Supernova sighting leaves astronomers agog

The death of a star has been witnessed as it occurred for the first time, a remarkable observation that will lead to new insights into this cataclysmic cosmic process, writes Science Editor Roger Highfield.

Galaxies typically host a supernova - star death - only once or twice every century, making it nearly impossible to capture the instant it starts to spew energy into space, briefly shining brighter than billions of stars combined.

But the lucky glimpse by a satellite of the spectacular outburst from the very beginning has allowed astronomers from around the world to quickly follow up with eight other orbiting and ground-based telescopes and collect a wealth of new information on what happens when a star is blown to smithereens.

"We usually only get to see them a good few days after they exploded, usually when they go bright at optical wavelengths," says Dr Kim Page, University of Leicester, one of the team that reports the discovery today in the journal Nature.

But this time they could pick up a tell- tale burst of X rays, which occurs at the outset of star death. Thanks to a satellite, "we got on to it in nine seconds," says Dr Page, who led the X-ray analysis.

"Now we have a much better idea of the star that exploded."

This new supernova, named SN2008D, intrigues scientists because it is closer to Earth - at 90 million light years - than any type of supernova ever observed in the act of exploding, giving them the clearest picture ever of a star at the moment of its explosive death.

They can, for example, deduce that a thin outer layer of the star must have been ejected at velocities up to about 70 per cent the speed of light, much higher than ever thought.

Supernovae are explosions of stars more than eight times the mass of the Sun whose cores run out of nuclear fuel and collapse in on themselves to form a neutron star, an ultradense object, or a black hole.

During the conflagration one can think of the dying star as a leaky bottle containing light radiation. It turns out it is leakier when it comes to higher energy radiation - X rays - than the reds, greens and blues of optical light.

Previous X-ray observations of normal supernovae have been at later stages in the explosion, when stellar material collides with the surrounding envelope of gas and dust shed earlier by the star, generating high-energy radiation.

This is the first time a normal supernova has been glimpsed at the moment of "shock breakout," when the shock wave rebounding from the collapsed core breaks though the star's surface to produce a shower of X-rays.

Dr Paul O'Brien, also from Leicester, says, "understanding supernovae is important as these nuclear furnaces make the heavy elements from which planets like ours form."

Astronomers were fortunate because when the star death occurred, on January 9, Nasa's Swift satellite was already monitoring X-rays from another supernova in the same galaxy when the second one exploded. At the same time, astronomers and their colleagues were monitoring the first supernova from the ground and caught images of the new one only hours after the explosion.

"We were in the right place, at the right time, with the right telescope on January 9th and witnessed history," remarks Dr Alicia Soderberg of Princeton University.

"We were looking at another, older supernova in the galaxy, when the one now known as SN 2008D went off. We would have missed it if it weren't for Swift's real-time capabilities, wide field of view, and numerous instruments.

"This newly born supernova is going to be the Rosetta stone of supernova studies for years to come."

Dr Soderberg adds that Swift and other planned X-ray satellites designed to scan the sky for such X-ray flashes should see hundreds more like this in the future, making all-wavelength, cradle-to-grave analyses of supernovae common.

"This first instance of catching the X-ray signature of stellar death is going to help us fill in a lot of gaps about the properties of massive stars, the birth of neutron stars and black holes, and the impact of supernovae on their environments," said Dr Neil Gehrels, principal investigator of the Swift satellite.

"We also now know what X-ray pattern to look for. Hopefully we will be able to find many more supernovae at this critical moment." - te