Redshift: The Basics
If you have ever stood by the side of the road as a car passed by, you have an idea of what redshift is. As the car moves toward you, its engine sounds higher-pitched than the engine of a stationary car. As the car moves away from you, its engine sounds lower-pitched than the engine of a stationary car. The reason for this change is the Doppler effect, named for its discoverer, Austrian physicist Christian Doppler. As the car moves toward you, the sound waves that carry the sound of its engine are pushed together. As the car moves away from you, these sound waves are stretched out.
The same effect happens with light waves. If an object moves toward us, the light waves it gives off appear shorter, so the light will become bluer. If an object moves away from us, its light waves appear stretched out, becoming redder. The degree of “redshift” or “blueshift” is directly related to the object’s speed in the direction we are looking. The animation below schematically shows what a redshift and blueshift might look like, using a car as an example..
The speeds of cars are much too small for us to notice any redshift or blueshift. Galaxies are moving fast enough with respect to us, but unfortunately, we can’t tell by looking at them if their colors are shifted because we don’t know what color they “should” be. Astronomers need a way to measure this shift using only the light that arrives here from the object. In fact, astronomers did not discover that galaxy light could be shifted until they started analyzing their spectra around the turn of the twentieth century. What did they notice?
A spectrum (the plural is “spectra”) measures how much light an object gives off at different wavelengths. The spectrum of a star is often displayed as a graph. See Preflight – SDSS Spectrum Graphs for more help. The spectra of stars and galaxies almost always show a series of peaks and valleys called “spectral lines.” These lines always appear at the same location along the spectrum graph, so they make good markers for redshift or blueshift. If astronomers look at a galaxy and see spectral lines at longer wavelengths than they would be on Earth, they know that the galaxy is redshifted and is moving away from us. If they see the same lines at shorter wavelengths than observed on Earth, they refer to the galaxy as blueshifted and conclude it is moving toward us.
The Sloan Digital Sky Survey has measured spectra for around a million galaxies. Each spectrum is put into a computer program that automatically determines its redshift. The program outputs a picture like the one below, with spectral lines marked. The “z” number at the top of the spectrum shows the redshift. Positive z-values mean the galaxy has a redshift; negative z-values mean the galaxy has a blueshift.