Photo of a dying giant star
This is the first image of a star beyond the Milky Way. The red supergiant WOH G64 is the most extreme of its kind: a cocoon of gas and dust indicates an impending supernova explosion
Thanks to a newly developed instrument of the Very Large Telescope Interferometer of the European Southern Observatory in Chile, a research team led by Keiichi Ohnaka has succeeded in taking the first-ever photograph of a red giant star beyond the Milky Way. Three researchers from the Max Planck Institute for Radio Astronomy were also involved in the study. The colossus, designated WOH G64, is a red supergiant orbiting the Milky Way in a satellite galaxy known as the Large Magellanic Cloud. It has already expanded to 2,000 times the size of the Sun and is on the verge of a supernova explosion. This makes it one of the largest stars ever to have been captured on camera. The dust that the outer stellar envelope repels in this phase of its life is also included in the image.
“For the first time, we have succeeded in taking a zoomed-in image of a dying star in a galaxy outside our own Milky Way,” says Keiichi Ohnaka, an astrophysicist from Universidad Andrés Bello in Chile. Unlike our home star, even the nearest stars usually appear only as bright dots in images from telescopes. Only in about two dozen cases have astronomers spatially resolved the stellar disc. Most of these are red giant stars, hundreds of times the size of the Sun. Such resolved images are mainly obtained using so-called interferometers, or arrays of telescopes. In the case of the star WOH G64, the four individual telescopes of the Very Large Telescope in the Atacama Desert in Chile form a virtual large telescope over a hundred metres in diameter: The Very Large Telescope Interferometer.
A monster in the Large Magellanic Cloud
Astronomers have known about this star for decades and have appropriately dubbed it the ‘behemoth star’. Ohnaka's team has already used the Very Large Telescope Interferometer in 2005 and 2007 to learn more about the properties of the star. However, it has not been possible to obtain an image of the star so far.
For the desired image, the team had to wait for the further development of the instrument called GRAVITY, which combines the light beams from the four telescopes of the interferometer. “We discovered an egg-shaped cocoon closely surrounding the star,” says Ohnaka. “We are excited because this may be related to the drastic ejection of material from the dying star before a supernova explosion.” The so-called superwind phase in which WOH G64 is currently in could indicate that the star will explode within the next thousand years.
Shortly before a supernova explosion
After comparing their new results with other previous observations of WOH G64, they were surprised to find that the star had become dimmer over the past decade. “We have found that the star has been experiencing a significant change in the last 10 years, providing us with a rare opportunity to witness a star’s life in real time,” says Gerd Weigelt, an astronomy professor at the Max Planck Institute for Radio Astronomy in Bonn, Germany and a co-author of the study. In their final life stages, red supergiants like WOH G64 shed their outer layers of gas and dust in a process that can last thousands of years. "This star is one of the most extreme of its kind, and any drastic change may bring it closer to an explosive end," adds co-author Jacco van Loon, Keele Observatory Director at Keele University, UK, who has been observing WOH G64 since the 1990s.
The team thinks that these shed materials may also be responsible for the dimming and for the unexpected shape of the dust cocoon around the star. The new image shows that the cocoon is stretched-out, which surprised scientists, who expected a different shape based on previous observations and computer models. The team believes that the cocoon’s egg-like shape could be explained by either the star’s shedding or by the influence of a yet-undiscovered companion star.
As the star becomes fainter, taking other close-up pictures of it is becoming increasingly difficult, even for the Very Large Telescope Interferometer. Nonetheless, planned updates to the telescope’s instrumentation, such as the future GRAVITY+, promise to change this soon. “Similar follow-up observations with ESO instruments will be important for understanding what is going on in the star,” concludes Ohnaka.
BEU, based on the original ESO press release
