STAR CLUSTERS
Stars rarely form in isolation but as part of a group of stars. Throughout the Milky Way galaxy, there are regions where many stars congregate in gravitationally-bound groups, known as star clusters. These stars were largely born together from the same interstellar nebulae. This is useful to astronomers because it gives them an opportunity to study stars with age and origin already accounted for.
OPEN CLUSTERS
Relatively young.
A few dozen to a few thousand stars.
Found in spiral arms.
1,000 to 2,000 open clusters in the Milky Way.
Stellar associations
Very young and more loosely bound than open clusters.
Stars identified by commonalities suggest they formed from the same source.
90% of all stars originate as part of an association.
Hundreds of light years across.
Moving groups
GLOBULAR CLUSTERS
Relatively old (10-13 billion years old).
Up to a million stars.
Found in galactic halo.
About 150 globular clusters in the Milky Way.