By: Dennis Overbye
Betelgeuse, the red supergiant star that marks the armpit of Orion the Hunter, has been dramatically and mysteriously dimming for the last six months.
Some astronomers and excitable members of the public have wondered if the star is about to explode as a supernova. Others have suggested more prosaic explanations, involving long-term cycles of variability, sunspots or dust.
Now new light, so to speak, has been shed on the mystery.
Recent high-resolution photographs of the star suggest that it is changing shape, astronomers from the European Southern Observatory said in a news release Friday. Instead of appearing round, the star now appears squashed into an oval.
A team led by Miguel Montargès of KU Leuven in Belgium used a special camera on the Very Large Telescope of the European Southern Observatory in Chile. The camera — called Sphere, for Spectro-Polarimetric High-contrast Exoplanet Research instrument — was designed to take pictures of worlds that orbit distant suns.
The result was high-resolution images of the surface of a star 700 light-years from Earth. Betelgeuse, one of the great beacons of the sky, is quite clearly going through some changes.
In January 2019, before all this began, the Betelgeuse that Montargès viewed through the camera was “a bright round disk,” he said in an email. A year later, all the brightness of the star had been squashed into an oval occupying the northern half of the star.
Montargès declined to discuss any deeper details, pending a peer-reviewed publication of his scientific conclusions.
“Well, what I mean is that in the visible we do not see anymore a bright round disk,” he said. “It could be either a local cooling of the surface that causes the star to look asymmetric or a dust cloud hiding part of the star.”
As supergiant stars like Betelgeuse evolve into supernova funeral pyres, they typically go through unstable periods in which they shed layers of gas dust into nearby space, shrouding themselves.
The possibility that dust might be responsible for Betelgeuse’s dimming was underscored by other infrared, or heat, images from the Very Large Telescope. Those images showed huge, flame-like protuberances of dust arcing out from the limb of Betelgeuse.
Edward Guinan, an astrophysicist at Villanova University who has been following Betelgeuse, called the new images of a squashed star “fantastic.” But based on his own observations he took exception to the idea that Betelgeuse was hiding behind a veil of dust.
“We think the star itself is doing this — not dust,” he said by email.
Like our own sun, Betelgeuse transfers its thermonuclear energy by convection from the centre, where it is generated, to its surface. Picture boiling oatmeal, with giant gobs of hot gas rising, radiating away their heat and energy and then cooling, turning over and sinking again.
Guinan said that the dimming of Betelgeuse was likely caused by the sinking and cooling of one of these giant globs or convective cells. Another, less likely explanation is a massive outbreak of starspots, akin to the dark blemishes that appear in great numbers on our sun every 11 years.
But the show might already be over. Guinan reports that the dimming of Betelgeuse has slowed and may have even stopped over the last week.
“We may be at/near the bottom of this ‘fainting’ spell,” he wrote.
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