What caused the dimming of Betelgeuse?

Betelgeuse's dimming sparked much debate in the astronomy community. Emily Cannon is part of a team trying to find out what caused it.

Astronomer Emily Cannon

Betelgeuse, Betelgeuse, Betelgeuse… Arguably the most famous of the red supergiants, Betelgeuse continues to give us a unique insight into the workings of the largest stars in the Universe.

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On 7 December 2019 an Astronomer’s Telegram (which these days is announced via email and Twitter) claimed that Betelgeuse was dimming, followed by news in February 2020 that it was brightening again. As Betelgeuse is a variable star it is not unusual to see variations in its brightness.

However, on this occasion the brightness of Betelgeuse was continuing to fall far below that of what was expected and the dimming became so apparent that it was even noticeable by eye in the night sky.

Within two weeks of the telegram, our team, led by Dr Miguel Montargès, submitted a proposal and was granted time to observe Betelgeuse with the SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) instrument, among others, at the European Southern Observatory’s (ESO’s) Very Large Telescope (VLT) in Chile’s Atacama Desert.

The high angular resolution capabilities of SPHERE allow us to resolve the surface of Betelgeuse, which means that the star no longer looks like a point source and we can distinguish surface features.

Two images of Betelgeuse captured by Emily’s team using the Very Large Telescope. Left shows the view in January 2019 and right shows the view in December 2019, revealing the star’s change in brightness over time. Credit: Credit: ESO/M. Montargès et al.
Two images of Betelgeuse captured by Emily’s team using the Very Large Telescope. Left shows the view in January 2019 and right shows the view in December 2019, revealing the star’s change in brightness over time. Credit: Credit: ESO/M. Montargès et al.

It was clear from the images we obtained on 27 December 2019 that there was a drop in brightness in the southern part of the star when compared to observations from January 2019.

But what could have caused this dramatic change? Much of the physics of red supergiants is still uncertain, so multiple causes are being explored.

Our team is currently investigating two main theories. The fact that part of the surface is at a cooler temperature could be due to the convective activity of the star, or because a clump of dust has been ejected in our direction.

This dust theory would not be too unexpected as Betelgeuse has already been observed to have a clumpy outer wind, just like many other red supergiants.

In order to test the viability of the theory that ejected dust is the cause, we are running many thousands of computer simulations that we can then compare to the observations.

Betelgeuse and Rigel. Credit: iStock
Stars Betelgeuse and Rigel as they appear in the constellation of Orion. Credit: iStock

Does this dimming mean Betelgeuse is about to go supernova? It is unlikely.

The final nuclear reaction sequences that happen within the star have such short timescales that their light would not reach the surface before the explosion occurred, so we don’t expect to see changes in brightness pre-supernova.

As we watch Betelgeuse regain brightness it is important to note that without the help of amateur astronomers in observing and documenting this star each day, much of this science would not be possible and events like this could be missed.

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Emily Cannon is an astrophysics PhD student from the Institute of Astronomy at KU Leuven in Belgium. This article originally appeared in the August 2020 issue of BBC Sky at Night Magazine.