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# Star Systems: Multiple Stars

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Author: Janice VanCleave

Most stars appear to be single stars. Telescopes have revealed, however, that more than half of all stars belong to double or multiple star systems in which stars appear to be close together.

In this project, you will model two types of double stars, optical and binary. You will learn how sky conditions affect observation of those doubles visible to the naked eye. You will also determine the orbital period of a pair of eclipsing binaries.

### Getting Started

Purpose: To make a model of an optical double.

### Materials

apple-size piece of modeling clay
3/8-by-36-inch (0.94-by-90-cm) dowel

### Procedure

1. Divide the clay by pulling off a golf ball-size piece. Form both the large and small pieces into balls.
2. Lay the dowel on a table. Place the clay balls next to the dowel as shown in Figure 26.l.
3. Close one eye and look at the balls at eye level. Move your head right or left until the balls appear to lie side by side.

### Results

The two clay balls appear close together.

### Why?

Two stars that appear to be close together are called double stars. If the stars are actually far apart and have no true relationship to each other, they are called optical double stars. Like the clay balls in this experiment, optical doubles appear to be close because they lie along the observer's line of sight.

### Try New Approaches

Binary stars are double stars that are relatively close to each other. Their mutual gravity binds them, and they revolve around a common point called the barycenter. To model binary stars, place a clay ball at each end of the dowel. (Note that distances between the stars are not being modeled, only their relationship.) Hang the dowel by a string so that it hangs level. Gently rotate the dowel about the supporting string (see Figure 26.2). The point where the string attaches to the dowel is the barycenter of this binary star system. To learn more, see Chapter 11, "Barycenter: The Balancing Point."