"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. Located a staggering 160 000 light-years from us, the star WOH G64 was imaged thanks to the impressive sharpness offered by the European Southern Observatory's Very Large Telescope Interferometer (ESO's VLTI). The new observations reveal a star puffing out gas and dust, in the last stages before it becomes a supernova.
"We discovered an egg-shaped cocoon closely surrounding the star," says Ohnaka, the lead author of a study reporting the observations published today in Astronomy & Astrophysics. "We are excited because this may be related 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, unveiling their properties, countless other stars dwell within other galaxies, so far away that observing even one of them in detail has been extremely challenging. Up until now.
The newly imaged star, WOH G64, lies within the Large Magellanic Cloud, one of the small galaxies that orbits the Milky Way. Astronomers have known about this star for decades and have appropriately dubbed it the 'behemoth star'. With a size roughly 2000 times that of our Sun, WOH G64 is classified as a red supergiant.
Ohnaka's team had long been interested in this behemoth star. Back in 2005 and 2007, they used ESO's VLTI in Chile's Atacama Desert to learn more about the star's features , and carried on studying it in the years since. But an actual image of the star had remained elusive.
For the desired picture, 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 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 VLTI. 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.
More information
ESO's Very Large Telescope Interferometer is able to combine light collected by the telescopes of ESO's Very Large Telescope (VLT), either the four 8-metre Unit Telescopes or the four smaller Auxiliary Telescopes, creating highly detailed pictures of the cosmos. Effectively, this makes the VLTI a "virtual" telescope with a resolution equivalent to the maximum distance between the individual telescopes. This process is highly complex and needs instruments especially dedicated to this task. Back in 2005 and 2007 Ohnaka's team had access to the first generation of these instruments: MIDI . While impressive for its time, those observations with MIDI only combined the light from two telescopes. Now, researchers have access to GRAVITY , a second-generation instrument able to capture the light of four telescopes. Its improved sensitivity and resolution made the image of WOH G64 possible. But there is more to come. GRAVITY+ is a planned upgrade of GRAVITY which will be able to take advantage of different technological updates performed at the VLTI and VLT. With these, the VLTI will be able to see objects fainter and farther than ever before.
This research was presented in a paper to appear in Astronomy and Astrophysics (https://www.aanda.org/10.1051/0004-6361/202451820).
The team is composed of: K. Ohnaka (Instituto de Astrofísica, Departamento de Física y Astronomía, Facultad de Ciencias Exactas, Universidad Andrés Bello), K.-H. Hofmann (Max Planck Institute for Radio Astronomy, Bonn, Germany [MPIfR]), G. Weigelt (MPIfR), J. Th. van Loon (Lennard-Jones Laboratories, Keele University, United Kingdom), D. Schertl (MPIfR), S. R. Goldman (Space Telescope Science Institute, Baltimore, USA).
The European Southern Observatory (ESO) enables scientists worldwide to discover the secrets of the Universe for the benefit of all. We design, build and operate world-class observatories on the ground — which astronomers use to tackle exciting questions and spread the fascination of astronomy — and promote international collaboration for astronomy. Established as an intergovernmental organisation in 1962, today ESO is supported by 16 Member States (Austria, Belgium, Czechia, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom), along with the host state of Chile and with Australia as a Strategic Partner. ESO's headquarters and its visitor centre and planetarium, the ESO Supernova, are located close to Munich in Germany, while the Chilean Atacama Desert, a marvellous place with unique conditions to observe the sky, hosts our telescopes. ESO operates three observing sites: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its Very Large Telescope Interferometer, as well as survey telescopes such as VISTA. Also at Paranal ESO will host and operate the Cherenkov Telescope Array South, the world's largest and most sensitive gamma-ray observatory. Together with international partners, ESO operates ALMA on Chajnantor, a facility that observes the skies in the millimetre and submillimetre range. At Cerro Armazones, near Paranal, we are building "the world's biggest eye on the sky" — ESO's Extremely Large Telescope. From our offices in Santiago, Chile we support our operations in the country and engage with Chilean partners and society.
