Huge blasts of high-energy radiation which sweep out across the Universe almost daily are most probably linked to the death throes of giant stars.

Astronomers have long been puzzled by what causes so-called gamma-ray bursts - colossal flashes of radiation which can, within a few seconds or minutes, match the energy our Sun will produce in its entire lifetime.

Now, work by two international research teams suggests hypernovae - exploding supermassive stars - may hold the key to at least some of these mysterious blasts.

The biggest stars in the Universe can explode at the ends of their lives to leave behind small, dense, rapidly rotating spheres of neutrons - neutron stars. The researchers think that if these neutron stars then later collapse in on themselves to form black holes they could give rise to the huge bursts of detected radiation.

The teams studied two recent gamma-ray bursts using the American Chandra X-ray Observatory and the BeppoSax satellite, an Italian project with Dutch participation.

Nuclear fusion

The data showed that, as the bursts expanded like an inflating bubble, they moved through nearby gas clouds enriched in iron. This element is produced in some of the nuclear fusion reactions which power stars and can be thrown out into space when the very biggest stellar objects explode.

The simplest explanation for these findings, the researchers suggest, is that a hypernovae can eject the iron-rich cloud shortly before the gamma-ray burst.

But, judging by the low density of one of the clouds, this may be several years after the primary explosive event.

Although what specifically initiates the burst is not yet clear, one possible scenario involves the collapse of the dense object, such as a neutron star or black hole, that is left behind in the aftermath of the initial explosion.

The international teams have published their research in the journal Science.

One team was led by Filippo Frontera, of the Universita' di Ferrara in Ferrara, Italy, and Istituto di Tecnologie e Studio delle Radiazioni Extraterrestri (TESRE), CNR, in Bologna, Italy.

The other was led by Luigi Piro, of the Istituto de Astrofisica Spaziale, CNR, in Rome, Italy.