An international team of astronomers has obtained the clearest images yet of the merger of two distant clusters of galaxies, calling it one of the most powerful cosmic events ever witnessed. The merging clusters bring together thousands of galaxies and trillions of stars into a single, bigger cluster, perhaps destined to merge yet again someday. Such a merger is the eventual fate of our own Milky Way.

Astronomers see clumps of galaxies everywhere in the sky. Yet they know that at the beginning of time, the universe was relatively smooth. There were no galaxies or even stars, only diffuse gas. They believe that over the ages, gravity brought the gas molecules together into brighter, warmer places that continued to attract matter until the first stars formed within the first several hundred thousand years. Stars are thought to have eventually gathered into galaxies, large gas clouds containing billions of stars.

University of Michigan astronomer August Evrard says the process continued to blend individual star clouds together into bigger clusters, becoming the most massive structures in the universe. "Just as wispy clouds formed in the morning can merge by afternoon into a huge cumulus thunderhead, so small clouds that assembled early in the history of the universe that might contain one galaxy merged together to create larger ones containing many hundreds of galaxies by today," he said.

The process continues. In the Astrophysics Journal, astronomers describe the current merger of a large galaxy cluster named Abell 754 in the constellation Hydra with a smaller cluster that its gravity is attracting. It and previous such mergers observed in the last 25 years confirm the theory of how star structures grew larger into super galaxies. But this observation of an event 800 million light years away is the clearest yet, obtained with the European Space Agency's orbiting XMM-Newton observatory. The researchers estimate that the merger began 300 million years ago.

The weather analogy for the event is appropriate, because scientists call it a cosmic storm. University of Hawaii astronomer Patrick Henry says the galaxy clusters are colliding like two high pressure weather fronts and creating conditions like a hurricane. The turmoil is tossing galaxies far from their paths and churning shock waves of superheated gas.

"A lot of fireworks happen when the most massive things into the universe slam into each other," he said. "Galaxies whiz by each other at one million miles [more than 1.5 million kilometers] per hour. The cluster gas is heated to hundreds of millions of degrees. The amount of energy released during one of these mergers is the same as that made by the stars in all the galaxies in the cluster during the entire age of the universe."

In fact, Mr. Henry says the amount of energy released during a galaxy cluster merger is second only to that released during the creation of the universe in the so-called Big Bang more than 13 billion years ago. A U.S. space agency scientist studying XMM-Newton data, Richard Mushotzsky, says it is the most powerful such merger on record and the second or third largest ever witnessed at three million light years in diameter.

"If you could see this image with your eyes, it occupies a size about roughly half the size of the moon, and it is many hundreds of millions of light years away," he said.

Mr. Mushotzsky says if scientists can understand the dynamics of merging galaxy clusters, they will better understand the evolution of the universe.

The astronomers say our Milky Way is facing such a fate in a few billion years because it is part of a small group of galaxies moving toward the Virgo Cluster. But the merger boom may be over in a few billion years beyond that. Scientists believe a mysterious dark energy appears to be accelerating the expansion rate of the Universe. Its objects are flying apart at increasing speeds and clusters may eventually never have the opportunity to collide with each other.

Long before that, the cluster merger involving Abell 754 will have been complete, according to Patrick Henry. "Although the weather map analysis shows that there is a large storm presently occurring in Abell 754, the long-term forecast is for fair weather about seven billion years in the future," he said.