Left: An image of the star WOH G64 taken with the Very Large Telescope Interferometer in Chile. Right: An artist’s impression of the star
ESO/K. Ohnaka et al., L. Calçada
Astronomers have taken the first detailed picture of a star in another galaxy, more than 160,000 light years away. The giant star may be showing signs that it is just years away from exploding, a process we have never seen in detail.
The largest stars we know of are red supergiants, which are stars that have run out of hydrogen fuel in their cores. A shell of hydrogen gas surrounding the core burns instead, massively expanding the volume of the star.
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One of the largest red supergiants we know of is WOH G64, sometimes called the behemoth star. It is between 1540 and 2575 times the size of the sun and resides in a satellite galaxy of the Milky Way, the Large Magellanic Cloud. The star has been a target for astronomers since it was discovered in the 1970s, but its distance has made it hard to examine closely.
Now, Jacco van Loon at Keele University, UK, and his colleagues have taken a close-up picture of WOH G64 using the Very Large Telescope Interferometer in the Atacama desert in Chile, a collection of four individual telescopes linked together to function as if they were a single 200-metre telescope. “In this image, we can see detail which would be equivalent to seeing an astronaut walking on the moon,” says van Loon. “You can’t see that through a normal telescope pointing at the moon.”
The image, which was taken using infrared light, shows a bright ball of gas and dust, more than 1000°C (1832°F), that the star has pumped out and that now surrounds it as a dense cocoon. “It’s really a structure we had not expected to actually see,” says van Loon. “We had expected just to see the star in the middle.”
The star appears dimmer than when it was last observed, so the gas and dust probably appeared relatively recently, says van Loon. It might have been produced by the star blowing off its outer layers, which astronomers have never captured in a red supergiant.
If that is what happened and the process resembles one seen in similar stars called blue supergiants, then it might be a sign that the star is decades or years away from exploding. “If we can see this star explode, we have much more detail about a star before it’s exploded than ever before,” says van Loon.
“It’s technically extremely impressive to be able to reconstruct an image of this object given its extreme distance,” says Paul Crowther at the University of Sheffield, UK.
However, it is harder to say for certain whether the observed gas and dust, and the associated dimming in brightness, are a sign of an imminent explosion. “Stars like this object are well known to be highly variable,” says Crowther. “It’s simply what happens in these objects where they have this dense, slow outflow that doesn’t go very far from the star. They’re well known to be dust factories.”
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