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Globular cluster dynamics.

Globular star clusters are so dense, why don't they collapse in on themselves from their own gravity?--Karry Ritter, Madison, Wisc.

Globulars only look like their stars are packed tightly. The stars are actually so small, compared to the spaces between them, that they almost never collide. So, with no way to "dissipate" (lose) orbital energy, stars just keep swarming around the cluster's center of gravity like bees. Stars in a globular often do swap orbital energy with each other during wide misses, but the total energy of motion remains the same, and this keeps the stars up.

But that's not the entire story. In the long run, random energy swaps tend to speed up low-mass stars and slow down high-mass ones. So low-mass stars tend to "float" to the outer parts of a cluster and may even fly off ("evaporate"), while the heavier stars sink closer to the center. About 20% of globulars have gone through a runaway "core collapse" in this manner before reaching a new equilibrium, so their inner regions are much more tightly concentrated than those of other globulars. Even here, though, star collisions are rare.


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Title Annotation:Astro Q&A
Author:MacRobert, Alan
Publication:Sky & Telescope
Article Type:Brief article
Geographic Code:1USA
Date:Feb 1, 2009
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