What happens when an enormous star blows up?

An international team, led by scientists in Israel, and including researchers from Germany, the US, UK and China, tracked a supernova -- an exploding star -- for over a year and a half, and found that it neatly fits the predictions for the explosion of a star of over 150 times the sun's mass. Their findings, which could influence our understanding of everything from natural limits on star size to the evolution of the universe, appeared recently in Nature.

'It's all about balance,' says team leader Dr. Avishay Gal-Yam of the Particle Physics and Astrophysics Department. 'During a star's lifetime, there's a balance between the gravity that pulls its material inward and the heat produced in the nuclear reaction at its core, pushing it out. In a supernova we're familiar with, of a star 10 -100 times the size of the sun, the nuclear reaction begins with the fusion of hydrogen into helium, as in our sun. But the fusion keeps going, producing heavier and heavier elements, until the core turns to iron. Since iron doesn't fuse easily, the reaction burns out, and the balance is lost. Gravity takes over and the star collapses inward, throwing off its outer layers in the ensuing shockwaves.'

The balance in a super-giant star is different. Here, the photons (light particles) are so hot and energetic, they interact to produce pairs of particles: electrons and their opposites, positrons. In the process, particles with mass are created from the mass-less photons, and this consumes the star's energy. Again, things are thrown out of balance, but this time, when the star collapses, it falls in on a core of volatile oxygen, rather than iron. The hot, compressed oxygen explodes in a runaway thermonuclear reaction that obliterates the star's core, leaving behind little but glowing stardust. 'Models of 'pair supernovae' had been calculated decades ago,' says Gal-Yam, 'but no one was sure these huge explosions really occur in nature. The new supernova we discovered fits these models very well.'

An analysis of the new supernova data led the scientists to estimate the star's size at around 200 times the mass of the sun. This in itself is unusual, as observers had noted that the stars in our part of the universe seem to have a size limit of about 150 suns; some had even wondered if there was some sort of physical constraint on a star's girth. The new findings suggest that hyper-giant stars, while rare, do exist, and that even larger stars, up to 1000 times the size of the sun, may have existed in the early universe. 'This is the first time we've been able to analyze observations of such a massive exploding star,' says Dr. Paolo Mazzali of the Max Planck Institute for Astrophysics, Germany, who led the theoretical study of this object. 'We were able to measure the amounts of new elements created in this explosion, including approximately five times the mass of our sun in highly radioactive, freshly synthesized nickel. Such explosions may be important factories for heavy metals in the Universe.'

This massive supernova was found in a tiny galaxy -- only a hundredth the size of our own, and the scientists think that such dwarf galaxies could be natural harbors for the giant stars, somehow enabling them to surpass the 150 sun limit.

'Our discovery and analysis of this unique explosion has given us new insights into just how massive stars can get and how these stellar giants contribute to the makeup of our Universe', says Dr. Gal-Yam.'We hope to understand even more when we find additional examples from new surveys that we have recently begun to carry out, covering large, previously unexplored areas of the Universe.'

Dr. Avishai Gal-Yam's research is supported by the Nella and Leon Benoziyo Center for Astrophysics; the Peter and Patricia Gruber Awards;William Z. & Eda Bess Novick New Scientists Fund; the Legacy Heritage Fund Program of the Israel Science Foundation; and Miel de Botton Aynsley, UK.