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Mysterious supernova in a class of its own

Astronomers have discovered what they think is a new type of supernova, which could have implications for how the universe is measured.

Dr Dovi Poznanski of the University of California, Berkeley and colleagues, identified the supernova event in the galaxy NGC 1821, some 160 million light years away.

Their results appear today in the journal Science.

Astronomers classify supernovae as one of two basic types.

Type 1 supernovae involve white dwarfs that explode after accreting, or gathering, material from companion stars in binary systems.

Eventually when they build up enough mass, they explode as a supernova.

Because type 1 supernovae usually explode at a set size, they're called standard candles and are used to gauge cosmic distance.

Type 2 supernovae occur when massive stars 10 to 20 times larger than the Sun explode after running out of fuel.

Astronomers determine whether a supernova is type 1 or 2 by examining the radiation they emit.

But when Poznanski and colleagues examined supernova SN 2002bj, they found it didn't fit either category.

The spectrum showed the presence of helium and intermediate-mass elements in SN 2002bj. But there was no clear sign of hydrogen or iron, which is typically found in a type 1 supernova.

Poznanski says, this suggests it evolved extremely fast, producing an unusual combination of elements.

"It only barely resembles a type 1a supernova."

Unique

Dr Stuart Ryder, an astronomer with the Anglo Australian Observatory, says, "a lack of iron usually means the progenitor, a white dwarf, didn't explode completely, but simply ignited the material it had sucked up from its companion star, which is what a 'nova' does."

Poznanski believes the answer may be a previously hypothesised event involving a helium detonation in a white dwarf, ejecting only a small envelope of material in the process.

In the case of SN 2002bj the visible ejecta was just a tenth the amount expected.

Ryder says the event could be something between a nova and supernova.

He says it raises questions as to whether type 1 supernovae are still useful as standard candles to measure cosmic distance, as well as estimating the amount of dark energy in the universe, thought to make up 70% of the mass of the universe.