Nature's Own H-Bombs

Puzzling bursts of energy in space may be thermonuclear blasts

Since 1963, the Pentagon's Vela satellites have been vigilantly circling the earth, using their electronic sensors to detect radiation from surreptitious nuclear explosions outside the U.S. Though these orbital watchdogs have identified only a few suspicious events, they have accidentally uncovered a major astronomical mystery: violent outbursts of energy, in the form of X rays and gamma rays, that are observable only above the earth's atmospheric shield. Such puzzling "highenergy transients," as scientists call them, rarely last more than ten seconds, yet they pack a wallop as great as a billion billion one-megaton H-bombs.* What could possibly cause such implausibly powerful explosions far out in space?

The scientists' tentative answers have ranged from warfare between extragalactic civilizations to the total annihilation that occurs when ordinary matter meets antimatter. Now they have settled on a simpler explanation. At a gathering of more than 100 astronomers and astrophysicists at the University of California at Santa Cruz, most of the experts agreed that the starbursts, at least those emitting X rays, are distant thermonuclear explosions. In effect, nature is setting off its own H-bombs. University of California Astrophysicist Stanford Woosley, the conference chairman, said: "It is as if an object 100,000 times brighter than the sun were there one second and gone the next."

Such cosmic violence, the scientists postulated, probably results from the stormy relationship between two companion stars that orbit each other. In this scheme, one member is an ordinary star like the sun. The other is a so-called neutron star; this is essentially a dead star that has run out of nuclear fuel. As its fires die out and the star's gases cool, they explode, with the remnants collapsing upon themselves, and the star shrinks to incredible density. Typically, such a star once had a girth of 100 million miles or more, but now is only a few miles in diameter. A teaspoonful of its material weighs as much as 100 million tons, and the star's gravitational force is so strong that it pulls away a steady stream of gases, mostly hydrogen and helium, from its larger companion. As the gases spiral toward the neutron star, they heat up, reaching such high temperatures (up to 10 millionDEGC) and densities that the atoms of hydrogen smash into each other and fuse. This causes a runaway thermonuclear explosion that spews a torrent of X rays. During the period of calm after the explosion, ranging from a few hours to a few days, enough fresh material piles up on the surface of the neutron star to fuel another giant blast.

Astronomers so far have identified about 30 X-ray bursters, all of them near the center of the Milky Way galaxy. Some have even been observed from the ground with conventional telescopes because the X rays they emit are frequently accompanied by flashes of ordinary light. By contrast, the more powerful gamma-ray bursts, apparently a different phenomenon, are harder to explain. For one thing, they are far more common; they are discovered at the rate of 150 a year. They also occur all over the sky, rather than just in the vicinity of the galactic center. One extraordinary event detected by an international network of satellites on March 5, 1979, still has scientists awed and baffled. It was 100 times as bright as any previous outburst and was spotted in the direction of the Large Magellanic Cloud, a satellite galaxy 150,000 light-years from our own galaxy. The great explosion may have been connected with a supernova, the cataclysmic death of a giant star. Unlike the X-ray bursts, the gamma rays never seem to strike from the same direction. Says French Astronomer Kevin Hurley: "Studying gamma-ray bursts is like trying to study a mirage."

Woosley, who was one of the first scientists to propound the H-bomb explanation for the X rays, thinks the same process is at work in the gamma-ray bursts. In the case of gamma rays, however, the neutron star has powerful magnetic fields that compress the incoming gases so densely that an even more powerful thermonuclear blast is triggered. Other researchers like M.I.T.'s Paul Joss speculate that a comet or asteroid careering into the neutron star might be the cause of an H-bomb-like explosion that produces gamma particles. Still another hypothesis suggests that the blast occurs when the neutron star is rocked by the stellar equivalent of an earthquake. Imaginative as these ideas are, not even their proponents are completely satisfied. "Nature is extraordinarily clever," says Joss. "Certainly she is far more clever than astrophysicists."

* A one-megaton bomb is equivalent to a million tons of TNT. This file is automatically generated by a robot program, so viewer discretion is required.