Star Clusters, Page 65: [Previous Page] [Next Page] [Class Home Page] Textbook pages 392 - 401

Star Clusters and Associations

Nearly all stars are born in large clusters, ranging in size from 10s to millions of stars. We see clusters through out our galaxy with a variety of ages and other important characteristics. Clusters are unique in that they identify a grouping of stars with the same age and distance. This can be exploited to better understand stars in general.





Important Types of Galactic Star Clusters

Name	Typical Age	Typical Members	Where Located	Typical Numbers
T Associations	Few Million Years	Low mass stars, T Tauri Stars	Near Dark Clouds	10-100 members
OB Associations	Few to 10 Million years	Stars of all masses	Near Giant Molecular Clouds	1000 to 100,000 stars
Open Clusters (or Galactic Clusters)	10 to 1000 Million years	B, A, F, Red Giants	Disk of Milky Way	1000 to 100,000 stars
Globular Clusters	few to 10 Billion years	Solar Type stars Red Giants	Halo of Milky Way	10,000 to 106 stars


The Young Clusters
The T associations (called 'T' for they contain T Tauri type stars), and the OB Associations are both very young star clusters, just a few million years old. The only real difference between the two is their size. T associations form from small dark clouds, while the much larger OB associations form from Giant Molecular Clouds.




The Older Clusters
There are two main differences between the two older clusters, Open and Globular: their location and their age. Globular Clusters are among the oldest objects in our Galaxy. Also, Globular Clusters appear to have a completely different spatial and kinematic signature compared to all the other star clusters listed, lying in the Halo of our galaxy, and rotating in random directions and inclinations. Open clusters and both kinds of associations lie in the disk of our galaxy and rotate in the same direction.