A very distant star, located more than 160,000 light years from our planet, has just become the star of a masterful photo: it is the first time that astronomers have succeeded in capturing a close-up of a star located in outside our galaxy.
The object in question, named WOH G64is a red hypergiant star located in the Large Magellanic Cloud, a dwarf galaxy located on the outskirts of the Milky Way. It is first distinguished by its rather phenomenal proportions. The least we can say is that it has not stolen its hypergiant status: it is approximately 2000 times larger than our Sun, hence its nickname “ Behemoth ». It's also incredibly bright — a particularly enticing combination of characteristics for astronomers, since it opens the way to very precise observations.
For around twenty years now, teams from the prestigious European Southern Observatory (ESO) have been regularly studying WOH G64 using the jewel in their scientific arsenal: the Very Large Telescopeor VLT for short. It is a huge interferometer made up of four state-of-the-art optical telescopes that is one of the most powerful observatories on the planet. We already owe him a myriad of particularly remarkable observations of exoplanets, extremely old galaxies, and even Sagittarius A*, the supermassive black hole at the center of the Milky Way.
This long observation campaign made it possible to rigorously document the characteristics of this cosmic colossus. But she ended up hitting a wall when astronomers wanted to get even closer to the Behemoth. Initially, even the VLT was not capable of capturing a detailed enough image to take this work to the next level. Heartbreaking, because close-ups of this kind are real scientific gold mines; they allow you to collect lots of data on the properties of the object, as well as its role in its local environment.
A close-up with incredible precision
However, the situation changed with the arrival of GRAVITY, the second generation of VLT scientific instruments which finally allowed astronomers to capture this exceptional close-up.
At first glance, this blurry image doesn't look much. But it is still a real technical feat which required absolutely incredible precision. To illustrate this, we can think on the scale of the Earth: capturing a close-up of a star located more than 1.5 billion billion kilometers away amounts to zoom in on a grain of sand located in Baghdad from Paris — although this would obviously be impossible in practice because of the curvature of the Earth's surface.
At the time of writing, astronomers have only captured around twenty such close-ups. And each time, these were stars located in our Milky Way. This is therefore a great first.
A dust cocoon with strange behavior
And beyond this innovative side, this image also met all the expectations of researchers at the scientific level.
To begin with, this close-up allowed the team to see that the brightness of WOH G64 had decreased significantly over the last decade. Data that is anything but anecdotal, because it is directly linked to the life cycle of the star. In their final stages of life, red supergiants like WOH G64 gradually lose their outer layers of gas and dust. This material tends to accumulate in a vast cloud around the star, which could explain the drop in brightness perceived by the VLT. This is therefore a superb opportunity to observe the decline of a true cosmic colossus live.
« We discovered that the star has undergone a significant change over the past ten years, giving us a rare opportunity to observe the life of a star in real time “, explains Gerd Weigelt, professor of astronomy at the Max Planck Institute and co-author of the study.
Beyond its potential impact on brightness, there is another characteristic of this cloud that surprised astronomers: its shape. These cocoons of dust and gas are never perfectly circular; but in the image, we can clearly see that it presents a extremely marked asymmetry, well beyond what the researchers' models predicted.
This is an interesting detail, because it points to the presence of a mechanism capable of modifying the structure of the cloud. According to the authors, it is for example possible that WOH G64 is part of a binary system, and that it is accompanied by another star whose gravitational influence could deform this dusty cocoon.
Unfortunately, this hypothesis will be difficult to confirm, because the drop in luminosity of the star will continue to complicate observations. To get to the bottom of the story, we will probably have to wait for GRAVITY+, the third generation of VLT scientific instruments which should take over within a few years… and cross your fingers that the Behemoth does not end up in a supernova until then!
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