The photosphere is only 310 miles (500 km) thick.This is the outermost layer of the Sun that emits energy in the visible light spectrum, and so it is the layer that seems to define the surface of the Sun.The photosphere maintains a steady average temperature of 9,900°F (5,500°C).The photosphere oscillates by 30 miles (50 km) in height, five or six times per half hour.These are giant acoustic compressions and are detected by the Doppler effect in wavelengths of solar spectral lines (see the sidebar "Doppler Effect,"below). Scientists from the SOHO (Solar and Heliospheric Observatory) mission have taken images showing "sunquakes" creating waves similar to those formed by a pebble falling into water, though these waves were formed by the eruption of a solar flare.The waves accelerated from 22,000 miles per hour (14,000 k/h) to more than 10 times that speed and carried the energy equivalent to 40,000 times the devastating San Francisco earthquake of 1906.

S ound is produced by waves passing through a substance. The sound of a voice is produced by vibrations of the vocal chords making waves in air. These waves strike the eardrum and allow the person to hear them. Sounds can pass through water as well, but no sound passes through space because space is a near vacuum with no substance to carry the sound waves. For more on waves, wavelength, and frequency, see appendix 2, "Light, Wavelength, and Radiation."

The term Doppler effect is most often used to describe the ways sounds appear to change when an observer is standing still and the object making the sounds is moving past. A good example is the sound of a car horn while the car is speeding past: As the car approaches, the car horn sounds shriller, until after it passes, when the sounds becomes deeper and deeper. The car horn produces a uniform sound, and to the driver, the sound does not seem to change: It only seems to change for the observer, standing still as the car rushes past.

In 1842, Christian Doppler, director of the Institute of Physics at Vienna University, explained this effect in a presentation regarding the colors of starlight to the Royal Bohemian Society of Sciences. Though he applied the science to stars moving toward or away from the Earth, the same physics apply to the sounds of objects moving toward or away from an observer. As the speeding, noisy object is coming toward an observer, the wavefronts of the sound it is producing are pressed closer together by the object's


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