1. Clusters Clusters and Age Stars are born from molecular clouds
Usually many stars born, not just one
A cluster is a group of stars that are together and were probably born at the same time
Some stars leave the cluster later – probably our Sun
Because they were all born at the same time, they will be the same age
By comparing the H-R diagram of a cluster to theoretical models, we can deduce the approximate age
The turn off point

2. Types of Clusters There are two types of clusters
Open clusters are loosely bound together
Up to a thousand stars
Generally younger stars
In the disk of galaxies (more on this later)
Globular clusters are more tightly bound
A few thousand to one million stars
Generally older stars
Not confined to disks of galaxies (more on this later)

3. Open Clusters
NGC 290
M35
NGC 2158
Pleiades M6
M36

4. Globular Clusters M3
M80
M10 M2
M13

5. Cluster Diagrams:
A cluster diagram is a Hertzsprung Russell diagram showing all the stars in a cluster
Recall:
Stars are “born” as Main Sequence Stars
Massive stars are the hot luminous ones
The most massive stars die first
Over time, the cluster diagram will change:

6. Cluster Diagram: 1 Million years
At 1 million years old:
Some stars aren’t even main sequence yet
The brightest stars, though rare, dominate the light
O and B stars
Blueish tint to the cluster
The Sun

7. Cluster Diagram: 10 Million Years
At 10 million years old:
Almost all stars are now main sequence
Some of the heaviest are in their supergiant phases
The transition determines the turnoff point
Some of them have died
Turnoff

8. Cluster Diagram: 30 Million Years
At 30 million years old:
More stars are supergiants
Turnoff point has moved
Mix of stars now
White color to cluster
Turnoff

9. Cluster Diagram: 200 Million Years
At 200 million years old:
Red giants, core helium- burning, and double shell-burning
Turnoff point moved farther
Yellow tint to cluster
Turnoff

10. Cluster Diagram: 2 Billion Years
At 2 billion years old:
G, K, M stars dominate
Yellow/orange tint to cluster
Turnoff

11. 10 Billion Years At 10 billion years old: K, M stars dominate
Red tint to cluster
Sun is about to turn off
Turnoff

12. The Turnoff Point Clusters move around on H-R diagram over time
The turnoff point tells us how old the cluster is
The color of a cluster changes from blue (young) to red (old) over time
The Sun is not in a cluster