The First Close-Up Picture of Star Outside the Milky Way

Like a performer preparing for their big finale, a distant star is shedding its outer layers and preparing to explode as a supernova.

Astronomers have been observing the huge star, named WOH G64, since its discovery in the 1970s. It’s one of the largest known stars, and also one of the most luminous and massive red supergiants (RSGs). The star is surrounded by an envelope of expelled star-stuff, which could indicate it’s getting ready to explode.

WOH G64 isn’t in the Milky Way; it’s in the Large Magellanic Cloud (LMC), the Milky Way’s largest satellite galaxy. Getting these detailed image is quite a feat for the ESO’s Very Large Telescope Interferometer. It’s also quite an accomplishment for the team of scientists behind the image.

They’ve published their images and the results of their observations of the star in the journal Astronomy and Astrophysics. Their research is titled “Imaging the innermost circumstellar environment of the red supergiant WOH G64 in the Large Magellanic Cloud.” The lead author is Keiichi Ohnaka, an astrophysicist from Universidad Andrés Bello in Chile.

“This star is one of the most extreme of its kind, and any drastic change may bring it closer to an explosive end.”

Jacco van Loon, study co-author, Keele Observatory

“Significant mass loss in the red supergiant (RSG) phase is of great importance for the evolution of massive stars before they end their life in a supernova (SN) explosion,” the researchers write in their paper. Understanding the progenitors to supernovae (SNe) is important because of the role they play in the Universe. These massive stars forge heavy elements through nucleosynthesis then spread them out into their surroundings when they explode. These heavy elements make rocky planets possible. SNe shockwaves can also compress gas in their vicinities, which can trigger the birth of new stars. Better images of stars approaching their explosive ends help astronomers understand them better.

“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,” lead author Ohnaka said.

WOH G64 (WOH hereafter) is a whopping 160,000 light-years away. Even though the red supergiant is a behemoth that’s 2,000 times larger than the Sun, that’s an enormous distance. It’s all because of the VLTI and one of its newer instruments, called GRAVITY. It’s a powerful instrument that was installed on the VLTI in 2015.

When Ohnaka and his colleagues saw the images, they were buoyed with excitement. The images show a cocoon of dust surrounding the star, evidence that it’s convulsed and shed some of its outer layers.

“We discovered an egg-shaped cocoon closely surrounding the star,” said lead author Ohnaka. “We are excited because this may be related to the drastic ejection of material from the dying star before a supernova explosion.”

Ohnaka and his colleagues have been observing WOH for a long time, but had to wait for better instruments to get a closer look.

Among other things, they noticed that the star has become dimmer over the last decade.

Gerd Weigelt is an astronomy professor at the Max Planck Institute for Radio Astronomy and a co-author of the research. “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,” Weigelt said. 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.

Jacco van Loon, the director of the Keele Observatory at Keele University in the UK has been observing WOH since the 1990s. “This star is one of the most extreme of its kind, and any drastic change may bring it closer to an explosive end,” Keele said.

With the more limited data available in the past, Ohnaka modelled what the dust environment might look like. Those models and observations predicted a different shape than the GRAVITY images reveal.

The images show an elongated, compact emission region in near-infrared (NIR) surrounding the star. This suggests that hot new dust has formed near the star, which helps obscure the star itself. The star’s NIR continuum has shifted in the last decade, which also supports the new dust hypothesis. Earlier images from before 2003 show more hydrogen absorption than recent images.

Other observations of RSG stars also show that their circumstellar environments aren’t spherical. For example, dust surrounding the remnant of SN1987A is also not spherical. Astrophysicists think that this dust was shed by SN1987A’s progenitor star before it evolved into a blue supergiant and exploded.

The elongated, cocoon shape of the emissions has two potential explanations. “The elongated emission may be due to a bipolar outflow along the axis of the dust torus,” the authors explain. “Alternatively, the elongation may be caused by the interaction with an unseen companion.”

The non-spherical structures are common, and researchers want to understand this phenomenon better. “Given the high multiplicity rate among massive stars, the asymmetric, enhanced mass loss in the RSG phase, which can be driven by binary interaction, is essential not only for better understanding the evolution of massive stars but also for interpreting early-phase SN spectra,” the authors explain.

Unfortunately, observing WOH is becoming more difficult. The dust is obscuring the star. “The formation of new hot dust also means that the central star is now more obscured than the epochs before 2009,” the authors explain, and if the star keeps shedding material, the star will become dimmer.

But new instruments might help. GRAVITY’s successor, GRAVITY+ is being rolled out incrementally and will be completed in 2026.

“Similar follow-up observations with ESO instruments will be important for understanding what is going on in the star,” concluded Ohnaka.

WOH G64 is getting ready to explode, but that doesn’t mean it’s imminent in terms of human lifespans. Nobody alive today will witness the explosion. However, in stellar terms, the star’s death could be imminent.

Maybe our distant descendants, if we have any, will witness it.