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Runaway Universe
Moving Targets
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Illustration: car with sound waves

Finding out how fast and in what direction a stellar object is moving relative to us involves the Doppler effect and the reading of absorption lines.

The Doppler Effect
The change in pitch you hear when a car speeds by happens because the sounds we hear are waves like the ripples in a pond. As the source of these waves moves by, it catches up with the waves it's already sent out, causing them to "bunch-up" on the leading side of the car and "spread-out" on the trailing side. This is known as the Doppler effect.


Illustration: high and low notes on piano, with monitors showing fast and slow sound waveforms

The closer together the waves, the greater their frequency—that is, the more often they hit our ears. It's the frequency of a sound that allows us to distinguish between different tones—it's what causes a C to sound different from an F. We hear the higher frequency as a higher pitch.


Illustration: two cars, fast and slow, with sound waves, fast and slow

The faster a source moves, the more exaggerated the effect. A sound coming from a faster moving car, for example, will have a higher pitch than the same sound coming from a slower moving car.

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