Astronomers observe the final acts of a dying giant star

Massive stars, as they near the end of their lives, undergo dramatic transformations. One of the most striking phases occurs when these stars become red supergiants (RSGs), shedding vast amounts of gas and dust in their final evolutionary stages.

This process is not only crucial for the star's ultimate fate—its explosion as a supernova—but also for understanding the broader mechanisms shaping our universe.

Red supergiants exhibit mass-loss rates that can reach an astonishing 10⁻⁴ solar masses per year. Recent advances in astronomical techniques have illuminated this process in unprecedented detail, revealing that the material ejected by these stars often deviates from a spherical shape.

Observations of the circumstellar dust rings, such as the one around SN1987A, suggest that the asymmetry of the mass-loss process may result from binary star interactions. Understanding these phenomena helps unravel the evolution of massive stars and informs the interpretation of early-phase supernova spectra.

Among red supergiants, WOH G64 stands out. Located 160,000 light-years away in the Large Magellanic Cloud (LMC), this star dwarfs our Sun, being about 2,000 times its size. With a high mass-loss rate and significant infrared brightness, WOH G64 has been the subject of extensive study.

Its circumstellar environment, previously modeled as a thick, geometrically toroidal dust cloud, has long fascinated astronomers. Yet until recently, direct imaging of this colossal star remained elusive.

Using the European Southern Observatory’s (ESO) Very Large Telescope Interferometer (VLTI), astronomers have, for the first time, captured a high-resolution image of WOH G64. Keiichi Ohnaka, an astrophysicist at Universidad Andrés Bello in Chile, described this milestone: “We have succeeded in taking a zoomed-in image of a dying star in a galaxy outside our own Milky Way.”

The observations, enabled by the VLTI's advanced capabilities, revealed an egg-shaped cocoon of gas and dust encasing the star. “We are excited because this may be related to the drastic ejection of material from the dying star before a supernova explosion,” Ohnaka added.

These findings offer an extraordinary glimpse into the star's final life stages, when red supergiants like WOH G64 expel their outer layers in a slow yet dramatic process.

Astronomers have monitored WOH G64 for decades, yet the new image marked a turning point. Previous efforts with ESO’s VLTI in 2005 and 2007 relied on earlier-generation instruments, which lacked the resolution to resolve the star’s intricate features. The team had to await the development of GRAVITY, a second-generation VLTI instrument that integrates light from four telescopes, achieving unprecedented sensitivity and clarity.

One unexpected finding was the dimming of WOH G64 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,” noted Gerd Weigelt, a professor at the Max Planck Institute for Radio Astronomy.

The dimming could stem from material shed by the star. The resulting dust cocoon, shaped more like an egg than a sphere, challenges previous assumptions about how red supergiants eject mass. Jacco van Loon, Director of Keele Observatory in the UK and a co-author of the study, emphasized, “This star is one of the most extreme of its kind, and any drastic change may bring it closer to an explosive end.”

ESO's VLTI represents a powerful tool for exploring the cosmos. By combining light collected from multiple telescopes, the VLTI creates detailed images with a resolution equivalent to that of a single telescope spanning the maximum distance between them. This unique capability has unlocked new insights into the lives of stars like WOH G64.

The GRAVITY instrument, key to imaging WOH G64, exemplifies the VLTI's technical evolution. Unlike its predecessor MIDI, which combined light from just two telescopes, GRAVITY utilizes four. This leap in technology has enabled astronomers to study fainter and more distant objects than ever before.

Further advancements lie on the horizon. GRAVITY+, an upgrade to the existing system, promises to enhance sensitivity and resolution, offering even greater potential for unraveling the mysteries of dying stars. “Similar follow-up observations with ESO instruments will be important for understanding what is going on in the star,” Ohnaka said.

The work on WOH G64 highlights the collaborative nature of modern astronomy. Teams from institutions such as the Max Planck Institute for Radio Astronomy in Germany, Keele University in the UK, and Universidad Andrés Bello in Chile have pooled their expertise to achieve these groundbreaking results. Their findings, published in Astronomy & Astrophysics, mark a significant step forward in our understanding of stellar evolution.

ESO, founded in 1962, continues to lead international efforts in astronomy. Its array of observatories, including the VLTI, operates under some of the clearest skies on Earth in the Atacama Desert. With ongoing projects like the Extremely Large Telescope and the Cherenkov Telescope Array South, ESO remains at the forefront of exploring the universe.

The discoveries surrounding WOH G64 not only illuminate the dramatic death throes of massive stars but also pave the way for future explorations of other distant celestial phenomena. For now, the enigmatic egg-shaped cocoon of this dying giant serves as a poignant reminder of the universe's dynamism and the relentless drive of humanity to understand it.

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