By Jon Cartwright of Nature magazine

Some of the brightest stellar explosions in the Universe should be classified together as a new type of supernova, according to an international collaboration of researchers. The group has catalogued six explosions that cannot easily be explained by any process yet known.

When stars several times more massive than our Sun die, they explode, forming supernovae. The process varies, but the result is a massive radiation of energy that can outshine an entire galaxy. Sometimes the radiation is produced by the radioactive decay of freshly generated elements, whereas in other cases it comes from an explosive release of heat or from a collision between debris ejected from the star and material surrounding it.

Robert Quimby, an astronomer at the California Institute of Technology in Pasadena, and his colleagues are presenting a new class of supernova that is not driven by any of these processes.

In a study published online today in Nature, the researchers describe four previously unidentified supernovae, along with two known events that had confounded astronomers: SN 2005ap, which in 2007 was identified as the brightest supernova ever detected, and SCP 06F6, which made headlines in 2008 because it had a spectrum that didn't match any known types of supernova.

The supernovae in the new class have several distinguishing features. One is that they are very bright--about ten times more luminous than type Ia supernovae, the most commonly recorded type. Another is that their main emission is not visible light, as for most supernovae, but ultraviolet radiation.

Unrivalled power

The question is what causes their brightness. In type Ia supernovae, the lasting glow comes from the radioactive decay of isotopes such as nickel-56. But Quimby's group doesn't think that this is the case with the new supernovae, because their ultraviolet light fades away about three times too fast to match the rate of nuclear decay.

The light could come from the explosion itself, but the researchers say that the sheer brightness would require the star to give off an "unrealistic" amount of energy. The only remaining conventional explanation--that the light is generated in interactions between debris from the star and hydrogen-rich surrounding material--seems unlikely because the light that they emit shows no indication of the presence of hydrogen.

Perhaps, says Quimby's group, the exploding stars were so big--say, 100 times the mass of our Sun--that they would become very unstable, throwing off bits of material before their final explosion. That final explosion would then interact with the material previously cast off, producing a dazzling light show.

One the other hand, the early stages of the supernovae might have created spinning, highly magnetized neutron stars or 'magnetars'. The very strong magnetic field of such stars would slow down their spin, and the excess energy of their motion would be released to make the supernovae unusually bright.

The process is likely to be debated for some time. "The death of very massive stars is still quite uncertain," say Hideyuki Umeda and Ken'ichi Nomoto, astronomers at the University of Tokyo. "How mass is ejected from these stars, and how long before the explosion, is still unknown and a controversial issue."

But the new supernovae don't have to be assigned to a named class to be useful to astronomers. Their extreme brightness means they should illuminate distant parts of the Universe, perhaps literally shedding light on the formation of very faint dwarf galaxies.

This article is reproduced with permission from the magazine Nature. The article was first published on June 8, 2011.