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The images on the right show the four supernovas discovered by the Palomar Transient Factory. On the left is a comparison image of the same part of the night sky before each supernova appeared. (Robert Quimby/Caltech/Nature)

A new kind of supernova 10 times brighter than other exploding stars has been discovered by an international team of astronomers.

"It's a big surprise that we not only found something new, but something so bright that you'd expect someone would have come across them before," said Robert Quimby, an astronomer at the California Institute of Technology and the lead author of the study published Wednesday online in Nature.

Supernovas are explosions of stars that appear as bright new objects in the sky. They typically last for several weeks and then fade away.

Quimby said it's "pretty unusual" to find a new kind of supernova, since astronomers had previously believed that all types of supernova in the part of the universe that we can see had been discovered (although they thought there might be undiscovered types at the very edge of the universe).

The new "superluminous" type of supernova is rare — Quimby estimates there is only one for every 10,000 regular supernovas. But because they are brighter, they can be seen from much farther away.

Just four supernovas of the new type were found in 2009 and 2010 using the Palomar Transient Factory telescope in southern California, among more than 1,000 supernovas detected by the telescope.

Nicholas Law, a postdoctoral fellow at the Dunlap Institute for Astronomy and Physics at the University of Toronto and the project scientist for the telescope, said looking for supernovae is a major portion of what the telescope does.

It takes 100-megapixel images of 150 different areas of the sky — each a few times larger than the full moon —  twice each night. Computers sort through the images to find bright spots that weren't there before.

The researchers analyzed the signals of the very bright supernovas, including how their brightness changed with time and what colours of light they shone.

"We found that they were rather odd and they didn't match any other type of supernova that we've know up until now," said Law, one of the co-authors of the study.

Unusually ultraviolet

The light shone by the new supernovas is more ultraviolet in colour than other supernovas. That indicates that the supernovas contain mainly oxygen and very little hydrogen or metals such as calcium or iron that are usually seen when a star explodes.

Quimby said the evidence shows they don't explode by the two methods that normally produce a supernova. One is a thermonuclear explosion, but the superluminous supernovas fade away too quickly to be powered by radioactive decay. The other method requires hydrogen.

He thinks the superluminous supernovae may be caused by the death of a supermassive star, about 100 times larger than a sun that throws off a bubble of material before it explodes. The brightness may be caused by the explosion crashing into the cast-off bubble.

Another possibility is that the supernova is powered by energy from a magnetar, a fast-spinning neutron star with a strong magnetic field that is sometimes produced by a previous supernova.