Ages from HR diagrams range from less than a million years up to several billion. Very few open clusters older than the age of our sun. Tens of thousands to millions of stars. 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.