Sunspots: Cooler Surface Regions (page 2)

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Author: Janice VanCleave

Try New Approaches

Photographs of the Sun's photosphere show that it has a grainy appearance called granulation. Covering the photosphere are bright spots resembling rice grains, called granules. A granule is the top of a rising current of hot gases from the convection zone. A single granule measures about 900 miles (nearly 1,500 km) across. Between the granules lie dark intergranular lanes where the cooler gases return to the convection zone. Also seen on the photosphere are large dark spots called sunspots. These are centers of intense localized magnetic fields that are thought to suppress the currents of hot gases. They are dark because they are cooler than the areas surrounding them. Find out more about granules and sunspots and add them to your Sun model. For information about the size of sunspots and their two observable regions, the umbra (darkest part) and the penumbra (grayish outer part), see Dinah Moche, Astronomy: A Self-Teaching Guide (New York: Wiley, 1996), pp. 102–105.

Design Your Own Experiment

The Sun rotates about once every 4 weeks. Design an experiment to confirm that the Sun rotates. Observe and record the location of sunspots over a period of time, but unlike Galileo, who looked directly at the Sun and in time lost his eyesight, you must not look directly at the Sun. A safe, indirect method of observing sunspots uses a telescope, but binoculars with one lens covered will work too. Without looking through the lens, point the telescope at the Sun and adjust the position of the instrument until its shadow is as small as possible. Then focus the Sun's image on a sheet of white poster board—the screen. Secure the telescope and screen so that they do not move by setting the telescope on a tripod and tacking the screen to a tree (see Figure 7.3). Cut a hole in a second piece of poster board large enough to fit around the end of the telescope that faces the Sun. This sheet of poster board casts a shadow on the screen, making the Sun's image easier to see. At the same time each day or as many days as possible for 2 weeks or more, tape a clean sheet of tracing paper to the screen. Mark the location of sunspots on the paper. Compare the drawings. How much do the sunspots move? Do some move faster than others? Does the path of sunspots reveal the location of the Sun's equator and poles? For more information, see Philip Harrington, Astronomy for All Ages (Old Saybrook, CT: Globe Pequot Press, 1994), pp. 77–79. CAUTION: Never look directly at the Sun. It can permanently damage your eyes.

Sunspots Cooler Surface Regions

Get the Facts

At any given moment, the number of visible sunspots can vary from several hundred to none at all. The number increases and decreases in a regular pattern called the sunspot cycle. How long does each cycle last? How does the Sun's activity relate to the number of sunspots? Get today's sunspot number at For more information, see Moche, Astronomy, p. 103.

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