Open cluster
From South Dublin Astronomical Society
| Open Cluster M45 |
|---|
 Image by Dave Grennan
|
Open clusters vary from the Pleiades, a reasonably close together grouping of stars with some nebulosity around them detectable in photographs to clusters like the Hyades in Taurus in which the stars appear so far apart that its not clear that its a cluster at all.
Contents |
Cluster Composition
Open clusters are relatively young groups of stars where only a few have left the Main sequence. This give us an age of open clusters of only up to several hundred million years. Some clusters have stars that have yet to all reach the main sequence e.g. NGC 2264, indicating an age of about 2 million years . In the case of the Pleiades above all stars have reached the main sequence and some have started to leave it, this indicates an age of about 50 million years.
Stellar Compositions
Using spectroscopy we can examine the properties of the stars within the clusters. Open clusters have relativly high metallically stars showing the presence of many heavier elements, these are classed as Population I stars.
Heavy elements can only be created in large quantities in supernova explosions, and open clusters must be formed around the diffuse nebulae resulting from such clouds. This shows that the earliest open clusters can only have formed after the first of the Population II stars had gone supernova, and again shows that open clusters are relatively young. The clusters with higher metallicities may be composed of material that has been recycled through supernovae several times.
Location, distribution, and size.
The open clusters are almost all confined to the spiral arms in the plane of the Milky Way, and there are over 1100 catalogued open clusters. As they reside on the galactic plane it is probable that many more are obscured by dust, and the galactic bulge, and there are estimates that there may be as many as 100,000 Milky Way open clusters.
The stars within an open cluster are generally all moving in the same direction with the same velocity. In some cases such as the Ursa Major moving cluster this is about the only way to define them as an open cluster. The more closely packed open clusters will experience more gravitational interaction between the member stars, but the cluster still travels as a unit.
Open clusters continue to be formed and give us key information into the process of star formation and evolution from the diffuse nebulae from which they originate.