Star clusters fall into two main types: open clusters, which are fairly young, and globular clusters, which can be as old as the Milky Way itself. The older the cluster, the less elements heavier than hydrogen it contains. Open clusters tend to scatter and spread out over time.
So what’s the deal with open cluster NGC 6791? Out of the 2,000-some known clusters, it is different. At 13,000 light years away in the constellation Lyra, and with around 60,000 members, NGC 6791 has both old and new characteristics, the first of its kind. The large open star cluster is twice as enriched in heavy elements as our sun, which makes it about 8 billion years old, but has stars belonging to both types of clusters, red and very blue stars, and bright horizontal branch stars that are normally found in globular clusters.
It’s an oddball that will keep the astronomers, like Imants Platais of Johns Hopkins University, busy for awhile.