Star clusters are as the name says, a cluster of stars. They come in different forms (spherical-clusters, open-clusters) and consist out of 1000’s of stars concentrating around one point.
Spherical clusters (or globular clusters) are still a create mystery to todays astronomers. They are in orbit around the milky way and seem to contain only old stars (it appears no new stars form there). Why are so many old stars attract to that spot?
More information on the specific cluster and image is available when clicking on the image.