Star Clusters

Star clusters are groups of stars that formed from the same molecular cloud and are gravitationally bound.

Open Clusters

• Contain dozens to a few thousand stars
• Loosely bound - stars can drift away over time
• Found in the plane of the Milky Way
• Relatively young (millions to billions of years)
• Examples: Pleiades (M45), Hyades, Beehive Cluster (M44)

Globular Clusters

• Contain tens of thousands to millions of stars
• Tightly bound in a spherical shape
• Found in a halo around the Milky Way, not in the disk
• Very old (10-13 billion years) - among the oldest objects in the universe
• Examples: M13 (Hercules Cluster), M22, 47 Tucanae, Omega Centauri

Why Study Star Clusters?

Stars in a cluster all have the same age and distance, varying only in mass. This makes clusters ideal for testing theories of stellar evolution. Plotting cluster stars on an H-R diagram reveals the cluster's age and composition.