The Small Sagittarious Star Cloud
Star clusters come in two types, "Globular" clusters and "Open" clusters (also called "Galactic" clusters). Globular Clusters surround the center of the galaxy in a spherical halo, whereas Open Clusters populate the relatively thin disk of the galaxy.
Globular Star Clusters
Globular clusters have a full, round appearance (like M 22) and contain up to a million or more stars. They are the oldest objects in the Galaxy, 10 to 15 billion years old. Most of the stars in a globular cluster are faint main sequence stars with a few hundred of them nearing the end of their lives as red giant stars.
The origin of globular clusters is mysterious. Some recient findings from the Hubble Space Telescope shows that some Globular clusters in other Galaxies are much younger that those in the Milky Way.
Photo of Globular Cluster M22
Open Star Clusters
The stars in open clusters are a lot younger than most of the stars in our galaxy and usually contain material that has been enriched in heavy elements by generations of stellar evolution. Open clusters usually contain 100 to 1,000 stars and are formed in the gas and dust rich arms of spiral galaxies. Our sun began it's life in interstellar conditions similar to those that produce open star clusters. One of the most famous open clusters is The Pleiades, which is 490 light-years distant and made up of nearly 500 stars, 6 of which are extremely bright blue-white giants.
Photo of Open Cluster M11
Images and Text: Copyright 1996, Christopher Galfo.