The shockwave generated by the explosion of an ageing giant star has been observed by an international team of astronomers.

Key points: A supernova occurs when a star collapses in on itself

A supernova occurs when a star collapses in on itself Astronomers have theorised a shockwave was part of this process

Astronomers have theorised a shockwave was part of this process The shockwave was observed when a red giant exploded

The shockwave was observed when a red giant exploded The star was 270 times the radius of our Sun and 750 million light years away



The discovery, accepted for publishing in the Astrophysical Journal, will help scientists understand the life cycle of stars, said study co-author Brad Tucker of the Australian National University.

"This is the first time we've seen this in the normal visible colours, and we now know it happens," Dr Tucker said.

"The fundamental way we believe that core collapse happens is related to this shockwave happening. So the physics has been around … for decades and we've finally now been able to physically examine and test what's going on."

The team of scientists observed the earliest moments of two old stars exploding using the Kepler Space Telescope.

They spotted the shockwave around the smaller of the two stars — a red supergiant over 270 times the radius of the Sun and 750 million light years away.

As the star ran out of fuel it began collapsing and compressing on its central core.

"It's like packing in dirt," Dr Tucker said. "You keep pressing it till it's so dense you can't get it in anymore, and that's when you create a neutron star.

"But you reach a limit when you can't pack it in anymore, and that force pushing in bounces back and it triggers a shockwave to go through the star, causing the star to actually blow up."

Challenge to catch explosion

That's the moment the supernova starts creating the heavier elements on the periodic table, such as gold, silver and platinum.

"It's that singular moment when we can see the periodic table happening, when we can see the process of creating these new elements, and also see a switch from fission to fusion all at the same time because of this residual shockwave going through this star," Dr Tucker said.

The shockwave that initiated the core collapse or type IIp supernova was seen as a quick brightening — or flash. The supernova itself also creates a brightening, but this fades over a longer period of time.

Because the shockwave doesn't last very long — typically hours to days — it's been a challenge to catch one.

A shockwave has been measured coming from a supernova similar to this one. ( Pete Challis (CfA, Harvard) and the High-Z SN Team, Hubble Space Telescope (NASA/ESA) )

Scientists have previously observed a shockwave in X-ray (as opposed to visible light) but that was just pure luck, Dr Tucker said.

"They were actually looking at another exploding star and one happened to go right off in the exact patch of sky in the exact moment they were looking at it. It was the definition of luck."

The Kepler Space Telescope enabled the astronomers to systematically scan the sky.

"Kepler is unique," Dr Tucker said, "because it's in space and the way it's set up, you can monitor the sky every 30 minutes. So you know that once the star blows up, you catch it within 30 minutes."

The second exploding red giant they observed, however, did not shows signs of a shockwave.

The researchers suggested this was because the huge size of the second star — with a radius of 400 times our Sun — made it hard for the shockwave to get through the star to escape out into space.

"Because it had to travel twice the distance [of the other shockwave] we believe it happened but it never made it outside the surface of the star and we never saw it," Dr Tucker said.

More supernovae detected by Kepler

The exploding red giants were found in the first Kepler space observatory mission, known as K1. Four other supernovae have been found in this mission — three previously discovered were caused by the collision of pairs of very old, dense stars called white dwarfs — and another star is yet to be analysed.

The second Kepler mission — known as K2 — began in 2014 following repairs to the space telescope, and has so far discovered nearly 20 supernovae, which are yet to be analysed.

"With the original Kepler mission we did 500 galaxies and we got six supernovae," Dr Tucker said.

"With K2 it is doing three to 5,000 galaxies at a time; we're increasing the amount of galaxies and we're hoping to increase the amount of supernovae we find."

The research is reported on the pre-press site arXiv.org.