From Dust to Dust: A Unique Supercomputer Provides a Glimpse of How Galaxies Evolve

By Peterson, Ivars | Science News, January 14, 1989 | Go to article overview

From Dust to Dust: A Unique Supercomputer Provides a Glimpse of How Galaxies Evolve


Peterson, Ivars, Science News


FROM DUST TO DUST

The images lack the flash and action of a video game. Nonetheless, the patterns weaving across the screen have a mesmerizing effect. Strands and filaments in speckled shades of red, orange and yellow float against a background of blue pinpoints. Lumps accumulate, then disappear. Bubbles collapse, then form again.

This "galaxy in a box," a remarkable computer simulation, suggests how a galaxy evolves, as marked by the ebb and flow of interstellar dust and gas against a background of stars forming and dying. In this simulation, minutes of computer time stretch into billions of years on a galactic time scale.

By matching such computer models with astronomical observations, scientists hope to determine the fundamental factors affecting galactic evolution. Already, the results of simulations show that a wide range of initial conditions produce plausible patterns of star formation and destruction within a galaxy.

What makes the galaxy-in-a-box simulation possible is the use of a one-of-a-kind, experimental supercomputer that combines the power of 576 individual processors, each one roughly as capable as the original Cray-1 supercomputer. But even with such a machine, it takes careful programming, tremendous attention to detail and a number of mathematical tricks to get the right images on a screen quickly enough so that scientists don't need to wait hours or even years for results.

Developed over the last three years, the galaxy-in-a-box simulation represents the point efforts of astrophysicist Kevin H. Prendergast and mathematicians David V. Chudnovsky and Gregory V. Chudnovsky, all from Columbia University in New York City, and M.M. (Monty) Denneau of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y., who designed and built the GF11 supercomputer.

Stars condense out of clouds of coalescing gas. During their lifetimes, fueled by internal nuclear engines, they slowly evaporate, or lose mass, by returning gas to the interstellar medium. Massive stars end their lives in supernova explosions, leaving behind small remnants in the form of neutron stars or black holes and scattering the bulk of their mass as dust and gas. Eventually, this ejected material gets recycled, becoming part of another generation of stars.

This picture of the birth and death of individual stars emerges from a combination of physical theory and observations of nearby astronomical objects, especially of regions where stars now appear to be forming. Missing is the bigger picture of how entire galaxies, consisting of billions of stars, form and evolve.

The Columbia-IBM collaboration attempts to construct a realistic computer model of galactic evolution embodying what is known about the interaction of stars and interstellar gas. The model incorporates Newton's laws of motion, star formation and evaporation, the effects of stellar winds and starlight in heating up and pushing around interstellar gas, and the cooling of interstellar gas by the emission of infrared radiation.

The initial computer model developed by Prendergast and his collaborators illustrates star and gas interactions in two dimensions, on a square grid consisting of 100 rows of 100 cells each. That grid represents a slice through a portion of a galaxy about 1,000 light-years across. On this scale, stars are too numerous to be pictured individually, and the stars and gas are treated computationally as two separate but intermingled fluids -- not unlike computer simulations in hydrodynamics showing the mixing of two different liquids. For each cell at every time step, the computer calculates the new gas and star densities, taking into account the effects of star formation, mass loss, and stellar heating and radiative cooling.

On a typical supercomputer, such computations would take days. But Denneau's GF11 supercomputer is special. The hardware fills a room. …

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