Two faint supernovas unlike any star explosions ever seen before may have exploded in the same way, or they may differ, but in either case are breaking down categories that distinguish one type of stellar death from another.

Supernovas are violent explosions that signal the death of stars, and scientists think such stars explode in two basic ways. Either the core of a star at least eight times as massive as our sun collapses violently and ejects the its outer envelope, or a small and aged white dwarf star sucks in material from a nearby companion until it grows too massive and explodes.

But at least one of two newfound supernovas appears to have broken those molds.

A partial explosion

One of the supernovas in question is SN 2005E. It is unusually faint and quickly fading, but also has properties commonly associated with the supernova core collapse method, such as a lack of hydrogen and silicon in its light spectrum. Such supernovas, called Type 1b, are thought to result from collapse of a star that has lost its hydrogen envelope.

However, SN 2005E occurred in the halo of an isolated galaxy known as NGC 1032, which no longer supports star formation. Outside of star forming galaxies, stars generally don't get massive enough to support core collapse.

"We didn't find any trace of star formation," said study researcher Hagai Perets of the Harvard University Center for Astrophysics. "That's the basic strange thing about this supernova."

Perets and his colleagues argue that SN 2005E must have occurred when a white dwarf star accreted helium-rich material from a nearby star and then partially exploded, as some models indicate is possible. Burning of helium would explain why 2005E was extremely rich in calcium — something that isn't normally seen in type 1b supernovas.

The researchers estimate that 2005E ejected about a third of a solar mass when it exploded, which is much less than supernovas of either the core collapse or regular white dwarf variety. Nearly half of that material was calcium.

Current models can't explain all the unusual features of the supernova, but Perets told that if partial explosions of white dwarf stars do happen, they might explain a number of puzzling facts in astronomy, such as why some stars and intergalactic dust are so rich in calcium and possibly even mysterious gamma rays that researchers have proposed might be coming from dark matter.

Supernova oddity

The second newly found supernova, SN 2005cz, was also relatively faint and rich in calcium for a type 1b.

But based on the pattern of light it emitted, Japanese researchers argue that it must have resulted from core collapse in the conventional way. They propose that a star of 10 to 12 times as massive of the sun lost its hydrogen envelope to a binary partner before exploding.

According to the researchers, modeling indicates that such a low-mass star would produce the right ratio of calcium to oxygen seen in the supernova.

"We can explain [it] in the context of traditional core-collapse scenario," said Koji Kawabata of Hiroshima Astrophysical Science Center.

A possible problem with this interpretation is that 2005cz occurred in an elliptical galaxy, NGC 4589, where stars of sufficient mass for core collapse would normally be rare. The researchers cite evidence that the galaxy shows signs of recent star formation.

Perets said he and his colleagues found no evidence of star formation near 2005cz. He said the supernova is most likely another partially exploded white dwarf, like 2005E.

Bolstering their argument, Perets and his colleagues identified a total of eight supernovas that match the general properties of 2005E, including faintness and high calcium content, and 2005cz is one of them.

"I would be surprised if '05cz turned out to be core collapse because of the statistical distribution of the class of eight," says David Branch, an astrophysicist at the University of Oklahoma in Norman, who was not involved in either study.

Branch says the white dwarf explanation is probably on the right track.

"We can't really sort this out right now," he said. "The main thing is it will stimulate observational and modeling advances, because people will want to get to the bottom of this."

The new results are published in the journal Nature.