It lies about 30,000 light years from Earth and contains about a million stars. It is just one of over 100 globular star clusters that roam the halo of the Milky Way galaxy. The stars in these clusters are all at least 12 billion years old--among the oldest known. Globular clusters probably appeared even before our Galaxy itself was fully formed. As such, these remarkable star systems offer vital clues to the earliest epochs of the universe.
Here is the cluster M15:
It lies 40,000 light years from Earth. This system has undergone core collapse, in which its central core has shrunk to the point where its density has reached millions of stars per cubic parsec. Under these conditions, physical interactions (i.e. collisions) between stars may be quite commonplace. M15 contains several millisecond pulsars--neutron stars spinning hundreds of times per second, the results of recent mass transfer in close binary systems.
The record holder for both numbers and rotation rates of millisecond pulsars is the marvelous southern cluster 47 Tucanae:
This cluster, which is well on its way toward core collapse, boasts some two dozen millisecond pulsars in its core, with periods as short as 2 ms. The reason why 47 Tuc has so many high-speed pulsars, even though its central density is substantially less than M15, is the subject of active research.