In the latest in a series of supernova firsts, scientists report in Science that they pinpointed a star that flared in the ultraviolet portion of the spectrum for several hours before blowing itself apart in a supernova. The researchers believe the finding represents the earliest visible sign of an imminent supernova—a surge in temperature as the expanding internal shock wave strains to break free of the star but has yet to shred it apart.

A type II supernova, or stellar explosion, occurs when a star of at least eight times our sun's mass runs out of nuclear fuel at its core. Without nuclear energy to puff it up, its massive gravity causes the core to implode, producing a shock wave that rips through the star's outer layer and finally blows it apart, releasing a flood of x-rays. But researchers believe that the first sign of death would actually be the heating of the star's surface as the expanding shock wave sends out hot radiation ahead of it.

"The first photons that would tell you this core collapse has happened are these UV photons," says astronomer Kevin Schawinski of the University of Oxford in England, who led the study. "It's really precious information, because when the shock wave then hits the surface of the star, there's very little information about the star that's left," he says, because the star itself is destroyed."

Astronomers typically detect supernovae as flashes of visible light, which occur a few days after the actual explosion as the radiation cools down. To probe deeper, they have to either get creative—or lucky. In March, astronomers reported that they had observed an outburst of x-rays from the supernova SN 2008D when it happened to go off while a telescope was trained on its home galaxy to study a previous supernova.

In the new study, Schawinski and his colleagues compared a database of supernovae spotted by their visible light with measurements of ultraviolet light from stars taken in early 2004 by NASA's GALEX (Galaxy Evolution Explorer) satellite. They found one explosion—dubbed SNLS-04D2dc—for which the parent star began to shine in the ultraviolet for seven hours. They estimated that the shock wave reached the surface of the star four hours into the UV flare-up.

Schawinski says the duration of the ultraviolet glow indicates the size of the star. In this case, it was apparently in a bloated form known as a red supergiant. He says the hot radiation from the shock wave would have rapidly heated the star, shifting its color from red to blue-white and finally into the ultraviolet.

So are researchers going to turn around next week and find an even earlier sign of an impending supernova? That's pretty unlikely. They believe there would indeed be two earlier signals of core collapse—a burst of ghostly particles called neutrinos that are generated during nuclear reactions and a clatter of spacetime ripples known as gravitational waves—but either would be challenging to detect. (Gravitational waves, in fact, still elude astronomers, although experiments are underway to find them.)

Too bad, because they would shed light on the central mystery of supernovae, Schawinski says: "How does the energy go from the core collapse to destroy the star?" People have ideas, he says, but "the step in between those two we still don't understand well at all."