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Open clusters are physically related groups of stars held together by mutual gravitational attraction. They are believed to originate from large cosmic gas and dust clouds (diffuse nebulae) in the Milky Way (or other parent galaxy), and to continue to orbit the galaxy within or near their parent galaxy's disk. In many clouds visible as bright diffuse nebulae, star formation still takes place at this moment, so that we can observe the formation of new young star clusters. The process of formation takes only a considerably short time compared to the lifetime of the cluster, so that all member stars are of similar age. Also, as all the stars in a cluster formed from the same diffuse nebula, they are all of similar initial chemical composition.

Over 1100 open clusters are known in our Milky Way Galaxy, and this is probably only a small percentage of the total population which is probably some factor higher; estimates of as many as about 100,000 Milky Way open clusters have been given.

Cassiopeia Clusters NGC 133, NGC 146, and King 14

Cassiopeia Clusters NGC 133, NGC 146, and King 14

NGC 663 in Cassiopeia

NGC 663 in  Cassiopeia

NGC 225 in Cassiopeia

NGC 225 in Cassiopeia

NGC 225 with VdB 4 and LDN 1302

NGC 225 with VdB 4 and LDN 1302

M45 - Pleiades in Taurus

M45 - Pleiades in Taurus

Double Cluster in Perseus - NGC 869 and NGC 884

Double Cluster in Perseus - NGC 869 and NGC 884

NGC 457 - "Owl Cluster" in Cassiopeia

NGC 457 - "Owl Cluster" in Cassiopeia

M11 in Scutum

M11 in Scutum

M11 Processed as RGB In CCDStack and CS3

M11 Processed as RGB In CCDStack and CS3

M11 Processed as LRGB in CCDStack and CS3

M11 Processed as LRGB in CCDStack and CS3

M26 in Scutum

M26 in Scutum

M25 in Sagittarius

M25 in Sagittarius

M18 in Sagittarius

M18 in Sagittarius

M23 in Sagittarius

M23 in Sagittarius

Double Cluster in Perseus - NGC 869 and NGC 884

Double Cluster in Perseus - NGC 869 and NGC 884

Kemble 1 - Kemble's Cascade

Kemble 1 - Kemble's Cascade

M44 - Beehive Cluster in Cancer

M44 - Beehive Cluster in Cancer

M45 - Pleiades in Taurus

M45 - Pleiades in Taurus

M39 in Cygnus

M39 in Cygnus

M52 in Cassiopeia

M52 in Cassiopeia