Researchers have offered a new explanation for an unprecedented stellar explosion caught in the act earlier this year.

A team of astronomers announced in May that they had witnessed the initial flash of x-rays of supernova 2008D, spotted in the spiral galaxy NGC 2770 some 90 million light-years away.

Their take: that it confirmed the long-standing x-ray breakout model of supernovae, in which the shock wave sent out from the core of a collapsing star sheds x-rays when it reaches the star's surface. But some were skeptical because the x-ray flash lasted for a relatively long time—several hundred seconds—compared with the tens of seconds expected for a typical supernova.

Another research team argues in Science today that the x-ray "breakout" was more likely the product of jets of high-speed material flung out when the star collapsed into a black hole. They base their conclusion on the optical afterglow of the supernova.

Researcher Paolo Mazzali of the Max Planck Institute for Astrophysics in Garching, Germany, and his colleagues were among those who focused optical telescopes on SN 2008D for several weeks after NASA's Earth-orbiting Swift Gamma-Ray Burst observatory happened to detect it while astronomers were monitoring an earlier supernova.

Based on its spectrum, they estimated that 2008D exploded with about six times the energy of a regular supernova and had a mass roughly seven times that of the sun. They say these features put it somewhere between typical supernovae and more powerful versions—sometimes referred to as hypernovae—that are thought to give rise to gamma-ray bursts, the most energetic explosions known.

The results confirmed that SN 2008D had apparently lost the hydrogen envelope that surrounds any star, as in a hypernova and certain less powerful supernovae. They detected only the kinds of elements found deep inside massive stars: silicon, calcium and oxygen appeared first, followed by helium.

The spectrum indicated that the shock wave that blew apart the star was moving at 10 to 20 percent of the speed of light—close to speeds seen in hypernovae, Mazzali says—but that it petered out. They interpret the pattern as evidence that the imploding star began to spray material in a weaker version of the high-speed jets that are thought to spring from black holes formed by hypernovae, but slowed when they encountered helium—an element absent in hypernovae.

"It almost made a gamma-ray burst, but didn't," Mazzali says. He says the event was mistaken for an everyday variety type Ib supernovabecause it didn't give off gamma rays. "This could actually be the way that most of these stars explode when they have sufficiently high mass to form a black hole," he says.

Unfortunately, researchers do not know for sure which supernovae form black holes, so it is premature to claim that SN 2008D produced one, says astrophysicist Adam Burrows of Princeton University, a co-author on the initial SN 2008D study.

That Harvard University–led group contended that the x-rays were produced not by jets but by the supernova shock wave as it pushed through the hydrogen that had blown from the dying star. Burrows, however, says that interpretation was itself questionable, because the data, although very high quality, was patchy due to limited telescope time.

"We don't really know which model could be correct," he says. "There may be objects where each model has its place." But without more data to go on, he wouldn't put his money on either of them for SN 2008D. "I think both interpretations are a stretch."