Red shift is a result of the Doppler Effect, as BlatantEcho mentioned. Imagine a wave of light being fired at a mirror. If the mirror is stationary, the waves reflect back toward the source at the same frequency as they arrived.
If the mirror is moving away from the source, though, the time it takes for consecutive peaks to hit and be reflected will increase, because the mirror is in motion relative to the wave. The reflecting wave will be stretched out, producing lower-frequency radiation. In the EM spectrum, this makes yellow and bluish light appear more red (per red-orange-yellow-green-blue-indigo-violet).
The same effect occurs when the mirror is moving toward the source, but in this case, the light waves are being compressed, creating a blue shift. Radar guns use the blue shift of reflected radio waves to measure the speed of cars, airplanes, etc.
In the case of stars, the source itself is in motion; instead of being red-shifted as it is reflected off something, the light is red-shifted as it leaves a moving star. Astronomers have identified a particular class of stars called Cepheid variables
, which have a strong correlation between the period of variability and the luminosity of the star.
Because we know how luminous a particular Cepheid star is by measuring its period of variability, we can calculate how far away it is by its apparent brightness; the farther away the star, the dimmer it will be. Measuring the Cepheids' red shifts has shown a very tight relationship, called the Hubble Constant, between a star's distance from us and how fast it is moving away from us.
Other celestial objects, like supernovae and quasars, can also be used to calibrate distances, which allows us to impute distances to extremely remote (~10-15 billion light years) objects whose light is so red-shifted that it is nearly infrared radiation. Residual EM energy from the Big Bang is even more red-shifted; it permeates space as low-level microwave radiation called the Cosmic Microwave Background. The CMB has been measured and matches theoretical predictions of the Big Bang theory.
[Edited 2004-01-22 05:10:10]
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