Supernova shockwave seen with visible light for first time

The final violent death throes of a star has been seen with visible light for the first time and provided a fresh mystery for astronomers.

Scientists think shock breakouts – a shockwave and flash of light that rocks a massive star just before it explodes into a “supernova” – allow the stars to finally explode, spewing out all the heavy atoms that exist in the universe.

But actually watching that process occur and seeing how it progresses has proved elusive, leaving scientists guessing about exactly how it happens.

By sifting through three years of data collected by Nasa’s now half-broken Kepler space telescope, an international team of scientists have now seen the elusive shock breakout occur. The problem is, it seemed to happen in only one of two exploding stars observed.

In data collected in 2011, they found two supernovae begin, potentially capturing the crucial moment. However only one star seemed to have the shockwave. An author on the paper, Brad Tucker from the Australian National University, said that was a mystery. He said the shockwave was thought to ripple across the surface and actually allow the supernova to explode.

“We’ve always thought that this is the physical mechanism that allows the star to blow up,” he said. “So gravity collapses the core down, and once the pressure is too much, you create a neutron star or sometimes a black hole, the rest of the energy rebounds and causes the star to blow up.

“It’s been this fundamental thing that we’ve always thought occurs but we’ve never seen it take place.”

Tucker said it had been seen by chance with x-ray telescopes before, but not in great detail.

The fact one of the supernova they saw with Kepler had the shock breakout and one didn’t, means there’s something to learn, he said.

Since the one where they didn’t see the shockwave was a bigger star – about 500 times the size of Earth’s sun – it could mean it wasn’t strong enough to escape the star’s gravity. “It could mean that the shockwave happened but it didn’t have enough oomph to get out,” he said.

Tucker said it was also possible that something like dust was blocking the view of the shockwave or, because it was further away (2,000 times further than the smaller one), it was just fainter and they missed it.

“It’s telling us something but we just don’t know what it is,” he said.

“That is the puzzle of these results,” said Peter Garnavich, an astrophysics professor at the University of Notre Dame in Indiana. “You look at two supernovae and see two different things. That’s maximum diversity.”

The shock breakout itself lasted only about 20 minutes, so catching the flash of energy was a milestone for astronomers, where things usually happen on the timescale of years, centuries or millennia.

“In order to see something that happens on timescales of minutes, like a shock breakout, you want to have a camera continuously monitoring the sky,” said Garnavich. “You don’t know when a supernova is going to go off, and Kepler’s vigilance allowed us to be a witness as the explosion began.”

Tucker said that as they push through more data from the Kepler missions, they will almost certainly see more of these events. He said from 500 galaxies they watched in the original Kepler mission, they found six supernovae, including these two. Kepler’s second mission – called K2 – aims to watch 5,000 galaxies, so should increase the odds, he said.

“While Kepler cracked the door open on observing the development of these spectacular events, K2 will push it wide open observing dozens more supernovae,” said Tom Barclay, director of the Kepler mission at Nasa Ames. “These results are a tantalising preamble to what’s to come from K2.”

Steve Howell, project scientist for Nasa’s Kepler and K2 missions, said: “All heavy elements in the universe come from supernova explosions. For example, all the silver, nickel and copper in the earth and even in our bodies came from the explosive death throes of stars. Life exists because of supernovae.”

That’s not quite as poetic as the way US astronomer Carl Sagan famously put it:

The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.

The findings have been accepted for publication in the Astrophysical Journal.