V Sagittae Could Explode Into a Daytime-Visible Supernova — What to Expect

An extraordinarily luminous binary star system, V Sagittae, may be headed for a spectacular ending: a thermonuclear explosion bright enough to be seen from Earth in daylight, according to recent research.

What V Sagittae Is

V Sagittae sits about 10,000 light-years away in the small constellation Sagitta. The system pairs a white dwarf — the dense remnant of a sun-like star — with a heavier stellar companion that is donating mass at an unusually high rate. The two stars orbit each other every 12.3 hours and are slowly spiraling together.

New Observations And What They Reveal

An international team led by the University of Turku used the X-Shooter spectrograph on the European Southern Observatory's Very Large Telescope (VLT) to collect spectra of V Sagittae over a 120-day campaign. Spectral analysis lets astronomers identify chemical signatures and measure velocities, helping to constrain the system's properties.

Earlier work from 1965 suggested component masses of roughly 0.7 and 2.8 solar masses, but the new study finds that conclusion controversial. Based on the short orbital period and current data, the team argues the total mass of the system may be below about 2.1 solar masses, with each star nearer to ~1 solar mass. However, the authors emphasize that the system's wildly variable light and fast outflows complicate direct measurements of the individual stars' orbital motions and masses.

Unusual Activity: Supersoft X-Rays, Chaotic Accretion, And A Gas Halo

V Sagittae has been identified as the brightest known supersoft X-ray source (SSS) in the Milky Way. In these systems, steady thermonuclear burning occurs on the white dwarf's surface as it accretes matter from its companion. V Sagittae is roughly 100 times brighter than comparable variable systems even in relatively faint phases.

An illustration of a nova: an explosion that occurs when a white dwarf star siphons too much material from its larger stellar companion. | Credit: International Gemini Observatory/NOIRLab/NSF/AURA/M. Garlick, M. Zamani

Observers recorded dramatic, unpredictable changes in the speed of material in the white dwarf's accretion disk — sometimes shifting within days — and signs that some matter has been expelled to form a circumbinary ring or halo. The team estimates this circumbinary disk may extend roughly two to four times the stars' separation.

Nova First, Then Possibly A Supernova

The researchers forecast that a nova outburst could occur in the coming years: when a white dwarf accreting material ignites thermonuclear burning on its surface and ejects accumulated layers. Classical novae do not destroy the white dwarf and can recur on long timescales.

After one or more nova events, the ongoing orbital decay and extreme mass transfer may culminate in a merger and a final explosion. The team and previous studies suggest that this merger could produce a supernova luminous enough to be visible in daylight from Earth. A 2020 study from Louisiana State University extrapolating the system's shrinking orbital period estimated the merger could happen as early as 2067, though the timing is highly uncertain.

“If the [observed] period decline continues then it must happen, but stellar evolution is hard to predict exactly, so that might easily change,”

— Phil Charles, professor emeritus of astronomy, University of Southampton

What To Watch For

A naked-eye nova would be an exciting precursor and could arrive within years. Professional and amateur astronomers are advised to monitor Sagitta for brightening events and evolving spectral signatures. While a daytime-visible supernova would be rare and spectacular, the exact timeline and outcome remain uncertain because of complex accretion physics and observational ambiguities.

Bottom line: V Sagittae is one of the Milky Way's most intriguing binaries — bright, chaotic, and potentially on the verge of explosive changes that could produce an unforgettable celestial show within decades.