NGC 6791 is an open star cluster in the Lyra
constellation. At roughly 8 billion years old, and with an Iron
to Hydrogen abundance ratio that is more than twice that of the
Sun, it is one of the oldest and most metal-rich clusters in the
Milky Way. This is contrary to the typical rule-of-thumb where
older means more metal-poor. Compounded with the fact that it
has an unusually high population of stars, NGC 6791 is among the
most studied clusters in the sky. Among the dimmest stars in the
cluster are groups of white dwarfs that are 6 billion years old
and another group that appear to be 4 billion years old. The
ages are out of sync with those of the cluster's normal stars,
which are 8 billion years old. This seeming contradiction in age
for this cluster has been studied and a solution proposed with
age of about 8 billion years.
