Scientists led by a Pasadena astronomer said Tuesday they have used an extraordinarily long exposure from the Hubble Space Telescope to create sharp images of a 4-billion-year-old galaxy cluster that confirms theories that the universe is evolving--"and at a pretty rapid rate."

At the same time, in the background of those images, the astronomers serendipitously stumbled upon what appears to be a previously unknown, 10-billion-year-old galaxy cluster. When studied further, that cluster may answer fundamental questions about how and why galaxies formed.

Along with other recent finds, the images should help partially rehabilitate the Hubble's reputation. The $1.5-billion space telescope had been written off as "virtually blind" after the National Aeronautics and Space Administration found a fundamental flaw in its main imaging mirror after its 1990 launch.

Salvaging images from the Hubble's current wide-field and planetary camera--an updated model is scheduled to be installed late next year--has not been simple, but it is possible, said Alan Dressler of Pasadena, the Carnegie Institution astronomer who announced the findings Tuesday.

For example, Dressler's team kept the space telescope fixed on one spot for six hours spread over 10 Earth orbits just to gather enough light to see the galaxy cluster clearly. Even then, the images had to be enhanced by computer to produce the photographs made public on Tuesday in Washington.

Daniel Weedman, an astronomy professor at Pennsylvania State University, praised the effort behind the discovery as "almost heroic."

These new images--produced by Dressler, Augustus Oemler of Yale University, James E. Gunn of Princeton University and Harvey Butcher of the Netherlands Foundation for Research in Astronomy--show some galaxies colliding and violently ripping one another apart. This phenomenon, which astronomers refer to as a "cosmic Cuisinart," could explain how some galaxies evolve.

"We have actually seen galaxies change over time. That's very important to proving the Big Bang theory," said Dressler, a pioneer in studying the morphology, or structure, of galaxies. "The universe was a much different place even 4 billion years ago than it is today. . . . The universe is evolving, and at a pretty rapid rate."

Other astronomers were equally enthusiastic about the images of the distant galaxies, which are so far away that light from them has taken 4 billion years to reach Earth--and, thus, give scientists the opportunity to see how those galaxies appeared about the time our own Milky Way galaxy was created.