A giant galaxy at the heart of a distant cluster may be the largest such object in the known universe, a mammoth group of stars with a diameter of 6 million light years - 60 times wider than Earth's Milky Way, astronomers said.

The object at the center of a galactic cluster known as Abell 2029 is some 1 billion light years from Earth, that is, the distance light travels in 1 billion years at a speed of 186,000 miles per second."This cluster was discovered by George Abell in the '50s," said Stephen Boughn, co-author of a paper appearing Thursday in the journal Science. "The central dominant galaxy at the middle of this cluster has long been known as one of the most luminous galaxies in the known universe."

Building on the work of others, Boughn and his co-workers - Juan Uson of the National Radio Astronomy Observatory and Jeffrey Kuhn of Michigan State University - set out to measure the dim, diffuse light given off by stars stripped away from galaxies in Abell 2029 by gravitational interactions.

Using a 36-inch telescope at the Kitt Peak National Observatory in Arizona, the astronomers were able to detect a "halo" of diffuse light surrounding the massive galaxy at the center of Abell 2029.

A mosaic of 16 images made with light-sensitive electronic "chips" called charge-coupled devices, or CCDs, showed the diffuse halo to be remarkably uniform, leading the astronomers to conclude they were looking at a single galaxy.

"Our contribution was primarily to get a technique for measuring diffuse light," Boughn, an astronomer at Haverford College in Haverford, Pa., said in a telephone interview.

"When we analyzed the data for this cluster, we indeed found diffuse light in the cluster. When we plotted that light as a function of distance from the center . . . we couldn't tell where the (central) galaxy ended and where this light began. It was just a perfectly smooth curve."

Measurements indicate the galaxy has a diameter of some 6 million light years, more than 4 1/2 times the diameter of the previous record holder and some 60 times wider than Earth's Milky Way, which has a diameter of about 100,000 light years.

"Whether the diffuse halo in Abell 2029 is considered to be an extension of the central . . . galaxy or a component of diffuse cluster light is, to some extent, a matter of semantics," the astronomers wrote in Science.

But the scientists believe they are seeing a giant galaxy, primarily because the halo's light spreads very smoothly outward from the central galaxy and because the halo has the same elliptical shape as the central galaxy.

"Quite simply, the central galaxy is indistinguishable from the diffuse light that extends from the cluster center," they said. "Thus this `object' is among the largest and most luminous galaxies observed."