These two stars are locked in an extraterrestrial tango so tight that they orbit each other in just 12.3 hours, swinging gradually closer with each orbit, according to the statement.

[...]

In a study published in November in the journal Monthly Notices of the Royal Astronomical Society, an international research team led by the University of Turku in Finland analyzed the light emitted by V Sagittae to better understand exactly what type of beast it may be.

These data were gathered over a 120-day observation period by the X-Shooter spectrograph at the European Southern Observatory's Very Large Telescope, situated at an altitude of 8,600 feet (2,600 meters) atop Cerro Paranal in Chile's Atacama Desert.

Spectrographs like X-Shooter collect incoming light from celestial objects and then separate that light into its constituent wavelengths. This provides a spectrum that reveals the object's chemical composition, since each atom and molecule absorbs and reflects a certain wavelength of light. For perspective, think of how a prism splits white light into its constituent colors to produce a rainbow.

This spectral data helped the researchers re-analyze V Sagittae's characteristics. Previously, in a study from 1965, astronomers calculated that its two stars were 0.7 and 2.8 solar masses, though this is a controversial conclusion.

To constrain stellar sizes, this more recent study considered factors like orbital period to suggest that the entire system may be below 2.1 solar masses, with both the white dwarf and its companion each weighing in at around 1 solar mass.

An orbiting nuke

The researchers also identified V Sagittae as a supersoft X-ray source (SSS), meaning it generates lower-energy X-rays compared with hard sources like active black holes and colliding neutron stars. Classical SSS are composed of an accreting white dwarf and a more massive star whose gas is overspilling and falling onto the white dwarf.

V Sagittae's prodigious gravitational appetite is causing a sustained thermonuclear reaction on the white dwarf's surface, turning it into an orbiting nuke and the brightest SSS in the galaxy, researchers said in a statement.

In fact, even during its fainter phases V Sagittae is 100 times brighter than other variable star systems. The speed of the infalling material in the white dwarf's accretion disk shifts dramatically and unpredictably, sometimes in just days, as it struggles to consume all the material it pilfers from its partner, the team said in a separate statement.

As a result, a significant amount of material has escaped and formed a ring, or halo, of gas that encircles both stars, composing a "circumbinary disk" with a radius that may span about two to four times the separation between the two stars.

[...]

When the stars spiral into each other and smash together, they'll produce a "supernova explosion so bright it'll be visible from Earth even in the daytime," adds Rodríguez-Gil.

This ultimately brilliant finale may occur as early as 2067, according to a 2020 study from Louisiana State University, which predicted V Saggitae's demise based on the decreasing orbital period of its stars. Charles concludes that if the "[observed] period decline continues then it must happen, but stellar evolution is hard to predict exactly, so that might easily change!"

So keep an eye tuned toward Sagitta for a nova and mark your calendars for the supernova that will spectacularly spell the end of one of our galaxy's most tantalizing star systems.