Star Systems: Multiple Stars
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.
Purpose: To make a model of an optical double.
- apple-size piece of modeling clay
- 3/8-by-36-inch (0.94-by-90-cm) dowel
- Divide the clay by pulling off a golf ball-size piece. Form both the large and small pieces into balls.
- Lay the dowel on a table. Place the clay balls next to the dowel as shown in Figure 26.l.
- 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.
The two clay balls appear close together.
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."
Design Your Own Experiment
- Investigate how sky conditions affect the identification of naked-eye doubles. Test your vision with the optical double stars Mizar and Alcor, which appear to be the second star from the end of the Big Dipper's handle (see Figure 26.3). Mizar is easy to see, but its double, Alcor, is much fainter. According to legend, before the invention of eyeglasses, Alcor was used by some cultures as a test of one's eyesight—only people with very good vision can see it with the naked eye. Look for Mizar and Alcor on different nights. Note the sky conditions, such as the phase of the Moon or the presence of clouds. How does the position of the pair in relation to the horizon affect your ability to see both stars?
- Repeat the previous experiment using binoculars. Note: If you use a telescope, Mizar and Alcor look rather widely separated and you will discover that Mizar is itself a double star with a tiny, close companion star that is not visible with binoculars. Mizar is a binary star. For information about multiple star systems, see star charts in Terence Dickinson, Nightwatch(Buffalo, NY: Firefly Books, 1998), pp. 100–119.
- In an eclipsing binary system, our line of sight from Earth happens to align with the orbital plane of the two binary stars. As they periodically move in front of each other, the star in front partially or completely blocks the light of the star behind it, causing the total light from the star system to dim periodically. Algol is the name of the eclipsing binary system in the constellation Perseus. The two stars of the Algol system are Algol A and Algol B. Design an experiment to determine the changes in the apparent magnitude (how bright a celestial body appears as viewed with the naked eye from Earth) of Algol and the time cycle of these changes. To judge Algol's magnitude, compare it to Mirfak, the brightest star in Perseus. Note: If Algol is not visible, observe the eclipsing binary system Beta Lyrae in the constellation Lyra. For information, see Fred Schaaf, 40 Nights to Knowing the Sky (New York: Henry Holt, 1998), pp. 110–113; and Janice VanCleave's Constellations for Every Kid (New York: Wiley, 1997), pp. 193–200.
Get the Facts
- The period of rotation of binary stars can range from less than an hour to thousands of years. What binary star system has the shortest known orbital period? Why is the longest orbital period difficult to determine? For information, see Brian Jones, The Practical Astronomer (New York: Fireside, 1990), p. 46.
- Stars in some binary systems are so close that they appear to be single stars even when viewed through a telescope. How do astronomers know there is a second star? For information, see spectroscopic binaries in Jones, Practical Astronomer, pp. 46–47.
Warning is hereby given that not all Project Ideas are appropriate for all individuals or in all circumstances. Implementation of any Science Project Idea should be undertaken only in appropriate settings and with appropriate parental or other supervision. Reading and following the safety precautions of all materials used in a project is the sole responsibility of each individual. For further information, consult your state’s handbook of Science Safety.