Symmetrical, spherical shape, concentrated to center in most cases. Around 20 to 100 pc in diameter. Brightest stars are red giants. Short main sequence, predominant giant branch. HR diagrams of clusters Mass of the brightest stars on the main sequence is a good indication of the age of the cluster.
Open clusters show a nice range of ages.
Few or no red giants. Many protostars still seen in vicinity Age a few million years. No O stars and few B stars, but lots of A and fainter. Age around 100 my. The Oldest open clusters are quite rare.