This is a simple question, about the movement of stars in globular clusters. I was thinking about it, and at first I assumed that they would be moving in a similar direction, like stars in a spiral galaxy. But then I remembered the problem about, "you can't comb hair on a spherical head." If the stars are all spinning in one direction, those on the axis of the spin won't be moving, and so will collapse toward the center. So it would end up as a donut.

So the simple question is, how do the stars move in a globular cluster? Are they moving randomly? Or is there some kind of systematic motion within the cluster?

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The motion of stars in a globular cluster is essentially random. Each star orbits the center of mass of the cluster. The inclinations are randomly oriented and the eccentricities are randomly distributed between 0 (circle) and 1 (parabola - escape trajectory).

Note that this is a first approximation, since the stars are so tightly packed that the stars rarely (if ever) repeat an orbit (another randomizing effect).

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T. Anderson

Thanks. I asked partly because I was wondering if globular clusters had any angular momentum of their own. So there's something very different between spiral galaxies, which seem to spin around together, and globular clusters? It seems that most astronomical objects have angular momentum, but I guess there are some things -- another that comes to mind is nebulae that maybe don't have angular momentum?

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