Clusters and Groupings

The following figure illustrates the difference between accidental groupings (constellations and asterisms), open clusters, and globular clusters.

Constellations and asterisms generally consist of stars that are widely separated in distance but are in approximately the same direction and so appear close together on the celestial sphere. In contrast, clusters correspond to real physical groupings of stars.

Open clusters and globular clusters have some distinguishing characteristics:

1. Open clusters are preferentially found in the plane of our spiral galaxy (thus they are also called galactic clusters); in contrast, the globular clusters are concentrated in a more spherical halo and can be very far out of the plane of the galaxy. We shall discuss these components of our galaxy more extensively in Chapter 23.

2. Open clusters tend to be more irregular in shape than the highly symmetric globular clusters.

3. Most open clusters contain no more than a few hundred stars, but large globular clusters may contain hundreds of thousands of stars.

The following table summarizes some of the basic characteristics of globular clusters, open clusters, and associations (associations are described in the right panel).

Characteristics of Clusters and Associations
Characteristic	Globular Clusters	Open Clusters	Associations
Diameter (pc)	20 - 100	< 10	30 - 200
Number of Stars	10⁴ - 10⁶	50 - 1000	10 - 100
Mass (solar units)	10⁴ - 10⁶	100 - 1000	100 - 1000
Density of Stars (solar masses / pc³)	0.5 - 1000	0.1 - 10	<0.01
Shape	Spherical	Irregular	Irregular
Color of Brightest Stars	Red	Red or Blue	Blue
Metal Content	Low	Higher	Higher
Where Found	Halo of Galaxy	Disk of Galaxy	Disk of Galaxy

In connection with the next to last line of this table, recall that for astronomers "metals" are all elements heavier than helium. We shall discuss some of the reasons for these characteristics of clusters and associations in the following chapters on the birth and death of stars (Chapters 20 and 21).