Chapter Objectives: Over half the stars in our sky are found in pairs (binary stars) or larger groups. Analysis of these binary systems provides us with our best means of determining stellar masses and sizes. In this chapter we will investigate the different types of binary stars and what can be learned from each of them about the physical properties of the stars. We will review Newton's generalizations of Kepler's laws as tools to determine stellar masses. The mass-luminosity relation that applies to most stars will be developed from these data on stellar masses. The Doppler effect will be applied to a study of binary star systems and also to the discovery of extrasolar planets. We will study the accretion of matter from one star onto another in some binary systems, and the relationship between that accretion and novae, supernovae, strong X-ray sources, and black holes.

The two types of star clusters -- the younger open clusters and the older, more crowded globular clusters -- will be introduced and the important information they reveal about stellar evolution will be discussed.

Chapter Skills: After studying this chapter you should be able to

1. Describe and understand the types of binary stars.
2. Use Newton's generalization of Kepler's laws to calculate stellar masses and center of mass distances.
3. Understand and use the mass-luminosity relation for main sequence stars.
4. Determine the radii of stars in eclipsing binary systems
5. Outline the steps by which astronomers determine the existence of extra-solar planets
6. Understand the role of angular momentum in binary star system and in accretion in accreting binary systems.
7. Understand how novae, type I supernovae, X-ray bursters, and black holes can occur in binary systems.
8. Describe the two types of star clusters in terms of their age, stellar populations, and location in our galaxy.
9. Discuss the formation and evolution of the galactic (open) and globular clusters of stars.