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Astronomers shocked by massive supernova that keeps exploding for more than 600 days

Sydney Morning Herald logoSydney Morning Herald 12/11/2017 Liam Mannix

This artist's representation of the supernova shows star-matter moving outwards from a series of explosions in the star's core over more than 600 days. © Carl Knox / Swinburne University of Technology This artist's representation of the supernova shows star-matter moving outwards from a series of explosions in the star's core over more than 600 days. Astronomers have observed what may be one of the universe's most powerful explosions.

They watched a star in a far-off galaxy first grow so huge it was bigger than our entire solar system - that's from the sun all the way out to far-off Pluto - and then collapse and explode.

The explosion, known as a supernova, was itself several times the size of our solar system. And it kept exploding, over and over again, for almost two years.

The explosion was so big that it has broken all existing theories of how stars die.

So far scientists don't have a complete theory explaining how it can be so big or why  it can continue to explode.

"I'm not going to lie – this is astounding," Professor Alan Duffy, one of Australia's foremost astronomers, told Fairfax after reading the scientists' findings.

Swinburne University's Igor Andreoni​, part of the team behind the discovery, said the work "challenges all the theories that we were so confident about".

"No theory predicts this specific behaviour we have observed," he said.

Supernovas are the largest explosions in space. Some are so bright they are visible with the naked eye.

The largest in recorded history was so intense that ancient Egyptian astronomers wrote that it lit up the night sky,  and glowed like a new moon during the day.

When a smaller star like our sun runs out of fuel, it fizzles out with a whimper.

But big stars like the huge one fuelling this supernova like to go out with a bang.

As they run out of fuel, they begin to swell,  growing to the size of a solar system. But the bigger they get, the more that gravity pulls on them. They eventually get too big, and gravity forces them to collapse in on themselves.

The collapse superheats the star's core to billions of degrees, and it explodes - an explosion so big it's often capable of initiating the formation of stars nearby.

But it's only supposed to explode once.

In a paper published in the journal Nature,  a team of astronomers from several countries, including Mr Andreoni, report on the supernova named iPTF14hls, which was first spotted by an  observatory in California about 509 million light years from Earth just over three years ago. 

Seen from Earth, supernovas tend to steadily grow brighter for a week - the explosion itself - and then lose their glow over the next roughly 100 days as the superheated matter is ejected out into the frozen depths of space.

But as the team observed iPTF14hls, they saw it was much more spectacular. "It was still bright after 600 days,"  Mr Andreoni said.

And the supernova wasn't fading – it appeared to still be exploding. "It was going bright, dim, bright, dim, over and over," he said. 

Six hundred days is already a record, but remarkably the star's explosive death may have even stretched over decades.

The team discovered some  archived images from 1954 that  appear to record a glow in the same region of the sky.

Unfortunately the techniques at the time were too imprecise to be sure if it was the supernova, but the possibility is tantalising.

"Some people believe it is the same one, others don't," Mr Andreoni said.

When the research team submitted their discovery to Nature, they originally included some theories on what might be going on.

But because the exploding supernova was so far outside what existing theories predicted, they dropped their possible explanation  and let people speculate on their own.

The theory that best explains what is going on is known as Pulsational Pair Instability.  Huge stars, ones hundreds of times the mass of the sun, weigh so much that when they collapse they actually force energy to turn into matter.

This can lead to a series of explosions that shake but don't totally destroy the star, before a single final explosion blows it to pieces.

That's their best theory, but it still does not fully explain what's going on, Mr Andreoni said.

"It's an event that produces more questions than answers – and as scientists we really like that."

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