Overview for Estimating Age of Star Clusters Look for a turn-off point from Main Sequence Determine spectral class of stars at turn-off point Estimate lifetime of this spectral class (see table next page) Use spectral class lifetime as estimated age of star cluster Sidebar If there is no turn-off point from Main Sequence it must be very young cluster No cluster can be older than the universe, which is ~13.7 billion years old Main Sequence Region O B A F G K M

Spectral Class ~T ~MS Lifetime =========== ========== ========== O 30,000 K 10 million years B 20,000 K 100 million years A 10,000 K 1 billion years F 8,000 K 5 billion years G 6,000 K 10 billion years K 5,000 K 25 billion years M 3,000 K >100 billion years