Rather than being relatively stable, galaxies constantly are colliding and merging to form different shapes in "a cosmic game of Russian roulette," new computer calculations suggest.

Astronomers long have understood that billions of years ago in the early days of the universe it was fairly common for galaxies - systems of millions of stars, gas and dust - to run into each other or to unite in other ways. But it was thought such interactions, and the resulting formation of new galaxies, dwindled as the universe matured.Now, a growing number of scientists believe "galactic mergers" are still occurring, and one astrophysicist has devised a new computer model adding considerable weight to such theories.

In a study published in the journal Nature, Joshua Barnes of the Institute for Advanced Study in Princeton, N.J., uses computer calculations to model a process by which groups of disk-shaped galaxies may merge to form elliptical galaxies.

Galaxies occur in myriad shapes. Many, such as the Milky Way, are disk-shaped spirals, while others look like ellipses or spheres and a relative few are totally irregular. They tend to be in large groups, most in relatively loose association. Astronomers also have found a lesser number of smaller galaxy clusters, called compact groups, held together by tighter gravitational bonds.

According to Barnes, compact groups are "short-lived things" in a universe that astronomers estimate to be from 10 billion to 17 billion years old.

Barnes's model indicates a compact group of six disk-shaped galaxies probably would merge in a few orbital times, or "about 100 million to 1 billion years," to form a "single star pile" - a bright, elliptical galaxy.

"As two galaxies pass by each other they exert tidal forces, forces that then slow them down," Barnes said in a telephone interview. This would disrupt galaxy rotation to push the galaxies and their halos of dark matter into what he called a "series of close encounters," multiple collisions and mergers.

The astrophysicist theorized a similar but weaker gravitational drag eventually would convert a "significant fraction" of loose groups into compact groups, which would be transformed into elliptical galaxies.

In an accompanying editorial, Francois Schweizer of Washington's Carnegie Institution said the computer model sheds new light on the puzzle of why galaxies display various shapes.

"It is becoming increasingly clear that gravitational interactions, collisions and even mergers continue to reshape and feed galaxies right to the present day," Schweizer wrote. "The special thrill provided by Barnes's numerical experiment is to make us bystanders in a cosmic game of Russian roulette, yet instead of death from hits, there is rebirth."