Supernova Caught Starting to Explode for First Time

Ker Than
for National Geographic News
May 21, 2008
A brilliant burst of light marking a dying star's final moments before exploding has been glimpsed by astronomers for the first time.

Called a shock breakout, the x-ray flash—detected in January—signals the destruction of a star several times more massive than our sun.

This "first light" is just the opening salvo of a larger supernova blast that tears the star apart from within.

"They come out at the moment the star cracks open," said study team member Alicia Soderberg, an astrophysicist at Princeton University in New Jersey.

Shock breakouts have been predicted for decades but are so short-lived that they have never been seen before.

The study authors think that future wide-field x-ray surveys could allow researchers to witness the starts of hundreds of supernovae each year.

The discovery could also be a boon to scientists searching for exotic phantom particles and ripples in space-time emitted when massive stars explode.

The research is detailed in the May 22 issue of the journal Nature.

Bouncy Neutron Stars

Stars that are eight times or more massive than our sun go supernova when they run out of nuclear fuel and collapse under their own weight, forming small, dense objects called neutron stars.

Like a squeezed rubber ball, a newborn neutron star compresses and then rebounds, generating a shock wave that ploughs through the star's outer gas envelopes and rips it apart.

A shock breakout occurs when the shock wave pierces the star's surface trailed by a torrent of x-rays heralding the supernova blast.

Although this theory has been widely accepted, finding an actual shock breakout was due to extreme serendipity.

Using NASA's Swift X-Ray Telescope, Soderberg and colleagues had been observing a supernova in the galaxy NGC 2770 that had exploded about two weeks earlier.

But they also detected an x-ray flash at the opposite end of the same galaxy.

The probability of two supernovae exploding in the same galaxy within weeks of one another is about one in ten thousand.

"I was looking at the data as it came off the satellite, so I knew something had happened right then," Soderberg told National Geographic News.

Soderberg immediately alerted astronomers around the world, who quickly trained their instruments on the new supernova, dubbed SN 2008D.

The combined data gave scientists a record of the explosion from its initial moments, making SN 2008D the best studied supernova of its kind.

Supernova Predictions

During the initial moments of the explosion, supernovae are thought to emit a flood of mysterious particles called neutrinos and create ripples in the fabric of space-time called gravity waves.

(Related: "Scientists Try to Pin Down Elusive Neutrinos" [May 21, 2002].)

However, most supernovae are detected a few weeks after they explode, when they are most visible in optical light.

By then any neutrino and gravity wave signals have become too weak to associate with specific supernovae.

Having data on shock breakouts could change that.

"What this enables you to do is pinpoint the time of a core collapse to within minutes," said Roger Chevalier, an astrophysicist at the University of Virginia who was not involved in the study.

"You could then go back and look at data from neutrino and gravitational wave experiments and match a detection with a supernova."

The work could also give scientists insight into the state of massive stars just before their cores collapse, leading to improved predictions about which stars will go supernova, Chevalier said.

"There may be changes in the late stages of stellar evolution that affect what [the star] looks like just before the explosion."

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