New information on cosmic mystery – large ring encircles binary star

V Sagittae is no ordinary star. For more than a century, this unusually bright and unpredictable object in the constellation of Sagitta has baffled astronomers. First noticed in 1902 and confirmed in the 1960s to consist of a pair of stars orbiting each other every 12.3 hours, its true nature has remained a cosmic mystery.

Two main ideas have vied to explain it. The first envisions two hot stars locked in a tight orbit. The second – now gaining strong support – sees a white dwarf star greedily pulling in matter from a more massive companion.

In this second scenario, the transfer of material is so extreme that it could lead to a violent stellar merger by the end of this century, possibly ending in a spectacular nova or even a supernova explosion visible from Earth.

Now, an international team of researchers from the Finnish Centre for Astronomy with ESO at the University of Turku in Finland, the University of Southampton and the University of Oxford in the UK, and the Instituto de Astrofísica de Canarias and the University of La Laguna in Spain has taken the closest look yet, using the European Southern Observatory’s Very Large Telescope in Chile and its X-shooter spectrograph.

For over four months in 2023, the team captured exceptionally detailed spectra – the “fingerprints” of light from the system – and made two key discoveries.

First, they found a large ring of gas encircling both stars – a so-called circumbinary disc. Second, they traced the motion of the inner disc of material swirling around the white dwarf, confirming its identity as the cosmic “thief” in the system.

However, the team also saw something strange: the speed of this material shifted dramatically and unpredictably over days to months, suggesting the system is experiencing chaotic behaviour due to extreme radiation, as it is heading towards its violent end involving a stellar merger and the explosion of the stars, which could very well be discernible from Earth.