Redshift (Encyclopedia of Science)
In astronomy, when matter moves away from an observation point, its light spectrum displays a redshift. A redshift is one type of Doppler effect. Named for Austrian physicist Christian Johann Doppler (1803853), this principle states that if a light (or sound) source is moving away from a given point, its wavelengths (distance between two peaks of a wave) will be lengthened. Conversely, if an object emitting light or sound is moving toward that point, its wavelengths will be shortened.
With light, longer wavelengths stretch to the red end of the color spectrum while shorter wavelengths bunch up at the blue end. The shortening of wavelengths of an approaching object is called a blueshift.
The first astronomer to observe a space object's Doppler shift was American astronomer Vesto Melvin Slipher (1875969) in 1912. His subject was the Andromeda galaxy, which was then believed to be a nebula, or a cloud of dust and gas (at that time it was not known there were other galaxies beyond the Milky Way). Slipher discovered that the spectrum of Andromeda was shifted toward the blue end, meaning that it was approaching Earth.
Two years later, Slipher analyzed the spectra of fourteen other spiral nebula and found that only two were blueshifted, while twelve were redshifted. The redshifts he observed for some spirals implied they were moving at enormous speeds....
(The entire section is 546 words.)
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