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The Question

(Submitted June 18, 2009)

What is known (or presumed) about the movement of stars within a globular cluster of stars, such as M13 in the constellation Hercules? Is the whole cluster rotating in the same direction, or are the stars randomly revolving around the center? Will the cluster eventually flatten out?

The Answer

Thank you for your question. Like other stellar spheroids (the halo and bulge of our galaxy, elliptical galaxies), globular clusters are supported against collapse via the mutual gravitational attraction of their constituent stars by random motions. This acts as a sort of pressure that opposes gravity, with the average velocity analogous to the temperature of a gas. This is in contrast to spiral galaxy disks that are rotationally supported. Globular clusters may also have a small net rotation, but at a speed that is significantly smaller than the random velocities -- otherwise globular clusters wouldn't be globular, but flattened.

Michael Loewenstein and Amy Fredericks
for `Ask an Astrophysicist'

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