Supernova "Shock Breakout" Seen From Red Giant -- A 1st

Ker Than
for National Geographic News

June 12, 2008

The ultraviolet flash that signals the explosion of a red supergiant star has been detected by astronomers for the first time.

"We have witnessed the violent death of a massive star in a galaxy almost a billion light-years away in unprecedented detail," said study team member Kevin Schawinski, an astrophysicist at the University of Oxford in the U.K.

The discovery comes just weeks after an independent team reported the first sighting of x-ray light from a star just as it was beginning to explode.

Seeing such "first light" from supernovae could help astronomers better understand what's happening inside massive stars in their final moments.

The research is detailed in tomorrow's issue of the journal Science.

Breakout Caught

Red supergiants are the most common stars that end their lives as supernovae.

These massive stars explode when the fuel that sustains nuclear reactions inside their cores is depleted.

The cores then collapse under their own immense weight, and the stellar breakdown generates a shock wave that plows outward through the star.

The moving shock wave heats material in front of it to several hundred thousand degrees.

For a brief moment, the stars shift from red-hot to white-hot and beyond, until they shine mostly in ultraviolet and x-ray light known as the shock breakout.

"For a moment the star is almost transparent, and this flash of light comes out," Schawinski told National Geographic News.

Shortly after, the shock wave pierces the star's surface, blasting it apart and strewing its luminous debris across light-years of space. (See related images of supernova remnants.)

"We caught the star while the supernova shock wave approached the surface of the star and then blew it apart," Schawinski said.

Peter Nugent is an astrophysicist at the Lawrence Berkeley National Laboratory in California who was not involved in the study.

Nugent called the latest breakout detection "incredibly exciting."

"You can literally see the shock coming through the undisturbed atmosphere of the red giant star and then the subsequent expansion of that star," Nugent said.

The discovery allows astronomers to analyze the properties of a dying star in the moments before it goes supernova, data that are almost impossible to measure, he added, "unless one went off in our own galaxy."

Star Detectives

The red supergiant's shock breakout was discovered using a combination of ground and space telescopes.

The Supernova Legacy Survey in Hawaii first detected visible light from the distant supernova, called SNLS-04D2dc, several weeks after it had already exploded.

To look for the earlier shock breakout, the team combed through a backlog of data collected by the Galaxy Evolution Explorer (GALEX), a NASA space telescope that scans in ultraviolet wavelengths.

Their search revealed a bright ultraviolet burst in the precise location of the supernova about three weeks before the explosion.

The previous team to detect a shock breakout witnessed the death of a rare compact blue star while examining images from NASA's Swift X-Ray Telescope.

But that discovery was very serendipitous and unlikely to be repeated, said Brian Schmidt, an astrophysicist at Australian National University who was not involved in either study.

"They were a million-to-one lucky," Schmidt said.

By contrast, the detection of the red supergiant shock breakout relied on diligent detective work.

"They did this without being lucky but by being systematic and sifting through huge amounts of data," Schmidt said.

The technique used by Schawinski's team could be more easily repeated and used to discover similar breakouts.