Astronomy 242: Foundations of Astrophysics II

9. The Milky Way: Star Clusters

Open clusters Globular clusters

Star Clusters

Two Types of Clusters

2. Open Clusters

1. Globular Clusters

— young, ‘metal’-rich stars

— contain 100 to 104 stars

— found in disk of Milky Way

— old, ‘metal’-poor stars

— contain 105 to 106 stars

— found in halo of Milky Way

Cluster Formation

Star Cluster R136 Bursts Out

Star clusters form in massive interstellar gas clouds.

— rapid formation ⇒ stars have similar ages

— cloud well-mixed ⇒ stars have similar composition

Evolution of HR Diagrams

So as a cluster ages, the main sequence ‘burns down’ in order.

lifetime: 107 yr

lifetime: 108 yr

lifetime: 109 yr

lifetime: 1010 yr

High-mass stars burn out first; low-mass stars die later.

(Note: this animation also shows stars after they leave the main sequence.)

Using the H-R Diagram to Determine the Age of a Star Cluster

Evolution of HR Diagrams

So as a cluster ages, the main sequence ‘burns down’ in order.

High-mass stars burn out first; low-mass stars die later.

Instead of plotting stars, we represent them with a line of constant age.

Using the H-R Diagram to Determine the Age of a Star Cluster

The Pleiades: A Young Cluster

Pleiades and Stardust

Using the H-R Diagram to Determine the Age of a Star Cluster

Star Cluster Messier 67

M67: An Older Cluster

Using the H-R Diagram to Determine the Age of a Star Cluster