The observational data indicate that all stars form in clusters. In a cloud of hydrogen gas,
laced with helium and a trace of other elements, something triggers a collapse. Stars form
with a range of different masses and begin their lives. Over time, the gravitational pull of
the parent galaxy will disrupt clusters but while they last, they present a snapshot of stars
that have formed more or less at the same time and at the same place. So it should not be
surprising that these clusters of stars are of great interest to astronomers.
We will be working with a type of cluster called an open cluster. All of these clusters are
in our own Milky Way galaxy: those in other galaxies are too faint to observe with the
0.9M telescope at Kitt Peak, where these data were taken. Open clusters are distinct, both
in appearance and properties, from the other common type of cluster, the globular
clusters.