“For the first time, we managed to take a zoomed-in image of a dying star in a galaxy outside our Milky Way,” says Keiichi Ohnaka, an astrophysicist at the Universidad Andrés Bello in Chile.
Located 160,000 light years away, the star WOH G64 was imaged using the impressive clarity of the European Southern Observatory's (ESO) “Very Large Telescope Interferometer” (VLTI). New observations reveal a star blowing gas and dust, in the final stages before becoming a supernova.
“We discovered an ovoid cocoon tightly surrounding the star,” says Keiichi Ohnaka, lead author of a study reporting the observations published today in Astronomy & Astrophysics. “We are excited because this could be linked to the drastic ejection of material from the dying star before a supernova explosion.” While astronomers have taken about two dozen zoomed-in images of stars in our galaxy, revealing their properties, countless other stars live in other galaxies, so distant that detailed observation of just one they proved extremely difficult. So far !
The newly imaged star, WOH G64, is located in the Large Magellanic Cloud, one of the small galaxies that orbit the Milky Way. Astronomers have known about this star for decades and have rightly nicknamed it “the giant star.” About 2,000 times the size of our sun, WOH G64 is classified as a red supergiant.
Keiichi Ohnaka's team has long been interested in this giant star. In 2005 and 2007, they used ESO's VLTI in the Chilean Atacama Desert to learn more about the star's characteristics, and continued to study it over the following years. But a true image of this star remained difficult to obtain.
To obtain the desired image, the team had to wait for the development of one of the VLTI's second-generation instruments, GRAVITY. After comparing their new results with other previous observations of WOH G64, they were surprised to find that the star had dimmed over the past decade.
“We found that the star has undergone a significant change over the past ten years, giving us a rare opportunity to witness the life of a star in real time,” explains astronomy professor Gerd Weigelt at the Institute Max Planck of radio astronomy from Bonn, Germany, and co-author of the study.
During their final phase of life, red supergiants like WOH G64 shed their outer layers of gas and dust, a process that can last thousands of years. “This star is one of the most extreme of its kind, and any drastic change could bring it closer to an explosive end,” adds co-author Jacco van Loon, director of the Keele Observatory at Keele University, at United Kingdom, which has observed WOH G64 since the 1990s.
The team believes this ejected material could also be responsible for the weakening of the star as well as the unexpected shape of the dust cocoon surrounding it. The new image reveals that the cocoon is stretched, which surprised scientists, who expected a different shape based on previous observations and computer models. The team believes that the ovoid shape of the cocoon could be explained either by the ejection of material by the star, or by the influence of an as yet undiscovered companion star.
As the star becomes dimmer, it becomes increasingly difficult to take further close-up photos, even with VLTI. However, planned updates to the telescope's instrumentation, such as the upcoming GRAVITY+, promise to change that soon. “Similar follow-up observations with ESO instruments will be important for understanding what is happening in the star,” concludes Keiichi Ohnaka.
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