Astronomers discover doomed pair of spiralling stars on our cosmic doorstep

Type 1a supernovae are a special class of cosmic explosion, famously used as 'standard candles' to measure distances between Earth and their host galaxies. They occur when a white dwarf (the dense remnant core of a star) accumulates too much mass, is unable to withstand its own gravity, and explodes.

It has long been theoretically predicted that two orbiting white dwarfs are the cause of most type 1a supernova explosions. When in a close orbit, the heavier white dwarf of the pair gradually accumulates material from its partner, which leads to that star (or both stars) exploding.

This discovery, published today in Nature Astronomy, has not only found such a system for the first time, but has found a compact white dwarf pair right on our doorstep in the Milky Way.

James Munday, PhD researcher at Warwick and leader of the investigation said, "For years a local and massive double white dwarf binary has been anticipated, so when I first spotted this system with a very high total mass on our Galactic doorstep, I was immediately excited."

"With an international team of astronomers, four based at The University of Warwick, we immediately chased this system on some of the biggest optical telescopes in the world to determine exactly how compact it is."

"Discovering that the two stars are separated by just 1/60th of the Earth-Sun distance, I quickly realised that we had discovered the first double white dwarf binary that will undoubtedly lead to a type 1a supernova on a timescale close to the age of the universe."

"At last, we as a community can now account for a few per cent of the rate of type 1a supernovae across the Milky Way with certainty."

Significantly, James's new system is the heaviest of its type ever confirmed, with a combined mass of 1.56 times that of the Sun. At this high of a mass, this means that, no matter what, the stars are destined to explode.

The explosion is not due for another 23 billion years, however, and despite being so close to our solar system, this supernova will not endanger our planet.

Right now, the white dwarfs are leisurely spiralling around each other in an orbit taking longer than 14 hours. Over billions of years, gravitational wave radiation will cause the two stars to inspiral until, at the precipice of the supernova event, they will be moving so fast that they complete an orbit in a mere 30 -- 40 seconds.

Dr. Ingrid Pelisoli, Assistant Professor at The University of Warwick and third author, added: "This is very significant discovery. Finding such a system on our galactic doorstep is an indication that they must be relatively common, otherwise we would have needed to look much further away, searching a larger volume of our galaxy, to encounter them.

"Finding this system is not the end of the story though, our survey searching for type 1a supernova progenitors is still ongoing and we expect more exciting discoveries in the future. Little by little we are getting closer to solving the mystery of the origin of type 1a explosions."

For the supernova event, mass will transfer from one dwarf to the other, resulting in in a rare and complex supernova explosion through a quadruple detonation. The surface of the mass-gaining dwarf detonates where it is accumulating material first, causing its core to explode second. This ejects material in all directions, colliding with the other white dwarf, causing the process to repeat for a third and fourth detonation.

The explosions will completely destroy the entire system, with energy levels a thousand trillion trillion times that of the most powerful nuclear bomb.

Billions of years into the future, this supernova will appear as a very intense point of light in the night sky. It will make some of the brightest objects look faint in comparison, appearing up to ten-times brighter than the moon and 200,000 times brighter than Jupiter.