• "We've never before seen a huge mass ejection of the surface of a star," says study's lead author
  • A plume more than 1 million miles across may have erupted from inside the star
  • This produced a shock that blew out a sizable part of the star's surface
  • Betelgeuse's interior still "bouncing"

Betelgeuse is one of the brightest stars in the night sky. But the supergiant, located 530 light years from earth, visibly dimmed in 2019. Theories have abounded since then about what caused Betelgeuse to dim, and even blink.

Scientists now think they know why it happened. Using the Hubble Space Telescope, scientists have found that the dimming of the star was the result of an enormous ejection of material from the star's surface, unlike anything seen before. And Betelgeuse is still recuperating from the shock of that massive eruption.

The scientists found that a plume more than 1 million miles across may have erupted from inside the star, producing a shock that blew out a sizable part of the its surface. The ejection was 400 million times larger than those usually seen in our Sun's coronal mass ejections.

The research team, led by Andrea Dupree, associate director of the Harvard & Smithsonian Center for Astrophysics, reported its findings in a paper published to the preprint database arXiv and accepted by The Astrophysical Journal for publication.

"We've never before seen a huge mass ejection of the surface of a star," Dupree said. "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."

For background, in late 2019, Betelgeuse's light started to faint. And by February 2020, it had lost two-thirds of its brilliance as seen from Earth -- which led to excitement and curiosity among scientists.

Another group of scientists said in a study last year that Betelgeuse is a senile star nearing its end, preceding which it will explode into a supernova in the next 100,000 years. A brilliant red giant, it forms the upper right shoulder of the winter constellation Orion The Hunter.

A still separate group of scientists assessed that the star itself was not going supernova before its time but that a giant dust cloud had blocked some of its light for the viewers on Earth.

The latest study factored in information from other stellar observatories, such as the STELLA Robotic Observatory in Spain's Canary Islands, and NASA's Earth-orbiting STEREO-A spacecraft. By putting together the different puzzle pieces, the scientists were able to create a credible narrative of the events that took place.

Dupree's team found that the blowout took a chunk of the star's lower atmosphere, the photosphere, leaving in its wake a cool spot that was further obscured by the dust cloud from the blowout, according to Hubblesite. This cool spot and dust cloud provide a good explanation for why the Betelgeuse's light dimmed.

The star is now showing the after-effects from the blowout.

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

Also, it has temporarily renounced its 400-day alternating cycles of dimming and brightening.

When Betelgeuse finally ends its life in a stellar explosion, the spectacle will be visible in the daytime from Earth. Fortunately, the star is too far away to have any impact on our planet.

Image: A graphic illustration of the event that caused Betelgeuse to dim. In the last panel, Betelgeuse is obscured by a giant dust cloud. NASA, ESA, and E. Wheatley (STScI)