In the interstellar medium--the vast, low-density regions of space between the stars--churn molecular clouds, turbulent masses of magnetized hydrogen gas and dust that ultimately produce new stars. Molecular clouds are extremely cold nebulae, with temperatures ranging from about -440 to -370 degrees Fahrenheit and consist mostly of hydrogen molecules.

When parts of a molecular cloud eventually collapse because of gravitational instability, they create a dense region warmer than the surrounding interstellar medium. This region, called a protostar, continues to increase in pressure and temperature until thermonuclear reactions at its core transform it into a new star.

But why, despite the mass and temperature of hundreds of observed molecular clouds in a given galaxy, don’t more of them simply collapse and form massive starbursts? "That would mean we would see much higher star formation rates in the galaxy than we actually observe," explains Fabian Heitsch, an astrophysicist at the University of Wisconsin at Madison (UWM). Heitsch says that, given the number of molecular clouds, one might expect to observe 200 newly forming stars per year, but actual observations only average around three per year.

Molecular clouds have been thought to be stable because of, among other things, the presence of magnetic fields, which, if sufficiently strong, could prevent the molecular clouds from collapsing under their own gravity. The interaction of magnetic fields with charged fluids (in this case, hydrogen) is called magnetohydrodynamics (MHD). In previous simulations, Heitsch, together with Mordecai-Mark Mac Low, Pakshing Li and Michael L. Norman, investigated the role of turbulence and magnetic fields in preventing the collapse of molecular clouds, relying heavily on NCSA's SGI Origin2000.

In his current work, Heitsch and his collaborators, Ellen Zweibel at UWM and Adrianne Slyz at the University of Oxford, are investigating how magnetic fields might diffuse in a turbulent environment, ultimately leading to the collapse within the molecular cloud. Heitsch recently ran two-dimensional simulations on NCSA's now-retired SGI Origin2000 and is currently preparing to run three-dimensional simulations on Copper, the center's IBM p690.

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Access Online | Posted 1-27-2004

The photogenic Horsehead Nebula, itself part of a molecular cloud. Stars form when regions of molecular clouds collapse from gravitational instability, as in the case of the young, bright star emerging at the top left edge. It is thought that the presence and resistance of magnetic fields prevent many more such collapses--and formations of new stars--than actually occur. Image courtesy of NASA, NOAO, ESA, and The Hubble Heritage Team (STScI/AURA).