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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