Frontiers | Pages 190-191 | See Linked Version

Two neutron stars complete their inexorable merging, beaming powerful jets of gamma rays in opposite directions. Such events may be a source of gamma-ray bursts, enormous outpourings of energy seen throughout the universe. Other gamma-ray bursts may arise from so-called hypernovas, theorized exploding stars displaying exceptional violence.

This variety of energy outputs doesn't make it any easier to figure out what's going on. Imagine seeing fireworks for the first time. You'd be hard pressed to come up with a single explanation for the giant red and green chrysanthemums, the white sparkles that glimmer for many seconds, and the orange squiggles that whistle as they plunge toward you--not to mention those concussive duds that leave your ears ringing. You would probably conclude that more than one explosive mechanism was at work.

The same may be true for gamma-ray bursts. Two popular models have emerged from the pack. To no one's surprise they both involve the densest forms of matter in the universe--the only kind capable of spawning such detonations. In one model two neutron stars whirl around each other in close orbit. We know that such systems can arise if each member of a binary pair of stars blows up in a supernova and leaves behind a neutron-star core. Einstein's general theory of relativity predicts that the two stars will slowly approach each other in a death spiral. The angular momentum lost from the system during this process gets converted into gravitational waves, which speed away from the stars like ripples on a pond. After hundreds of millions of years, the neutron stars will merge. Computer simulations show that the last few seconds of this event may liberate more energy than all the stars in the universe shining during those seconds. Most of that energy will be in gamma rays.

Other researchers think the most powerful gamma-ray bursts of all come not from neutron stars but from black holes, at the instant of their birth. This model calls for an exotic fate to befall certain collapsing stars. If the original star is especially massive and spins especially fast, it may not explode when the core collapses into a black hole. Rather, the model claims, the bulk of the star's gas would spiral into a disk around the black hole at nearly the speed of light. Within about 20 seconds the hole would gulp the entire disk. This ferocious process would heat the gas to 20 billion degrees and shoot stupendous jets of energy out the top and bottom of the disk, where the gas is least dense. Particles within these jets would crash violently into one another and into nearby clouds of gas in space, sparking the bursts of gamma rays. Because "super" lacks enough power as an adjective to describe such chaos, astrophysicists coined the word "hypernova" for a black hole born in this way.

There is some observational evidence that gamma-ray bursts do indeed funnel their energy into tight channels. The jets from gamma-ray bursts are similar to the (continued)