172 Star Clusters
Learning Objectives
By the end of this section, you will be able to:
- Explain how star clusters help us understand the stages of stellar evolution
- List the different types of star clusters and describe how they differ in number of stars, structure, and age
- Explain why the chemical composition of globular clusters is different from that of open clusters
The preceding description of stellar evolution is based on calculations. However, no star completes its main-sequence lifetime or its evolution to a red giant quickly enough for us to observe these structural changes as they happen. Fortunately, nature has provided us with an indirect way to test our calculations.
Instead of observing the evolution of a single star, we can look at a group or cluster of stars. We look for a group of stars that is very close together in space, held together by gravity, often moving around a common center. Then it is reasonable to assume that the individual stars in the group all formed at nearly the same time, from the same cloud, and with the same composition. We expect that these stars will differ only in mass. And their masses determine how quickly they go through each stage of their lives.
Since stars with higher masses evolve more quickly, we can find clusters in which massive stars have already completed their main-sequence phase of evolution and become red giants, while stars of lower mass in the same cluster are still on the main sequence, or even—if the cluster is very young—undergoing pre-main-sequence gravitational contraction. We can see many stages of stellar evolution among the members of a single cluster, and we can see whether our models can explain why the H–R diagrams of clusters of different ages look the way they do.
The three basic types of clusters astronomers have discovered are globular clusters, open clusters, and stellar associations. Their properties are summarized in Table 1. As we will see in the next section of this chapter, globular clusters contain only very old stars, whereas open clusters and associations contain young stars.
| Table 1. Characteristics of Star Clusters | |||
|---|---|---|---|
| Characteristic | Globular Clusters | Open Clusters | Associations |
| Number in the Galaxy | 150 | Thousands | Thousands |
| Location in the Galaxy | Halo and central bulge | Disk (and spiral arms) | Spiral arms |
| Diameter (in light-years) | 50–450 | <30 | 100–500 |
| Mass MSun | 104–106 | 102–103 | 102–103 |
| Number of stars | 104–106 | 50–1000 | 102–104 |
| Color of brightest stars | Red | Red or blue | Blue |
| Luminosity of cluster (LSun) | 104–106 | 102–106 | 104–107 |
| Typical ages | Billions of years | A few hundred million years to, in the case of unusually large clusters, more than a billion years | Up to about 107 years |
Globular Clusters
Globular clusters were given this name because they are nearly symmetrical round systems of, typically, hundreds of thousands of stars. The most massive globular cluster