

They focused on three quasars, luminous objects thought to be powered by massive black holes, from that early time; one was the most distant quasar ever observed. They stretched their horizons at the sight of a new image from the Hubble Space Telescope showing a glittering expanse of galaxies so remote that some of them had not been seen before; a few may be the earliest in the universe.

For other scientists here at the winter conference of the American Astronomical Society, there was astonishment and mystery enough closer to home, in Earth's own Milky Way galaxy. X-ray emissions revealed the messy eating habits of the black hole at the galactic center, and radio waves showed the magnetic field there to be surprisingly tangled.

The newest and most impressive discovery was made on the fringes of the Milky Way. Two teams of astronomers have observed a previously unseen band of hundreds of millions of stars orbiting beyond the galaxy's main disk, more than twice as far from the galactic center as the solar system.

The most likely explanation, astronomers say, is that the ring stars are remnants of smaller galaxies that came too close and were captured by the Milky Way's overpowering gravity. This is more evidence, then, that in part the galaxy grew to its present size at the expense of less fortunate neighbors.

''This is a vivid smoking gun of the disruption of a satellite galaxy,'' said Dr. Bruce Margon of the Space Telescope Science Institute in Baltimore, who was not on either discovery team.

Part of the ring was observed by scientists from the Sloan Digital Sky Survey, a project for mapping one-quarter of the sky in three dimensions that is based at Sunspot, N.M. A team of Australian, British and Dutch astronomers, working at a telescope in the Canary Islands, then saw other sections, leading scientists to recognize that the band of stars appeared to reach around the entire galaxy.

The ring's encircling diameter is estimated at 120,000 light-years, the teams reported. Its thickness is about 10 times that of the rest of the galaxy, extending well above and below the galactic plane. Gravity, primarily from unseen dark matter of an unknown nature, holds the ring of up to 500 million stars -- about the stellar population of small galaxies -- in a nearly circular orbit.

Such a vast congregation of stars in a coherent ring had remained hidden from view because it lies in the same plane as the Milky Way disk and so was obscured by intervening stars, gas and dust. It was hard to distinguish the ring stars from the other matter and impossible to recognize their number or their organization in a discrete torus.

''Our entire picture of the Milky Way is being changed with this discovery,'' said Dr. Brian Yanny of the Fermi National Accelerator Laboratory outside Chicago, who was co-leader of the Sloan Survey group that detected the ring. Stars are more numerous and more closely spaced toward the center of galaxies and are expected to thin out more or less evenly in the outskirts.

The other leader, Dr. Heidi Jo Newberg of Rensselaer Polytechnic Institute in Troy, N.Y., said, ''When we find large groups of stars formed into rings, it's an indication that at least part of our galaxy was formed by a lot of smaller or dwarf galaxies mixing together.''

The Milky Way originated about 10 billion years ago, and it probably captured some neighbors several times since then, growing into a collection of about 400 billion visible stars. Other researchers at the meeting reported seeing a faint trail of stars that appear to be leftovers from a similar capture by Andromeda, a nearby galaxy not unlike the Milky Way.

But Dr. Annette Ferguson of the University of Groningen in the Netherlands, a member of the Canary Islands observing team, offered a possible alternative explanation for the ring. The stars there might have come from inside the galaxy's disc, she suggested, and gravitational interactions could have disturbed their orbits, causing them to migrate outward and into the ring.

In any case, scientists said, the ring promises to be an ideal place to study the mysterious dark matter, which makes up most of the mass of the universe, and its role in shaping cosmic structures.

Other researchers have been taking a closer, sharper look into the heart of the Milky Way, a region hidden from ordinary view by a fog of dust and gas. What they are finding in radio and X-ray observations surpasses immediate understanding.

The magnetic field near the galactic center, it seems, is more chaotic than previously thought.

Using the radio telescopes of the National Science Foundation's Very Large Array in New Mexico, a team of astronomers studied strange filaments produced by the interaction between the galaxy's magnetic field and high-velocity electrons. The filaments seen before were all aligned in nearly the same direction, like iron filings near a bar magnet.

But in the new observations, astronomers were surprised to see many more filaments oriented in various directions, tangled, they said, ''like a bowl of spaghetti.''

Dr. Joseph Lazio of the Naval Research Laboratory, a leader of the observing team, said, ''The magnetic field in the center may not be all that organized or all that strong.''

Another team leader, Dr. Namir Kassim, also of the Naval laboratory, could only conclude, ''The Milky Way's center is an exciting, mysterious region that, once again, has given us a surprise.''

And then there was the latest report on the eating habits of the black hole believed to lurk at the Milky Way's core.

A black hole, by definition, cannot be observed directly. It is an extremely compact massive object so dense that nothing, not even light, can escape its gravity. Scientists infer its presence by observing the turbulence and high-energy emissions its gravity creates on surrounding material.

Some black holes in other galaxies may weigh as much as several billion times the mass of the Sun, scientists think, but the Milky Way's is thought to be a puny three million times the solar mass.

In the longest and most sensitive X-ray observations made so far of the Milky Way's center, NASA's Earth-orbiting Chandra X-ray Observatory zoomed in on the region of the suspected black hole.

''We are getting a look at the everyday life of a black hole like never before,'' said Dr. Frederick K. Baganoff of the Massachusetts Institute of Technology. ''We see it flaring on an almost daily basis.''

What did this mean? The hot gases of these X-ray flares could be crumbs from the black hole's last meal of stellar matter, scientists said. Its table manners are atrocious.

The frequency of the X-ray flares suggests that the black hole eats often, scientists further said, but their weak intensity suggests that its meals are more like snacks than banquets.

''Although it appears to snack often, this black hole is definitely on a severe diet,'' Dr. Baganoff said. ''This could be because explosive events in the past blew away much of the gas from the neighborhood of the black hole.''

Photos: Acting as a cosmic lens in space, the gravity of a cluster of galaxies has amplified the light from galaxies near the dawn of time and smeared them into arcs of red and blue in this picture taken by the Advanced Camera for Surveys on the Hubble Space Telescope and released last week. ABELL 1689, whose galaxies appear large and yellowish, is about 2.2 billion light-years away. DISTANT GALAXIES would be invisible without the amplification of the gravitational lens. They appear as small red dots. ARCS AND DOTS One galaxy, about 12 billion light-years away, has been split by the lens into two images, A and B. Many blue arcs, C, representing galaxies at a range of about 10 billion light-years, form a partial bull's-eye pattern around the center of the cluster. By analyzing these images, astronomers can map the mass, including unseen dark matter in the cluster.; The region of the black hole at the center of the Milky Way, captured by the Chandra X-ray Observatory. Scientists say the black hole makes a mess as it eats stellar matter. (Top, Hubble Space Telescope/NASA; below left, Chandra X-ray Telescope/NASA)