Open Clusters are moderately close-knit, irregularly shaped groupings of stars. They usually contain 100 to 1000 members and are usually about 4 to 20 pc in diameter. Our Sun is possibly inside or on the edge of a loose open cluster centered only about 22 pc away toward the constellation Ursa Major. The Pleiades is an example of an open cluster. About 900 open clusters are concentrated along the Milky Way band, indicating that they lie in the plane of our galaxy.
Associations are cousins of open clusters.
They often have fewer stars but are larger in size and have a looser structure. Some large associations include an open star cluster within them. They may have 10 to a few hundred members and diameters of about 10 to 100 pc. Associations are classified as O association or T association, depending on whether the prominent stars are O and B blue stars or T Tauri variables. Over 100 O associations have now been cataloged.