An artist's impression of two white dwarf stars orbiting each other, with helium being drawn from one to the other. (Tony Piro)

Astronomers examining data from a supernova first observed in 2002 have determined that it represents a new class of rapidly exploding star.

The explosion may have resulted from a binary star system where helium flowed from one white dwarf star to another, building up a layer of gas that detonated in a thermonuclear explosion.

The supernova, dubbed SM 2002bj, was three to four times faster than a standard supernova, disappearing in about 20 days, compared to three to four months for a typical supernova.

"This is the fastest evolving supernova we have ever seen," said Dovi Poznanski, an astronomer with University of California, Berkeley, and Lawrence Berkeley National Laboratory. "Its brightness just dropped like a rock."

The spectrum of the explosion had a strong signature for helium and none for hydrogen. It also suggested the possible presence of vanadium, an element never before seen in a supernova.

All of these qualities led the researchers to conclude that this type of supernova had never been seen before. Their work was published online this week in Science Express.

"This supernova is qualitatively different from the complete disruption of a white dwarf, known as a Type Ia supernova, or the collapse of an iron core and rebound of the surrounding material, so-called 'core-collapse supernovae,'" or Type II supernova, said co-author Alex Filippenko of UC Berkeley.

This type of explosion was first predicted two years ago by theoretical physicists at the University of California, Santa Barbara. Because this new type of supernova is similar to a Type Ia but is about one-tenth as bright and lasts about one-tenth as long, a Harvard physicist jokingly called it a Type ".Ia" (point one A), and the name has since stuck.

When the supernova was first observed in 2002, it was erroneously classified as a standard Type II supernova and filed away. Poznanski found the spectrum of SM 2002bj while looking through data on Type II supernovae.