Astronomy & Cosmology -
Open clusters are smaller than globular clusters, usually containing just a few hundred to a few thousand stars. The stars would all have formed from the same cloud of gas, so would all have very similar ages. Open clusters seem only to occur in galaxies undergoing active star formation, usually spiral and irregular galaxies. They hold together much more weakly than globular clusters, so tend to become disrupted and merge into the main galaxy on timescales of only a few hundred million years.
Caldwell 28 (NGC 752) is quite an old open cluster which may be as much as 2 billion years old. It is about 1,300 light-years away. In contrast, NGC 7822 is a very young cluster, perhaps as little as 2 million years old and around 3,000 light-years away.
Pictured to the right, NGC 2244 (Caldwell 50) is an open cluster within the Rosette Nebula, (Caldwell 49) also shown to the right. This is another very young object, probably 2 to 5 million years old, around 50 light-years across, and about 5,200 light-years away. The cluster is actively producing new stars. It also contains many large, hot, and very luminous stars that are ionizing the gas in the nebula producing the distinctive red color. The pressure from the highly energetic photons is also clearing away the center of the nebula.
NGC2244, top image, and the Rosette Nebula in which it is found.