Doomed 'cannibal' star could soon explode in a supernova so bright it would be visible during the day

Betelgeuse may have competition for the most exciting star about to go nova near Earth.

Astronomers have discovered the secret of a strange star system that has baffled them for years, finding it contains a dead star about to erupt after overfeeding on a stellar companion. The supernova explosion of this cosmic cannibal could be as bright as the moon, making it visible with the naked eye over Earth even in broad daylight.

The system in question is the double star V Sagittae located around 10,000 light-years from Earth, containing a white dwarf stellar remnant and its victim companion star, which orbit each other roughly twice every Earth day. The new research and the revelation of this white dwarf's imminent catastrophic fate answer questions about V Sagittae that have lingered for 123 years!

"V Sagittae is no ordinary star system - it's the brightest of its kind and has baffled experts since it was first discovered in 1902," team member and University of Southampton researcher Phil Charles said in a statement. "Our study shows that this extreme brightness is down to the white dwarf sucking the life out of its companion star, using the accreted matter to turn it into a blazing inferno.

"It's a process so intense that it's going thermonuclear on the white dwarf's surface, shining like a beacon in the night sky."

Final fate of a cosmic cannibal

White dwarfs represent the final stage of stars with masses around that of the sun, occurring when they run out of fuel for nuclear fusion. Indeed, our star will end its life as a cooling white dwarf when it runs out of hydrogen in around 5 billion to 6 billion years.

While this smoldering cosmic ember state represents the end for single stars going out with a whimper rather than a bang, white dwarfs that have a stellar companion can get a second lease on life and a more conclusive and explosive end. This happens when its dense stellar corpse is close enough to its companion star to allow its gravity to begin stripping away the partner's stellar material.

This material can't fall straight to the white dwarf because it has angular momentum, or spin. That means it forms a swirling, flattened cloud of matter around the white dwarf called an accretion disk, which gradually dumps matter to its surface.

This situation continues, and the stolen stellar material piles up on the surface of the white dwarf until it pushes this stellar remnant past the so-called Chandrasekhar limit of 1.4 solar masses. This is the mass limit that a stellar remnant has to exceed to trigger a supernova. The result is a Type Ia supernova that usually completely destroys the greedy white dwarf star.

However, this team found something very different and extraordinary happening with the stellar material being stolen by the white dwarf in V Sagittae.

The team uncovered the violent nature of V Sagittae using the Very Large Telescope (VLT), comprised of four individual telescopes located almost 9,000 feet (2,636 meters) on Cerro Paranal in the Atacama Desert of northern Chile.

This investigation revealed that there is a giant halo of gas comprised of material stolen from the companion star wrapped around both the cannibal white dwarf and its stellar victim. This is the result of the incredible amount of energy being generated in the system by the white dwarf as it strips material from its companion star.

This vast system-wide gas halo indicates that the white dwarf is snatching way more matter than it can handle. It also implies that this situation isn't going to continue for long, though when the end will come for this white dwarf isn't quite certain.

"The white dwarf cannot consume all the mass being transferred from its hot star twin, so it creates this bright cosmic ring," team member Pasi Hakala from the University of Turku said. "The speed at which this doomed stellar system is lurching wildly, likely due to the extreme brightness, is a frantic sign of its imminent, violent end."

"The matter accumulating on the white dwarf is likely to produce a nova outburst in the coming years, during which V Sagittae would become visible with the naked eye," Pablo Rodríguez-Gil from Spain’s Instituto de Astrofisica de Canarias said. "But when the two stars finally smash into each other and explode, this would be a supernova explosion so bright it'll be visible from Earth even in the daytime."

The team's research was published on Thursday (Sept. 11) in the journal Monthly Notices of the Royal Astronomical Society.