrea Dupree, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Massachusetts, in a statement.
“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.”
Our sun regularly experiences coronal mass ejections in which the star releases parts of its outer atmosphere, known as the corona. If this space weather hits Earth, it can have an impact on satellite-based communications and power grids.
But the surface mass ejection Betelgeuse experienced released more than 400 billion times as much mass as a typical coronal mass ejection from the sun.
The lifetime of a star
Observing Betelgeuse and its unusual behavior has allowed astronomers to watch what happens late in the lifetime of a star.
As Betelgeuse burns through fuel in its core, it has swollen to massive proportions, becoming a red supergiant. The massive star is 1 billion miles (1.6 billion kilometers) in diameter.
Ultimately, the star will explode in a supernova, an event that could be briefly visible during the daytime on Earth. Meanwhile, the star is experiencing some fiery temper tantrums.
The amount of mass that stars lose late in their lives as they burn through nuclear fusion can affect their survival, but even losing a significant amount of its surface mass isn’t a sign that Betelgeuse is ready to blow, according to astronomers.
Astronomers such as Dupree have studied how the star behaved before, during and after the eruption in an effort to understand what happened.
Scientists believe that a convective plume, stretching more than 1 million miles (1.6 million kilometers) across, originated from inside the star. The plume created shocks and pulsations that triggered an eruption, peeling off a chunk of the star’s outer shell called the photosphere.
The piece of Betelgeuse’s photosphere, which weighed several times as much as the moon, was released into space. As the mass cooled, it formed a large dust cloud 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 dimming — which lasted for a few months — was noticeable through observatories and backyard telescopes alike.
Recovering from the blast
Astronomers have measured Betelgeuse’s rhythm for 200 years. This star’s pulse is essentially a dimming and brightening cycle that restarts every 400 days. That pulse has ceased for now — a testament to how consequential the eruption was.
Dupree believes that the star’s interior convection cells that drive the pulsation are still reverberating from the blast and compared it to the sloshing of an unbalanced washing machine tub.
Telescope data has shown that the star’s outer layer has returned to normal as Betelgeuse slowly recovers, but its surface remains springy while the photosphere rebuilds.
“Betelgeuse continues doing some very unusual things right now,” Dupree said. “The interior is sort of bouncing.”
Astronomers have never seen a star lose so much of its visible surface before, suggesting that surface mass ejections and coronal mass ejections could be two very different things.
Researchers will have more follow-up chances to observe the mass ejected from the star by using the James Webb Space Telescope, which could reveal additional clues through otherwise-invisible infrared light.