(July 12) - Like a cosmic Grim Reaper, a blast of ultraviolet light signals the violent death of the universe's most massive stars. Now astronomers have viewed this heavenly harbinger for the first time.

"Astronomers have been dreaming about seeing the first light from the violent death of a star for over 30 years," said lead researcher Kevin Schawinksi of the University of Oxford. "Our observations open up an entirely new avenue for studying the final stages in the lives of massive stars and the physics of supernovae."

Kevin Schawinski, Oxford / NASA / GALEX / Reuters

This ultraviolet flash of light was produced from inside this dying star just before it exploded. It marked the first time scientists observed what happened in the final moments before a doomed star burst into space

Schawinksi and his colleagues detected the ultraviolet signal of a hefty star on the verge of explosion, which they detail in the June 13 issue of the journal Science.

Usually, when astronomers see a supernova, the star has already been destroyed. "It's very hard to tell much about precisely the kind of star that actually died there," Schawinski told SPACE.com. "The really cool thing about our observations is this light traveling ahead of the shock wave traveled through the star before it was destroyed."

He added, "It's telling us about the properties, the conditions, of the star at the moment it died, but before the shock wave actually disrupted it."

Doomed star

When a massive star, weighing at least 10 suns, runs out of nuclear fuel, it can collapse under its own weight, triggering an explosion called a supernova. The explosion sends the stellar guts spewing away at 20 million mph (10,000 km/sec) in a fireball that's a billion times brighter than the sun, the researchers say.

It's this fireball that scientists observe. What they haven't seen until now are the final moments of the doomed star just before the visible explosion. For the past 30 years or so, theorists have predicted a surge of ultraviolet light should come before the actual visible explosion.

There are several problems for actually seeing this phenomenon. "By the time you see the supernova, it's already days or weeks in the past," Schawinski said. "If you see a supernova you'd have to go back in time. You'd have to be already looking at the position."

The other issue is the fact that Earth's atmosphere absorbs ultraviolet light, and so you'd need a space telescope to actually be able to view the death beacon. The space telescope GALEX, which orbits Earth about every 98.6 minutes and views the universe in ultraviolet, was the answer.

With GALEX, researchers recently got front-row seats to the pre-show of what they suspect was a red supergiant star measuring somewhere between 500 and 1,000 solar radii on the verge of explosion. A red supergiant is a hefty star nearing the end of its life that can swell to 100 times its original size before exploding.

Schawinski and his colleagues looked at GALEX images taken at the positions of supernovae previously identified with optical telescopes in Hawaii.

"We found a new source at the location of one supernova, suddenly outshining its galaxy host in the UV," said Mark Sullivan of the University of Oxford. "It appeared a couple of weeks before the optical discovery of the supernova and marked the first stage in the death of the star."

The final hours

The UV peak represented a unique phase in the formation of the supernova SNLS-04D2dc, just before the shock wave from its collapsed core reached the star's surface to violently eject its shell of hot gas.