Open clusters are commonly found in the disk of a galaxy.

Examples of Open Clusters:

The Pleiades open cluster is a group of newly formed B-type stars. This 115 million year-old open cluster is easily visible with the naked eye. The brightest stars have a surface temperature of 30,000 Kelvin, however there are other stars that are a part of the cluster that have a variety of temperatures much cooler than these.

(© 2005 Russell Croman, www.rc-astro.com)

The Persius double-cluster is a remarkable site. This is actually a two open clusters.

(© 2005 Russell Croman, www.rc-astro.com)

Star clusters give astronomers a unique opportunity. All we know about stellar evolution come from star clusters (both open and closed). Stars in a cluster form at different times because of a variety of masses, but all contain nearly the same spectroscopic data. By careful observation, a Color-Magnitude Diagram can be created to plot the progression of evolution. A Color-Magnitude Diagram (or CMD) is really a specialized H-R diagram:

(Image credit: Brooks/Cole Thomson Learning)

The organization of stars on the diagram give us the age of the cluster. By evaluating when stars enter the later stages of evolution - called the Main-Sequence Turn Off Point - we can determine the age of the cluster.

Sometimes the mutual gravity of a cluster cannot hold the cluster together. This results in an stellar association. The most common member of a stellar association are O and B type stars. As a result, the correct term is OB Association.

Here is a summary chart of star clusters:

Characteristic:	Open Clusters:	OB Associations:	Globular Clusters:
Diameter (pc):	<10	30 - 200	20 - 100
Number of Stars:	50- 1000	10 - 100	104 - 106
Mass (Solar):	100 - 1000	100 - 1000	104 - 106
Density (Solar Mass/pc3):	0.1 - 10	<0.01	0.5 - 1000
Shape:	Irregular	Irregular	Spherical
Color (Common):	Red or Blue	Blue	Red
Metallicity:	High	High	Low
Location:	Disk of Galaxy	Disk of Galaxy	Halo of Galaxy

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