Three years ago, amateur astronomers around the world watched as the usually bright light of Betelgeuse dimmed overnight and remained that way for several months.

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Now a group of astronomers have determined that Betelgeuse's dimming was caused by a piece of the star’s atmosphere being ejected into space.

"We’ve never before seen a huge mass ejection of the surface of a star," says Andrea Dupree from the Center for Astrophysics, Harvard and Smithsonian, who led the study.

Betelgeuse is a red giant star over 1.6 billion kilometres wide – meaning if it replaced our Sun it would swamp Jupiter and almost reach Saturn.

It has ballooned in size as it is approaching the end of its life and will eventually go supernova.

Though this is unlikely to happen for the next 10,000 years, it gives astronomers a unique view of a star in its final centuries.

After blowing its top, the red supergiant star’s brightness dimmed dramatically behind the resulting dust cloud. Credit: NASA/ESA/Elizabeth Wheatley (STScI)
After blowing its top, the red supergiant star’s brightness dimmed dramatically behind the resulting dust cloud. Credit: NASA/ESA/Elizabeth Wheatley (STScI)

Over 200 years of observations have shown that the star’s brightness slowly pulses according to a 400-day cycle, but the scale and speed of the 2019 dimming was unprecedented.

Drawing together observations from all over the world, Dupree has determined that material bubbling up within the star blasted off a piece of the photosphere.

The ejection had a mass several times that of our Moon – a colossal 400 billion times more than what our Sun typically gives off during coronal mass ejections.

As this fractured piece cooled, it formed a dust cloud that blocked Betelgeuse’s light from Earth, causing it to appear dimmer.

It also appears that Betelgeuse’s 400-day brightness cycle has stopped, or at least paused.

These images, taken with the SPHERE instrument on ESO’s Very Large Telescope, show the surface of the red supergiant star Betelgeuse during its unprecedented dimming, which happened in late 2019 and early 2020. The image on the far left, taken in January 2019, shows the star at its normal brightness, while the remaining images, from December 2019, January 2020, and March 2020, were all taken when the star’s brightness had noticeably dropped, especially in its southern region. The brightness returned to normal in April 2020.
These images, taken with the SPHERE instrument on ESO’s Very Large Telescope, show the surface of the red supergiant star Betelgeuse during its unprecedented dimming, which happened in late 2019 and early 2020.

Spectral observations from the Hubble Space Telescope taken by Dupree showed signs that this event and the star’s attempts to rebuild its photosphere have disrupted the internal motions that drive the cycle.

"Betelgeuse continues to do some very unusual things right now; the interior is sort of bouncing," says Dupree.

"We’re left with something we don’t completely understand. It’s a totally new phenomenon that we can observe directly and can resolve surface details with Hubble. We’re watching stellar evolution in real time."

The team will continue to observe the red giant and hope to use the James Webb Space Telescope to watch as the cooling material moves away from the star.

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Authors

Elizabeth Pearson
Ezzy PearsonScience journalist

Ezzy Pearson is the Features Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.