The Second-Biggest Bangs

If

a star exploded somewhere in space with a violence dwarfed only by the Big Bang, you'd think folks would notice. Sometime yesterday, however, just such a cosmic detonation probably took place, and almost nobody on Earth was the wiser. Sometime today there's likely to be another one.

The cosmic cataclysms are known as gamma-ray bursts-- distant explosions, invisible to the human eye, that in seconds release more energy than our own little sun will put out in its 10 billion-year lifetime. Though astronomers have studied hundreds of gamma bursts, they have never determined what they are. Soon that may change. Last week astrophysicists from around the world gathered in Huntsville, Ala., to discuss the gamma-ray phenomenon and plan for the launch of a satellite that will turn the sharpest eye ever on the puzzling blasts.

Thirty years ago, military satellites first noticed flashbulb-like gamma bursts going off throughout the cosmos. The Compton Gamma Ray Observatory, opened in 1991, discovered that the bursts were surprisingly common--300 or so occur each year--and remarkably distant. "They are more than halfway to the edge of the visible universe," says NASA astrophysicist Neil Gehrels.

Although they're not certain what is causing the blasts, astronomers attending last week's conference have some ideas. When ordinary stars burn out, they may become neutron stars, dense bodies with gravity fields so powerful that a marshmallow falling into one would release as much energy as a thousand hydrogen bombs. If two of these bodies began orbiting each other, they would ultimately collide, leading to titanic gamma-ray blasts. Other researchers believe the bursts are due to especially large supernovas, great stellar outbursts called hypernovas.

In January, NASA will launch the HETE-2 satellite, which will study not only gamma-ray bursts but also their lingering afterglow of X rays and optical light. Three years later, a larger satellite with keener vision will conduct similar work in more depth. "Classical astronomers thought stars produced a steady emission in one wavelength," says Gehrels. "Now we realize we have all these flashing, transient things going on." Modern astronomers --with their modern machines--may at last determine what some of those strangest things are.