New research reveals why the star suddenly became significantly less bright at the beginning of this year and indicates that its brightness is decreasing again.

It is one of the brightest stars in the sky: the red giant Betelgeuse. But last October, the star’s brightness suddenly began to wane. And in February 2020, the star was no less than two-thirds less bright than normal, allowing Earthlings to even see with the naked eye that something was going on on the red supergiant. But what? Researchers were also not sure at first. But now the mystery has been solved. Observations from the Hubble Space Telescope indicate that the decrease in brightness can be traced back to a dramatic eruption on the star.

Dust
Betelgeuse ejected an enormous amount of warm material into the room, it turns out. “With Hubble, we see that material as it leaves the star’s visible surface and moves through the atmosphere,” says researcher Andrea Dupree. Once it reached the colder outer layers of that atmosphere, the material cooled and formed a cloud of dust. And that cloud was so massive that it blocked much of the light from Betelgeuse, making it appear as if the star suddenly became much less bright when viewed from Earth.

Exactly how the eruption originated is unclear. But it was probably a combination of circumstances. Betelgeuse naturally has a 420-day cycle in which it swells and contracts again and becomes brighter and less brighter, respectively. The swelling of the warm material that later grew into a cloud of dust at a greater distance from the star coincided with the swelling of the star. And the swelling of the star would have given the warm material that boost it needed to penetrate the atmosphere and cool enough to form a cloud of dust.

Less bright again
After the brightness of Betelgeuse declined sharply between October and February, the star then began to brighten again. And in April 2020, the brightness was back to normal. But recent observations indicate that the brightness of Betelgeuse is now declining again. And this again baffles researchers. “Betelgeuse normally has a cycle of 420 days and since the previous brightness minimum was in February 2020, this new decrease in brightness is more than a year too soon,” said Dupree. Reason enough for astronomers to keep a close eye on the red supergiant for some time to come.

Betelgeuse is a so-called red supergiant. A very bright star that is rapidly consuming its fuel. Without that fuel – or source of energy – the star can no longer compete with gravity and part of its own mass flows towards the core. That core gets heavier and heavier and is ultimately doomed to collapse. The result is a gigantic explosion, a supernova, which in the case of Betelgeuse is expected to be seen from Earth even in broad daylight. Based on what we know about Betelgeuse, researchers expect that explosion to occur within about 100,000 years. That means it could take tens of thousands of years, but it could also happen soon. When the brightness of Betelgeuse declined considerably at the end of last year, some thought it was a precursor to a supernova explosion. Scientists were not convinced, and now that the decrease in clarity has been explained in an alternative way, this exciting hypothesis seems to be off the table. Although .. “Nobody knows how a star behaves in the weeks before it explodes,” emphasizes Dupree. “And some did indeed predict that Betelgeuse was going to be a supernova. However, there is a good chance that it will not explode during our lifetimes. But who knows?” “And some did indeed predict that Betelgeuse was going to be a supernova. However, chances are that it will not explode during our lifetimes. But who knows?” “And some did indeed predict that Betelgeuse was going to be a supernova. However, chances are that it will not explode during our lifetimes. But who knows?”

It is certain that what happened on Betelgeuse is quite special for the star. “We know that other super-bright stars lose material and that material quickly converts to dust, which makes the stars appear less bright,” said Dupree. “But we have not seen this happen on Betelgeuse in the past century and a half. It’s very unique. ”

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