White dwarf pair is spiraling toward a spectacular explosion

Stars don’t live forever – some end quietly and others explode. While not every explosion in the universe is noteworthy, those associated with type 1a supernovae make their presence known across galaxies.

These massive events can light up the sky, help measure the universe’s rate of expansion, and tell us about the life and death of stars. For years, scientists believed certain conditions led to these explosions, but they lacked information from a nearby system to prove this.

Now, astronomers at the University of Warwick have changed that. They have discovered a rare, compact binary star system only 150 light years from Earth. This pair of white dwarfs, which are the dense remains of dead stars, is locked in a deadly embrace.

Eventually, their spiral will end in a spectacular explosion. Although that day lies billions of years ahead, the discovery itself has immediate and far-reaching impact.

White dwarfs headed toward explosion

Type 1a supernovae have long been a tool for cosmic measurement. Astronomers use them as “standard candles” to calculate distances to faraway galaxies.

That’s because these explosions tend to release consistent amounts of light. If experts know how bright something should be, and they measure how bright it appears, they can determine out how far away it is.

These explosions happen when a white dwarf, which is already an ultra-dense star remnant, takes in too much mass. It might pull material from a nearby star, eventually reaching a tipping point. When gravity overwhelms its internal forces, the star explodes in a sudden release of energy.

The new discovery shows this process can also occur when two white dwarfs orbit each other. In such pairs, the heavier star slowly pulls mass from its partner. This continues until one, or both, explode.

Evidence of a double white dwarf system

For years, this idea had strong support in theory but lacked direct proof. Astronomers suspected that most type 1a supernovae began with two orbiting white dwarfs. But they had never found a nearby system that confirmed it – until now.

The newly identified binary system is not just close. It’s also exactly the type of high-mass pair that scientists hoped to find. Published in Nature Astronomy, the discovery gives clear evidence of a double white dwarf system that meets all the conditions for a type 1a explosion.

“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,” stated James Munday, Ph.D. researcher at Warwick and lead investigator.

Chasing the pair across the sky

Once Munday and his colleagues detected the system, they acted fast. Working with international astronomers – four from Warwick – they turned powerful optical telescopes toward the pair. Their goal was to measure how close the two stars were and confirm the system’s nature.

“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,” explained Munday.

The results were striking. The stars are separated by just 1/60th the distance between Earth and the Sun. That distance, tiny by cosmic standards, revealed the system’s destiny.

“Discovering that the two stars are separated by just 1/60th of the Earth-Sun distance, I quickly realized 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,” said Munday.

The heaviest system ever confirmed

This binary system stands out for its enormous combined mass: 1.56 times that of our Sun. That might not sound like much, but it’s the heaviest white dwarf pair of its kind ever observed. When it comes to type 1a supernovae, mass is everything.

“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,” said Munday.

Even though the explosion won’t happen for another 23 billion years, its certainty is no longer in question. Because of the stars’ masses and distance, they are already on a path of no return.

Despite how near the stars are to Earth, the future explosion poses no danger. The force will be incredible, but space is vast. The blast, while bright, will spread its energy across an enormous volume.

How the explosion will unfold

Right now, the white dwarfs circle each other in a slow spiral, completing an orbit over 14 hours. But gravitational wave radiation is at work. Over billions of years, this silent force will draw the stars closer.

When the moment comes, they will orbit one another every 30 to 40 seconds – mere blurs of light racing around one another – before detonating.

“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,” noted Dr. Ingrid Pelisoli, assistant professor at The University of Warwick.

The mystery of supernovae

The discovery may be thrilling, but the work isn’t over. Pelisoli notes that this binary is just one piece of a larger puzzle.

“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.”

The system’s eventual fate involves an unusual and violent mechanism: a quadruple detonation. First, the surface of the heavier white dwarf explodes as it accumulates mass. Then its core follows.

The blast sends debris outward, which then strikes the companion star. That impact causes a third, then a fourth explosion.

A powerful white dwarf explosion

When this quadruple detonation occurs, the energy released will exceed anything humans can create. The explosion will destroy both stars entirely, with a force billions of times greater than that of the most powerful nuclear weapons.

From Earth, the supernova will shine as a brilliant, steady point of light. It will appear up to ten times brighter than the Moon and 200,000 times brighter than Jupiter. It will change the night sky for weeks, if not months, before fading back into darkness.

Though it will likely happen long after humanity’s time, the light from this explosion will travel across space and be a reminder of this rare discovery that was made in our corner of the galaxy.

The study is published in the journal Nature Astronomy.

Image Credit: Credit University of Warwick/Mark Garlick

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