An illustration of the pair-instability supernova. (NASA/CXC/M.Weiss)

Astronomers have identified a massive supernova as a new type of violent stellar explosion predicted by physicists to exist, but never before seen.

Researchers say that the supernova 2007bi, first observed 2½ years ago, was the result of the collapse of a star 200 times the size of the sun and the resulting nuclear explosion.

The supernova took place in a dwarf galaxy about 1.35 billion light-years away.

Avishay Gal-Yam of the Weizmann Institute of Science in Israel and his colleagues say that the supernova's extreme brightness and its evolution over time are best explained by a pair-instability supernova. Their research is published this week in Nature.

The type of supernova is predicted to occur in super-massive stars that don't form a dense iron core but an oxygen core.

Particles of light — photons — in the oxygen core interact with the nuclei of atoms to form electrons and their equivalent anti-matter particle, positrons.

The conversion of photons to pairs of electrons and positrons causes a violent contraction in the star, leading to a runaway nuclear explosion.

Stars as big as the one that led to this supernova are thought to have been common in the early universe, so pair-instability supernovas may have played an important role in determining how the young universe evolved.

Another supernova, 2006gy, the brightest ever recorded, is also speculated to have been a pair-instability supernova.