Astronomers spot two white dwarfs doomed to die in a quadruple detonation

By Will Dunham

WASHINGTON (Reuters) -Astronomers have spotted two hefty white dwarf stars - highly compact stellar embers - orbiting close together that appear destined to die in an extraordinarily violent quadruple detonation.

The two stars, gravitationally bound to each other in what is called a binary system, are located in our Milky Way galaxy about 160 light-years from Earth - relatively close in cosmic terms. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

White dwarfs are among the most compact celestial objects. Stars with up to eight times the mass of our sun appear destined to end up this way. They eventually burn up all the hydrogen they use as fuel. Gravity then causes them to collapse and blow off their outer layers in a "red giant" stage, eventually leaving behind a compact core roughly the diameter of Earth - the white dwarf.

"White dwarfs are the stellar remnants of the vast majority of stars, and from time to time we find systems where two white dwarfs closely orbit each other," said James Munday, a PhD researcher at the University of Warwick in England and lead author of the study published in the journal Nature Astronomy.

The researchers used data from four ground-based telescopes to study this binary system. One of the white dwarfs has a mass about 83% that of our sun and the other about 72%. No other known white dwarf binary has a larger combined mass, Munday said.

"They are both about as big as the Earth. One has a diameter about 20% larger and the other about 50% larger. That gives you an idea of how dense they are. It's the sun compressed onto the size of Earth. Their masses when they were regular stars were probably around three to four times the mass of the sun," said University of Warwick astrophysicist and study co-author Ingrid Pelisoli.

A few hundred binary systems composed of two white dwarf stars are known. These two orbit closest to each other of any of them. They are about 25 times closer to each other than our solar system's innermost planet Mercury is to the sun, completing an orbit every roughly 14 hours.

With the gap between them very gradually narrowing as the binary system loses energy, the fact that they are so massive and so close ensures their demise over a large timescale.

When they get closer to each other, the heavier of the two white dwarfs, because of its greater gravitational strength, will begin to draw material from the lighter one's outer layer and increase in mass past the threshold beyond which a white dwarf experiences a thermonuclear explosion.

This will set the stage for a complex explosion called a type 1a supernova, in this instance involving a quadruple detonation.

"White dwarfs are made up of layers, much like an onion. Their inner layer is a core of carbon and oxygen, surrounded by a helium layer and finally by a hydrogen layer," Pelisoli said.

"The less-massive star will transfer mass to the massive one when they start interacting. This will lead the helium layer (of the heavier one) to become too massive, triggering an explosion. This then triggers a second explosion in the carbon-oxygen core. The shock wave from these explosions in turn triggers a third explosion in the remaining helium layer of the companion, which triggers a fourth explosion in its carbon-oxygen core," Pelisoli added.

This quadruple detonation is expected to take about four seconds, start to finish. But it will not come anytime soon.

The researchers calculate that it will occur approximately 22.6 billion years from now. The universe is about 13.8 billion years old. When the explosion occurs, it would appear from the perspective of Earth about 10 times brighter than the moon in the night sky - if Earth, now about 4.5 billion years old, still exists.

This is the first time a binary system apparently headed for such a fate has been identified. If the two white dwarfs were far enough apart that the heavier one would not siphon material from the lighter one, they could survive in perpetual peace.

"In a wider orbit, they could indeed live stably without any catastrophic future, but here we know that the explosion will light up our side of the galaxy," Munday said.

(Reporting by Will Dunham, Editing by Rosalba O'Brien)