It's a bird! It's a plane! No, it's the hugest, most spectacular star explosion ever recorded, and astronomers are marveling at what they've just seen.

When a really big star (some are 10 times as big as our sun) runs out of fuel, it dies in a dramatic explosion called a supernova. Scientists have observed supernovas before, but this latest outburst had 100 times as much energy as a typical explosion. In just 2 months, it spit out more radiation than the sun will emit during its 10-billion-year lifetime.

Calculations suggest that the star that exploded had more than 150 times as much mass as our sun. That makes it the heaviest star on record (For comparison, our sun is more than 333,000 times more massive than Earth.).

Two teams of scientists—one led by researchers from the University of California, Berkeley, the other led by researchers from the California Institute of Technology in Pasadena—recently announced their observations of the eruption, now called SN 2006gy. (SN stands for "supernova.") Astronomers had first observed the SN 2006gy last September. It lies in a galaxy 240 million light-years from Earth.

Because of SN 2006gy's immense size, astronomers' usual theories of star death don't work. One popular theory, for example, says that after a supernova has blown off its steam, its core collapses into a superdense body like a neutron star or a black hole. This model can't account for the extreme brightness of SN 2006gy.

Instead, the two teams that reported the blast propose that the huge star's core was unusually hot. That heat, they think, caused high-energy gamma rays inside the core to destroy each other. Gamma rays help keep stars intact, so their destruction made the star unstable. The star would have then collapsed and blown up, leaving nothing behind but a huge, very bright explosion.

This type of supernova, called a pair-instability supernova, was probably common among the universe's very first stars, scientists suspect. SN 2006gy may then be a rare, modern example of an ancient phenomenon. Eta Carinae, a star in our own galaxy, might be poised to undergo the same type of explosion, researchers say.

But there are also some problems with the pair-instability model, and still other theories are possible. For example, theorist Stan Woosley of the University of California, Santa Cruz proposes that a series of instabilities over the course of a year, rather than a single huge explosion, spewed material out of the dying star. The final blast would have then lit up all the previously ejected material. Woosley's theory would explain the brightness without requiring that the star be so freakishly large.

Whatever the explanation for the explosion, the display it left behind was out of this world.—Emily Sohn