In a few years, we will witness a star’s dramatic end in a massive thermonuclear explosion.

In the silent immensity of the Milky Way, certain stars experience extraordinary destinies. They do not die out discreetly like the others, but prepare, in the shadows, upheavals of unimaginable magnitude. Among them, one candidate is now attracting all eyes. Its accelerated and chaotic evolution suggests a stellar explosion so powerful that it could illuminate our sky in broad daylight.

When two stars dance until they crash

At the heart of the discreet constellation Arrow, on the border between the summer stars, a foretold cosmic tragedy is playing out. The binary system V Sagittae, made up of a white dwarf and a more massive companion star, is engaged in a slow orbital standoff. In just 12.3 hours, the two stars complete a complete revolution around their common center of gravity. This gravitational embrace intensifies, drawing them inexorably closer to each other.

If this scenario is reminiscent of other stellar duos, one element radically sets it apart. The white dwarf does not just gravitate around its companion, it engulfs it. With each orbit, it tears material from its sister star at a rate never before observed. The study coordinated by the University of Turku and the University of Southampton, relayed by LiveScience, showed that this frantic absorption causes a rapid and chaotic increase in the brightness of the system, a sign of a major gravitational disruption.

Since 1902, the date of its discovery, astronomers have tried to characterize this atypical star. Thanks to observations carried out in 2023 by the Very Large Telescope in Chile, a network of clues has been put in place. The emission lines reveal the formation of a giant gas disk around the stellar couple, called a circumbinary disk, witness to the excess mass that the white dwarf is no longer able to absorb.

The most spectacular stellar explosion in our galaxy?

What awaits V Sagittae is not a simple burst of activity, but a stellar explosion of extremely rare magnitude. Initially, researchers expect the appearance of a nova, an explosion caused by the excess material accumulated on the surface of the white dwarf. This phenomenon could occur within a few years, making the star visible to the naked eye from Earth, even without a telescope.

But this first explosion will only be a prelude. According to calculations from a differential model published in Monthly Notices of the Royal Astronomical Society, the system should merge around 2083, with a margin of error of 16 years. The study reveals that the orbital period is rapidly decreasing, while the system's luminosity has doubled in less than a century, evidence that the orbit is contracting at high speed.

When the white dwarf and its companion star finally crash into each other, they will generate a thermonuclear supernova of such intensity that it will become visible in broad daylight. This light cataclysm could last several days, even weeks. And unlike novae, the ensuing supernova will completely destroy the system, leaving behind a stellar residue that is difficult to predict.

What V Sagittae reveals about the limits of cosmic stability

The story of V Sagittae is not just about celestial fireworks. It questions more broadly the current models of stellar evolution. Most known binary systems follow a stable pattern where the exchange of matter is self-regulated. This is not the case here. With an unusual mass ratio (the companion being almost four times more massive than the white dwarf), V Sagittae is located in a zone of theoretical instability rarely observed in reality. This imbalance causes a feedback effect. The more material the white dwarf absorbs, the tighter the orbit becomes, further accelerating the absorption.

Such an outburst has until now been considered as a borderline, almost abstract case. But V Sagittae provides the empirical demonstration, making tangible what the models predicted without ever being able to observe it. According to Professor Phil Charles, co-author of the study, this system represents a brief and violent phase of binary evolution, which could shed light on the mechanisms leading to certain Type Ia supernovae, often used as benchmarks to measure the expansion of the universe.

V Sagittae is therefore much more than an announced cosmic spectacle. It is a natural, rare and ephemeral laboratory, which tests our understanding of stellar cycles, matter and time. Looking up at the Arrow constellation in the years to come will not only be a matter of peering into a light in the sky, but of witnessing the spectacular culmination of a collapse orchestrated by gravity itself.