Among the members of the Milky Way are globular star clusters. An easy way to see how helpful they are, take the globulars listed in Messier's catalog, and plot them on a grid representing the sky. Despite the lack of objects to the far south, a function of Messier's northerly latitude, it becomes apparent that most of the clusters are gathered in the area centered in western Sagittarius. It is globular star clusters that point the way to the center of the galaxy. Globular star clusters are very old, in the neighborhood of 10 billion years.
Astronomers can measure a cluster's age by analyzing the light of the stars within it.
The brightest stars in a globular cluster are red giant stars. Stars past their prime, no longer using hydrogen for fuel, but helium. It is thought that these clusters were formed when the gasses of the Milky Way were in roughly a spherical shape before or during the collapse into a disk. In forming early the globular star clusters took themselves out of the galaxy's evolution, and follow the same paths they did when they formed, swarming, like bees, around the center of the galaxy. Stars are very massive for their size, and feel only the gravitational force of the rest of the galaxy.