Open Star Clusters
Open Star Clusters
| Group Name | oc |
| Reference | Catalog of Optically Visible Open Clusters
Wilton Dias (Universidade Federal de Itajubá) Jacques Lépine (Universidade de São Paulo) André Moitinho (Universidade de Lisboa) Bruno Alessi |
| Prepared by | Brian Abbott (AMNH/Hayden) |
| Labels | Yes |
| Files | oc.speck, oc.label |
| Dependencies | none |
| Census | 1,041 clusters with labels |
An open star cluster is a loose assemblage of stars numbering from hundreds to thousands. Unlike in an OB association, the stars in an open cluster are bound by their mutual gravitation. Astronomers know from the stellar spectra that stars in open clusters are typically young. (With the star's spectrum, we can determine the spectral type and the luminosity class, revealing the age of the star.)
Since these are young stars, we expect to see them in the star-forming regions of our Galaxy, namely in the spiral arms. For this reason, open clusters exist, for the most part, in the plane of the Galaxy where we view the arms edge-on as the band of light in the night sky. Because of this, open clusters were originally known as Galactic clusters, but this term fell out of favor once astronomers began to understand that the Galaxy includes objects beyond the Milky Way's disk.
Source Catalog
The open cluster catalog was compiled by Wilton Dias and collaborators in Brazil and Portugal. It is a comprehensive collection of data from other catalogs coupled with the latest science and data from ground- and space-based observatories.
| Data Variables for Open Star Clusters | |||
| Number | Name | Description | Units |
| 0 | diam | Angular diameter | arcminutes |
| 1 | logage | Log of the cluster's age | years |
Exploring the Data
As previously mentioned, open clusters are good tracers of local spiral structure. We have included a parameter in the data file to emphasize this point visually. The logage data variable is an indication of where the most recent clusters are forming. A preset selection expression (in mw.cf), called “young,” will remove the older clusters from view, leaving those clusters younger than 20 million years. To see this, type
see youngat the Partiview Command Line (be sure oc is the active group). The remaining clusters highlight the local spiral arms nicely. From outside the Galaxy, you should see three distinct arms: the Sagittarius Arm toward Galactic center; the Orion Arm, where we live; and the Perseus Arm, toward the outer edge of the Galactic disk. To emphasize the point further, turn on the OB associations. These are in exact agreement with the open clusters, each providing a measure of the Galactic structure in our part of the Galaxy.
| Selection Expressions for the Open Clusters | ||
| Alias | Partiview Command | Description |
| young | thresh logage 6 7.3 | Show clusters that are less than 20 million years old |
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott