Globular Cluster Orbits
The Milky Way galaxy has 157 known globular clusters. They take several hundred million years to orbit the centre of the galaxy along elongated ellipses.
In 1914, Harlow Shapley began a series of globular cluster studies, published in about 40 scientific papers. He examined cepheid variables in the clusters and used the period–luminosity relationship to establish their distances. He used Doppler shift to establish their velocities. He characterised their orbits, located the centre of the Milky Way and established its approximate size.
Source: Globular Cluster - Wikipedia