Population II Stars
On the left is a picture of the Globular Cluster, M 10.
These are extremely old clusters,
which have very densely packed centers. They live in the Halo, though some are seen
in and near the Disk as they pass through to the other side. These stars are part
of the Population II stars.
Population I Stars
On the right is a picture of the Open Cluster, M 6. Note that the cluster is much, much
looser (the stars more spread out). It can not possibly be very old since the stars are
barely gravitationally bound, and will likely soon disperse, no longer being a cluster.
Note that there are a lot more blue (hot, young) stars compared to the Globular Cluster to the left. This cluster lies in the Disk of our Galaxy. It rotates clockwise around the center of the Galaxy, just like the Sun and is an example of a Population I cluster.
Which cluster has more heavy elements (C, N, O, etc) in it's Stars??
Why?
Recall that by the time to sun formed 5 billion years ago, previous supernova (going off for the previous 10 billion years) had increased the number of metals in the interstellar medium.