Every day, the newly remodeled Hayden Planetarium in New Yorks Museum of Natural History shows a sell-out crowd the wonders of the galaxy and the universe beyond. Most nights, its dark.

Over the past few months, though, the dome and its computer equipment have fired up again after hours, to track the motions of stars that never were. Thats when the planetarium, instead of showing what scientists already know, becomes their instrument for finding out more.

The nighttime visitors are astrophysicists who study globular clusters -- 10 billion-to-12 billion-year-old clumps of thousands of stars, all pulling and pushing and bashing each other in an area that would more typically contain a dozen. In this "giant mosh pit in space," as the Museums Carter Emmart puts it, every stars motion is affected at every moment by every other.

Not wanting to wait hundreds of thousands of years to see what happens, stellar dynamists simulate these motions on computers. But the sheer amount of arithmetic in such a "multiple-body problem" is beyond what all but the speediest computers could ever do.

"A good globular cluster has a million stars, so for each time step, you need to work out a million times a million forces," said Piet Hut, an astrophysicist at the Institute for Advanced Study in Princeton, New Jersey. "And you want to model a billion years. Thats a lot of time steps."

So in the 1980s cluster dynamists made themselves experts in super-fast computers. In fact, the most powerful devices in the world are the ones they have built for their simulations. Their next machine, set to start crunching numbers next year, will perform a hundred trillion simple arithmetical calculations per second.

"That is how we get simulations," Hut said. "We cant do a million stars yet. We do 30,000. Then well work up to hundreds of thousands. And well do a million stars by 2005."

The results of all this data crunching are long columns of numbers that map the position and velocities of moving stars. That translates into video images of moving dots that swirl on the scientists desktop computers like a strangely purposeful screen-saver. The screen images, though, are two dimensional, and clusters exist in three. The little dots are only a rough approximation.

After visitors leave the Rose Center (Big Bang Theater within, shown here), astrophysicists have been powering up the state-of-the-art Hayden Planetarium to test their models of starcluster dynamics.

A few months ago, Hut, who is a fellow of the museum, was attending a conference at the planetarium. He realized that the software that turns computer code into a Milky Way for the daytime audience could transform the cluster simulators columns into a sky full of careening, caroming stars. Sitting under the dome, the scientists could see whats happening as if they were hovering outside the cluster or even sitting inside it.

"Seeing it in three dimensions is different, said Simon Portegies Zwart, a stellar dynamist at the Massachusetts Institute of Technology.

Hut got in touch with Emmart, a computer and graphics expert who designs and runs the Planetariums simulations. Certainly doable, Emmart said. In fact, the project, which began last summer, has gone so well that the scientists are working with the creators of Virtual Director -- the software the museum uses to run its shows -- and theyre starting to think about ways the model could depict non-stellar processes, such as gases in turbines, global economies or panicky crowds, for instance.

"I think the basic issues we are trying to address -- large ranges in spatial and temporal scales, and the need to handle huge volumes of data -- are by no means unique to starcluster dynamics, so we would hope that many of our techniques would find application well beyond astronomy," McMillan said.