ANTIMATTER PLUME FOUND IN MILKY WAY.

Byline: Malcolm W. Browne The New York Times

Astrophysicists announced Monday that they had discovered what appears to be a monster fountain of antimatter erupting outward from the core of the Milky Way.

They said the discovery would compel them to alter their image of the disk-shape galaxy. In the revised image, it is as if a burst of steam were spurting upward from the yolk of a fried egg.

The discovery, reported at a meeting here, was made using the Compton Gamma Ray Observatory, a satellite launched by the National Aeronautics and Space Administration six years ago. The four instruments aboard the observatory detect, measure and record gamma rays: invisible rays that have higher energies than all other forms of radiation, including X-rays.

The antimatter was discovered as a result of a series of observations made by the satellite since November.

Antimatter - a form of matter in which the electrical charge or other property of each constituent particle is the reverse of that in the usual matter of our universe - cannot be directly detected in space. But when antimatter comes into contact with ordinary matter, the two kinds of matter instantly annihilate each other, producing gamma rays, which can be detected by instruments outside Earth's shielding atmosphere.

The newly discovered plume of antimatter rises some 3,500 light-years above the disk of Earth's galaxy, which is about 100,000 light-years across. But even if this cloud of antimatter were to reach Earth, the scientists reassured their audience, it would cause no harm, because the concentration of antimatter particles in the cloud are extremely diffuse.

Moreover, only positrons are believed to be present, not antiprotons or entire antimatter atoms. Although forms of antimatter other than positrons have been created by laboratories on Earth, they have never been unequivocally identified elsewhere.

A prevailing theory is that the Big Bang of creation produced approximately equal amounts of matter and antimatter, which promptly annihilated each other, but that a small excess of ordinary matter was enough to create the universe as we know it, with very little surviving antimatter.

Astrophysicists representing the Naval Research Laboratory, Northwestern University and the University of California at Berkeley, who collaborated in the discovery announced Monday, said the cause and the nature of the antimatter fountain were puzzling. It might be a more or less continuous shaft of antimatter streaking northward from the galactic center, or it might be a cloud, separated from the main part of the galaxy.

Charles Dermer of the Naval Research Laboratory surmised that the fountain might be a mixture of gas, boiling away from violently dying stars near the center of the galaxy, and a stream of positrons.

When positrons (also called positive electrons) collide with ordinary, negatively charged electrons, they mutually destroy each other and spawn gamma rays that have a very specific energy: 511,000 electron volts. The Compton satellite is able to identify the specific energies of the gamma rays it sees, and the ``fountain'' was seen by tuning the instrument to the characteristic electron-positron annihilation energy.

It has long been known that Earth's galaxy looks something like a fried egg with pinwheel spirals. Earth lies in one of these spirals, and as we look up at the Milky Way on clear nights, we look inward toward the galactic center. But dust and gas obstruct any light from the galactic center, and so astronomers depend on other types of radiation to deduce the galaxy's innermost structure.

Earlier gamma-ray observations have suggested that there is an enormous black hole at the center of the galaxy and that as matter is drawn into the hole, it is so intensely heated that antimatter positrons are formed.

This would account for some of the bright gamma rays flowing from the galactic center, but scientists realized that it would not explain the much larger flow of gamma radiation coming from the Milky Way as a whole. Even after a relatively small black hole, dubbed the ``Great Annihilator,'' was discovered fairly near the galaxy's center several years ago, there still seemed to be too few gamma rays to account for all the gamma rays seen by wide-angle detectors in space.

The latest discovery, by the 6-year-old Compton satellite, could account for the missing gamma rays.