Starship Asterisk*

bosonm wrote:The August 6th APOD showed the Garrad Comet and Messeir 15, a closed globular cluster. My question is: what keeps the cluster so tightly packed? Is there a massive star in the center that acts as a strong gravitational attractor, or could there be a black hole there?

There could be a massive black hole, but it so, it would have little to do with keeping the stars together. A globular cluster is held together by the same thing that keeps a single star together: the mutual gravitational attraction of each of its parts to the rest. In a globular, each star is in orbit around a point that represents a center of mass from its current position. There need be no physical object at that dynamical center of mass.

bosonm wrote:The August 6th APOD showed the Garrad Comet and Messeir 15, a closed globular cluster. My question is: what keeps the cluster so tightly packed? Is there a massive star in the center that acts as a strong gravitational attractor, or could there be a black hole there?

A good place to start for your answer is the wikipedia article on globular clusters.