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The easiest way to experience the Doppler effect is to listen to a moving train. As the train moves towards a person, the sound it makes as it comes towards them sounds like it has a higher tone, since the frequency of the sound is squeezed together a little bit. As the train speeds away, the sound gets stretched out, and sounds lower in tone. The same happens with light when an object that emits light moves very fast. An object, like a star or a galaxy that is far away and moving toward us, will look more blue than it normally does. This is called blue shift. A star or galaxy moving away from us will look more red than it should, which is where red shift got its name, since the colors are shifted red.
The reason astronomers can tell how far the light gets shifted is because certain chemical elements, like the calcium in bones or the oxygen people breathe has a unique fingerprint of light that no other chemical element has. They can see what colors of light are coming from a star, and see what it is made of. Once they know that, they check to see the difference between where the fingerprint, called spectral lines, are actually at, and then look at where they are supposed to be. When they see that, they can tell how far away the star is, whether it is moving toward us or away from us, and also how fast it is going, since the faster it goes, the farther the distance the spectral lines are from where they should be.
- Ned Wright's Cosmology tutorial
- Article on redshift from SPACE.com
- Cosmic reference guide entry on redshift
- Mike Luciuk's Astronomical Redshift tutorial
- Animated GIF of Cosmological Redshift by Wayne Hu