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# Bong!: How do Astronomers use Vibrations to Determine the Nature of the Sun's Interior?

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Author: Janice VanCleave

### PROBLEM

How do astronomers use vibrations to determine the nature of the Sun's interior?

### Materials

• Stemmed water glass
• Tap water
• Metal spoon

### Procedure

1. Fill the glass about three-fourths full with water.
2. Gently tap the side of the glass with the spoon. As you listen to the sound, observe the surface of the water.

### Results

When a sound is produced, the water has tiny waves on its surface.

### Why?

Tapping the glass makes the glass and its contents vibrate (move quickly back and forth). The back-and-forth motion of the glass pushes on the water inside the glass, causing waves to form on the water's surface. The motion also causes movement in the air outside the glass, which causes sound waves to travel through the air to your ears.

One back-and-forth motion is called a vibration, and the number of vibrations in a specific time period is called frequency. Astronomers (scientists who study celestial bodies) use vibrations to study the interior of the Sun. Vibrations pass from the Sun's interior to its gaseous surface causing it to move up and down.

While the frequency of the vibrations produced by striking the glass is many vibrations in 1 second, the frequency of some solar sounds is only 1 vibration in 5 minutes. You are able to see the waves on the water and hear the sound produced by the vibrating glass. But computers are needed to distinguish between the Vibrations produced by the Sun. The study of the interior of the Sun by observing how its surface vibrates is called helioseismology.

### LET'S EXPLORE

By observing the different vibrations that occur on the Sun's surface, astronomers can determine different things about the Sun, such as its motion, temperature, pressure, composition, and the density of its inner layers. Density is a measure of the amount of mass (measure of the amount of matter in an object) in a specific volume (space occupied by a three-dimensional object).

Demonstrate how the density of a fluid (gas or liquid) affects the frequency of its vibrations by repeating the experiment using glasses filled with liquids of increasing density, such as water (least dense), cooking oil, and ketchup (most dense). It may be easier to detect a difference in sound than a difference in the number of waves on the surface of the liquids.

### SHOW TIME!

1. Prepare a display of the structure of the Sun's layers similar to the one shown here. Add a legend providing information about the different layers. For information about the Sun's layers, see pages 95–96 in Dinah Moche's Astronomy (New York: Wiley, 1996).
2.
1. Look for sunspots (cool, dark spots on the Sun's surface) on a projected image of the Sun. CAUTION: Never look directly at the Sun. It can permanently damage your eyes or blind you. It is safe to study the Sun's surface by using binoculars or a telescope to project the Sun's image onto a paper screen. Without looking through the instrument, point it at the Sun, and then focus the image of the Sun on a sheet of white poster board. Design a way to secure the instrument and poster board so that they do not move. This will allow you to draw the image and mark the sunspots. One way is to secure the poster board to a tree or building and use a tripod to hold the telescope. If binoculars are used, position them on a stool. NOTE: Ask an adult to make sure the lenses of the instrument are clean.
2. The Sun's image will be easier to see if you cut a hole in a second piece of poster board, 12 inches (30 cm) square. The hole should be large enough to fit around the front (Sun-facing) telescope lens or one of the binoculars' front lenses. Insert the lens of the instrument through the hole. This sheet of poster board casts a shadow on the poster board screen behind the telescope, which will make it easier to see the Sun's image.

Tape a sheet of tracing paper to the poster board screen. Trace the Sun's image on the paper and mark any sunspots. You may have to make observations for several days before you see any sunspots. As you observe the Sun's image, note that it moves to one side of your drawing as Earth rotates. For more information about sunspots and another method of viewing them, see pages 94–95 in Heather Couper and Nigel Henbest's How the Universe Works (Pleasantville, NY: Reader's Digest Association, 1994).

3. Repeat the previous experiment over several days. Compare the drawings to determine in which direction the sunspots move. What causes sunspots? How big are sunspots? Are sunspots permanent? For information, see pages 60–61 in Ann-Jeanette Campbell's Amazing Space (New York: Wiley, 1997).

### CHECK IT OUT!

The surface of the Sun is not smooth, but bumpy, because of the movement of the many gases that make up the Sun. The bumps on its surface are called granules and are the tops of rising currents of hot gases. How large are granules? How long do they last? What are supergranules? For information, see pages 101–102 in Dinah Moche's Astronomy (New York: Wiley, 1996).