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Stars - Stellar Populations - Open Clusters

Star clusters form from the same molecular cloud. An open cluster of stars is one that can contain about a dozen to a few hundred members. In addition, they are spaced loosely and are generally younger, hot stars.

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,

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

( 2005 Russell Croman,

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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