Astronomers have captured the first close-up image of a star beyond our galaxy, taken during a dramatic moment near the end of the star’s life. In it, the “behemoth” star appears to puff out gas and dust, a critical final step before exploding in a supernova.
The star, known as WOH G64, is 160,000 light-years from Earth in the Large Magellanic Cloud, a small neighbouring galaxy that orbits the Milky Way. Researchers took the picture using the European Southern Observatory’s Very Large Telescope Interferometer, located in Chile’s Atacama Desert.
The unprecedented image, combined with observations of WOH G64 over the past decade, has provided astronomers with the opportunity to witness key moments during the violent life cycle of a star. A study describing a comparison of the observations published Thursday in the journal Astronomy & Astrophysics.
“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,” said lead study author Keiichi Ohnaka, an astrophysicist from Andrés Bello National University in Chile, in a statement. “We discovered an egg-shaped cocoon closely surrounding the star. We are excited because this may be related to the drastic ejection of material from the dying star before a supernova explosion.”
A fading red giant
WOH G64 is a red supergiant star 2,000 times larger than our sun, which has led astronomers to call it the “behemoth star.” During the final stages of their lives, red supergiants shed their outer gas and dust layers, a process that can last for thousands of years.
Scientists first discovered the star in 1981, and observations by the InfraRed Astronomical Satellite later in the decade revealed WOH G64 was incredibly luminous and one of the most extreme examples of a red supergiant, said study coauthor Jacco van Loon, Keele Observatory director at the UK’s Keele University.
Van Loon has observed WOH G64 since the 1990s and studied it as a student at the European Southern Observatory. He was able to measure the rate at which the star was releasing material into space and realized it wouldn’t be able to keep up the ejection rate for long before it had nothing more to shed.
Over time, van Loon has watched the star’s brightness vary, suggesting it was changing from a massive red supergiant to a smaller and hotter star.
Ohnaka’s team used the Very Large Telescope Interferometer to study the star in 2005 and 2007, but the researchers waited years for one of the telescope’s second-generation instruments, GRAVITY, to be developed before an image of the star was possible.
The scientists compared the latest observations made via GRAVITY with previous ones, revealing the star has dimmed significantly 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,” said study coauthor Gerd Weigelt, an astronomy professor at the Max Planck Institute for Radio Astronomy in Bonn, Germany, in a statement.
The astronomy team believes the gas and dust released by the star is responsible for how much it has dimmed in recent years as well as the unusual, stretched-out shape of the dust cocoon that has formed around WOH G64. It’s also possible that an unseen companion star has influenced the shape of the cocoon, the study authors said.
WOH G64 is one of the largest and most luminous stars in the Large Magellanic Cloud, and if it replaced our sun, it would extend well past the orbit of Jupiter, said Edward Guinan, professor of astronomy and astrophysics at Villanova University in Pennsylvania. Guinan was not involved in the new study. If the star does explode in the future, it would be visible to the naked eye.
“They captured an amazing image using the VLTI/Gravity instrument of the star and its complex huge, ejected gas/dust torus,” Guinan said in an email, referring to the doughnut shape of the cloud around the star.
Destined to explode
It has taken the light from the moment of the star’s life captured in the new image 160,000 years to reach Earth. WOH G64 was likely 25 to 40 times the mass of the sun when it was born, and such massive stars die when their cores collapse, leading to the creation of a black hole, van Loon said. While the star is likely to go supernova, it’s impossible to say when the explosion could occur.
“We have never been able to tell precisely when a red supergiant is going to explode, as we can’t look inside it,” van Loon said. “What we do know, is that one such supernova seen in the same galaxy in 1987 to every astronomer’s surprise was a blue supergiant that had exploded. The Hubble Space Telescope soon revealed it had indeed been a red supergiant in the past, perhaps 20,000 years before the explosion. We could be seeing WOH G64 go through such (a) metamorphosis, in which case it might still take thousands of years before we would see it explode.”
Rare blue supergiant stars are some of the hottest, brightest stars in the universe. But other distant supernovas have shown that before they exploded, stars ejected dense clouds decades beforehand.
“If this is what we are seeing (WOH G64) doing, then a spectacle awaits us soon,” van Loon said.
In recent years, astronomers studying the red supergiant Betelgeuse, about 645 light-years away from Earth in the Milky Way, have seen that star unexpectedly dim, which led some to believe it was on the brink of exploding. Some astronomers believe it was just a dust cloud ejected by the star that caused the dimming. It’s nowhere near as bright or variable as WOH G64, van Loon said, and only experienced a brief hiccup compared with what WOH G64 is undergoing.
“While Betelgeuse is unphased by its display, WOH G64 might have blown off its mantle, and have changed forever,” he said in an email.
Ohnaka’s team plans to capture more images of the star in the future, which will be challenging as it grows fainter. But planned updates to the telescope’s instruments will help, Ohnaka said.
Over the past decade, van Loon ventured into other fields of astrophysics. But WOH G64 pulled him “back to studying these monster stars again,” van Loon said.
“The detail we can now see is astonishing, comparable to seeing a person on the Moon,” he said. “But what’s more, we caught it when it must have just done something dramatic. The elongated blob we see in our image is a dense cloud of dust. This makes it hard to see the star itself, but all indications are that it is indeed a much hotter star now. I feel very fortunate to witness this, as we could have been observing it for thousands of years without seeing much change.”
