A luminous flash in the sky that appeared in 2007 seems to fit the bill for an unusual but long-predicted type of supernova, according to a new study. So-called pair-instability supernovae, in which stars more than 140 times the mass of the sun collapse and ignite a thermonuclear explosion, have been predicted by theory for years, but actually observing one requires finding these massive stars that existed early in the universe but are much rarer now.

Astrophysicist Avishay Gal-Yam of the Weizmann Institute of Science in Rehovot, Israel, and his colleagues report in the December 3 Nature that a blast detected in April 2007 by a supernova survey likely originated from a star with a mass roughly 200 times that of the sun. (Scientific American is part of Nature Publishing Group.) Such large stars stave off gravitational collapse by relying on counterbalancing outward pressure of photons emanating from the core.

Equilibrium is lost, however, if the photons become so energetic as to give off pairs of electrons and their antimatter counterpart, positrons. That particle production reduces the outward pressure exerted by the photons, and gravitational collapse wins out, leading to a swift contraction and subsequent nuclear blast.

By tracing the slow fade of the 2007 supernova, known as SN 2007bi, over an observation campaign lasting more than 18 months, Gal-Yam and his colleagues were able to determine the elements in the light spectra and deduce the kind of progenitor that powered the explosion. The ejected matter, the study's authors conclude, was illuminated by at least three suns' worth of radioactive nickel, synthesized in the initial explosion. About half the progenitor star's mass was contained in its core. So, with the detonation of a 100 solar-mass core, the authors write, "theory unambiguously predicts a pair-instability supernova."

In at least one other case, researchers had expressed "good reason to suspect" that a supernova was of the pair-instability variety, but the claim for SN 2007bi appears more robust. In an accompanying commentary in Nature, Norbert Langer of the University of Bonn in Germany notes that Gal-Yam and his co-authors "report that SN 2007bi provides the first evidence of such an explosion." And University of Texas at Austin astronomy professor Volker Bromm told Sky & Telescope that "SN 2007bi is the first really convincing case of a pair-instability supernova."

But the case is not settled for all pair-instability experts. "I give it a 50 percent chance of being a pair-instability supernova," astrophysicist Stanford Woosley of the University of California, Santa Cruz, told Sky & Telescope. "The case has to be very tight. I'm not sure yet that it is, though in deference to the authors, I think they have tried and passed every test they could think of."

Artist's conception of a pair-instability supernova progenitor: NASA/CXC/M. Weiss