Twenty years ago this month, a blast of gamma rays struck nine spacecraft scattered across the solar system, a flash so intense that it swamped their radiation detectors.

Scientists scrambled to discover its origin. They knew exactly when the blast hit the widely scattered satellites and probes, so they could triangulate to find the source. Astonishingly, it came from beyond our Milky Way galaxy.The source was the remains of a supernova in the Large Magellanic Cloud, a dwarf galaxy that orbits the Milky Way. Estimates of its distance range from 160,000 light-years to 180,000 light-years; a light-year is the fantastic stretch that light travels in a year at 186,000 miles per second.

"It emitted more energy in one second than the sun emits in a thousand years," said Gerald Fishman of NASA's Marshall Space Flight Center, Huntsville, Ala.

For the gamma radiation to be that fierce after traveling so far, the cause had to be a previously unknown type of object.

Not only did the satellites record a tremendous jolt of gamma rays, but the emission continued at lesser levels for several minutes with fluctuations that repeated every 8 seconds. Nobody had ever seen this kind of periodicity before in gamma rays.

What strange kind of celestial object could have done this?

A scientific adventure unfolded as teams of experts tried to find an explanation. The quest ended last year with the discovery of the source: a rare, previously unknown type of star called a magnetar.

That theory and observation came together perfectly to solve the mystery is one of the triumphs of science.

"It's one of the most exciting things in high-energy astrophysics in the last couple of years, proving the existence of magnetars," Fishman said.

Chryssa Kouveliotou, a graduate student in Germany, began studying the radiation readings and looking for data elsewhere in the sky, using every available satellite and monitor.

"Every time we detected an emission that was remotely similar to a soft gamma repeater emission," her team would check it out, said Kouveliotou, who is now with the Universities Space Research Association at Marshall Space Flight Center.

Two additional sources were discovered releasing the same kind of repeating gamma radiation, both in the Milky Way.

Then in 1992, Robert Duncan of the University of Texas at Austin and Chris Thompson of the University of North Carolina at Chapel Hill announced a theory that the cause could be a spectacular type of neutron star, one equipped with an almost unbelievable magnetic field. This imagined beast was dubbed a magnetar.

When a large, aging star reaches the end of its life, it may use up its fuel and explode in a blast bright enough to be seen in distant galaxies. As the outer layers fly off into space, its core shrinks and collapses.

If the original star is at least 10 to 15 times the mass of our sun, the collapse is unstoppable and the star becomes a black hole -- a point in space whose mass is so concentrated that it sucks in anything nearby and distorts time and space. Its gravity is so strong that it sucks in anything nearby, and not even light can escape from it.

However, if the original star is at least five times the size of our sun but not big enough to form a black hole, the core collapse does stop. The atoms that made up the star break apart into protons, electrons and nuclei. Because of the monstrous pressure, all of the protons and electrons crush together, forming neutrons.

The star, about 10 miles across, is amazingly dense. A half-mile solid crust of iron nuclei forms on the surface. The interior is a liquid metallic material.

"It's an incredible object," Fishman said. "One cubic centimeter about the size of a sugar cube weighs 100 million tons. We're talking about a whole star about the size of Salt Lake City made up of this material.

"And they're rotating very rapidly, once every several seconds. And every time they rotate they give off pulses of X-rays, and we see those pulses."

With about 10 percent of neutron stars, an even stranger situation occurs. If the star is rotating swiftly during the collapse -- going at least 200 rotations per second -- currents in the metallic liquid create a dynamo. The dynamo effect sets up a ferocious magnetic field.

The object is a magnetar.

According to Fishman, if you were to replace the moon with a magnetar, its magnetic field would pull keys out of your pockets here on Earth. The magnetic field of Earth is 0.6 Gauss, while a magnetar would have a magnetic field of 10 to the 14th power or 15th power Gauss.

Ordinary neutron stars have a stable crust. According to Duncan and Thompson, the magnetic field should stress a magnetar, sometimes cracking the crust in a ferocious quake. When that happens clouds of particles burst out, releasing gamma and X-rays.

The theory predicts that a magnetar's spin should be slowing as the magnetic field creates a breaking effect.

Analyzing data from satellite readings, Kouveliotou and her colleagues found that one source of gamma ray emissions was slowing its spin by 1 second every 300 years. An immense magnetic field was slowing the neutron stars.

She had proven that magnetars exist.

In August 1998, a month after Kouveliotou published her analysis in Nature, a fourth magnetar was discovered by the Ulysses probe, out near the orbit of Jupiter. Two other satellites also recorded it. Its period was about 7.5 seconds.

The changes in the bursts seemed to show that a powerful, distorted magnetic field was readjusting. The fourth magnetar had been discovered.

"They emit randomly, which is unexpectedly," Kouveliotou said. "You cannot predict when they're going to be active, and they just emit in bunches."

One telling factor is that fluctuations in the bursts resemble the way energy is released in earthquakes, she said. There might be a large quake, then several smaller aftershocks.

"We were even able to determine the rate for slow-down of the star," she said. With that assumption, it's possible to calculate the strength of the magnetic field. The fields are the most powerful ever observed.

"It was extremely exciting," she said. "It's wonderful when we can solve mysteries, especially one that you work on for 10 years."