By Richard D. Jacobs, M.D.
Star clusters are simply visual groupings of stars. When stars group together in tight balls, they are called "globular clusters." Globular clusters are very interesting because they are said to be among the oldest structures in our galaxy. They tend to orbit in great loping circles around the core of the galaxy. Globular clusters are usually pretty bright and many can be imaged easily from a bright city location.Open, or galactic, clusters are irregular in size and shape and gravitationally bound together.