Features:

1. Open clusters are loosely-bound, irregularly-shaped groups of stars consisting of order 100 to 1000 stars with a size scales of order 4 to 20 pc (HI-392--393).

2. The stars in an open cluster form at about the same time (i.e., same time to within a few million or tens of millions of years: HI-338) in a star formation region.

   Note relative to age of the observable universe = 13.797(23) Gyr (Planck 2018), tens of millions of years is almost an instant in cosmic time. Of course, tens of millions of years is very long relative to all of human history.

3. Open clusters are mostly found only in disk galaxies (e.g., Milky Way) and mostly only in the galactic disks of disk galaxies (e.g., the Milky Way disk).

4. Both internal and external astronomical perturbations (mainly gravitational perturbations) tend to disperse open clusters and they probably only survive for a few hundred million years at most (Wikipedia: Open cluster: Eventual fate). The dispersion process due to internal astronomical perturbations is sometimes called gravitational evaporation.

5. Note the smaller stars that start in an open cluster will live longer than the open cluster: i.e., live after gravitational evaporation. To be explicit, stars ⪅ 3 M_☉ live ⪆ 4 gigayears (Gyr), and so usually will outlive the open cluster they belonged to, the smallest ones by thousands of gigayears (see Star file: star_lifetimes.html).

6. Probably the best known example of an open cluster is the Pleiades in Taurus. The Pleiades are well known in many cultures because they form a distinct group on the sky. For more on the Pleiades, see Star file: pleiades.html.