Quick Lab
Calculating the Sun's Power
Materials
portable socket with clear light bulb, paraffinblock photometer, meter stick, calculator
Procedure 
You can estimate the sun's power by comparing it to the known power of a light bulb. Because a bulb's efficiency is about 10 percent, its actual power (Pbulb) is the rated power in watts divided by 10.
Place the socket on a table so that the bulb filament is perpendicular to the direction of the sun. Turn the bulb on. CAUTION The bulb will become very hot. Do not touch it.
A photometer is used to measure light levels. Hold the photometer directly between the bulb and the sun as shown. CAUTION Do not look directly at the sun.
Look at the narrow edge of the photometer. Move the photometer toward or away from the bulb until the slabs of paraffin on the two sides of the foil seem equally bright.
Have a partner measure the distance (Dbulb) in centimeters from the foil to the filament.
Analyze and Conclude
Calculating Earth is about 150 million km from the sun. What is the distance (Dsun) between Earth and the sun in centimeters?
Calculating How bright a light source appears depends directly on its actual power and inversely on the square of its distance from the observer. When the photometer is positioned so that the bulb and the sun appear equally bright, the following ratios are equal:
Square your values for Dbulb and Dsun. Then insert these values and your value for P bulb into the formula. Use the formula to calculate the actual power of the sun (Psun) in watts.
Figure 6 Sunspots are relatively cool areas on the sun's surface. Interpreting Photos How do sunspots appear different from the surrounding photosphere?
Features of the Sun's Atmosphere
The sun's magnetic field produces striking features on and above its surface. 
Features of the sun's atmosphere include sunspots, prominences, and solar flares.
Sunspots
Look at Figure 6. The relatively small, dark-looking regions on the sun's surface are called sunspots. Sunspots are areas of gas in the photosphere that are cooler than the surrounding gases. However, sunspots aren't really small. Some are larger than Earth! They are also not really dark. If sunspots could be seen apart from the rest of the sun, they would shine brightly. But because they are cooler, sunspots give off less energy than the surrounding photosphere, and so they appear dark by comparison. Sunspots are often found in groups and are associated with intense magnetic fields on the sun. Sunspots last from a few hours to a few months. The number of sunspots varies in cycles, with about 11 years separating one sunspot peak from the next. During peak periods, there may be a hundred sunspots on the sun.
Table of Contents