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VLT sees surface of dim Betelgeuse

We stargazers have spent our entire lives seeing the star Betelgeuse as a bright star in the very noticeable constellation Orion the Hunter. But, in late 2019, Betelgeuse began an unprecedented dimming. Now Betelgeuse is at about 36% of its normal brightness, a change noticeable even to the unaided eye. That is interesting and exciting, in part because it’s known that Betelgeuse is the sort of star that will explode someday. The notion that the dimming of Betelgeuse might be tied to an impending explosion has set the astronomy world buzzing. On February 14, 2020, the European Southern Observatory (ESO) released new before and after images of Betelgeuse – before the dimming and after the dimming began – showing not only that the star has decreased in brightness but also that its shape has changed.

Astronomer Miguel Montargès at Katholieke Universiteit Leuven in Belgium led these observations, using ESO’s Very Large Telescope (VLT) in Chile. ESO said in a statement that his team has been watching the star:

… since December, aiming to understand why it’s becoming fainter. Among the first observations to come out of their campaign is a stunning new image of Betelgeuse’s surface, taken late last year with the SPHERE instrument.

The team also happened to observe the star with SPHERE in January 2019, before it began to dim, giving us a before-and-after picture of Betelgeuse.

Taken in visible light, the images highlight the changes occurring to the star both in brightness and in apparent shape.

Round orange sphere next to squashed-looking darker orange spheroid.
This comparison image shows the star Betelgeuse before and after its unprecedented dimming. The observations – taken with the SPHERE instrument of the Very Large Telescope in January and December 2019 – show how much the star has faded and how its apparent shape has changed. Image via ESO/ M. Montargès et al.
Star chart of Orion.
Although it’s fainter now than it was at this time last year, Betelgeuse is easy to spot in the noticeable constellation Orion the Hunter. Notice Betelgeuse and the bright star Rigel on either side of the short, straight row of three medium-bright stars. That row of stars represents Orion’s Belt. Betelgeuse is said to represent the right shoulder of the Hunter.

Betelgeuse is a red supergiant star. It is part of well established astronomical theory that stars like Betelgeuse will one day go supernova. But, ESO said:

… astronomers don’t think this is happening now. They have other hypotheses to explain what exactly is causing the shift in shape and brightness seen in the SPHERE images.

Montargès explained:

The two scenarios we are working on are a cooling of the surface due to exceptional stellar activity or dust ejection towards us. Of course, our knowledge of red supergiants remains incomplete, and this is still a work in progress, so a surprise can still happen.

ESO said that Montargès and his team needed the VLT to study Betelgeuse, which is over 700 light-years away.

Another new image, obtained with the VISIR instrument on the VLT, shows the infrared light being emitted by the dust surrounding Betelgeuse in December 2019. These observations were made by a team led by Pierre Kervella from the Observatory of Paris in France, who explained that the wavelength of the image is similar to that detected by heat cameras. The clouds of dust, which resemble flames in the VISIR image, are formed when the star sheds its material back into space.

Yellow and orange blobs surrounding a black circle with a tiny bright dot in the middle.
This image, obtained with the VISIR instrument on ESO’s Very Large Telescope, shows the infrared light being emitted by the dust surrounding Betelgeuse in December 2019. The clouds of dust, which resemble flames in this dramatic image, are formed when the star sheds its material back into space. The black disk obscures the star’s center and much of its surroundings, which are very bright and must be masked to allow the fainter dust plumes to be seen. The orange dot in the middle is the SPHERE image of Betelgeuse’s surface, which has a size close to that of Jupiter’s orbit. Image via ESO/ P. Kervella/ M. Montargès et al.

Emily Cannon is a Ph.D. student at KU Leuven, working with SPHERE images of red supergiants. She commented:

The phrase ‘we are all made of stardust’ is one we hear a lot in popular astronomy, but where exactly does this dust come from? Over their lifetimes, red supergiants like Betelgeuse create and eject vast amounts of material even before they explode as supernovae. Modern technology has enabled us to study these objects, hundreds of light-years away, in unprecedented detail giving us the opportunity to unravel the mystery of what triggers their mass loss.

Star chart of Orion.
This chart shows the location of the very bright red supergiant star Betelgeuse (Alpha Orionis) in the famous constellation Orion the Hunter. This map shows most of the stars visible to the unaided eye under good conditions. Betelgeuse is marked with a red circle. Although the star itself is clearly visible to the unaided eye, the nebula around it cannot be seen visually with any telescope. Image via ESO/ IAU/ Sky & Telescope.

Bottom line: The European Southern Observatory’s Very Large Telescope obtained high resolution images of Betelgeuse in December 2019. The star looks remarkably different from images taken in January 2019. Astronomers are still trying to figure out what is happening to the star.

Via European Southern Observatory

February 16, 2020

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