Redshift: The Basics
You may have noticed that as an ambulance approaches you and then passes you by, the pitch of its siren changes. The reason for this change is called the Doppler effect, named for its discoverer, Austrian physicist Christian Doppler. If something moves toward you while emitting a noise, the sound waves that carry the sound are compressed, causing you to hear a higher pitch noise. Similarly, as something moves away from you while emitting a noise, the sound waves you receive are stretched out, causing you to hear a lower pitch noise.
A similar effect happens with light waves. If a luminous object moves toward us, the light waves we receive appear shorter, or bluer; and if object were moving away from us, its light waves appear stretched out, or 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..
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Cars travel too slowly for us to notice any redshift or blueshift in the light from their headlights. Galaxies, on the other hand, do move fast enough relative to the Earth for the shift of their light to be detected. But, some galaxies are naturally red or blue, and it’s hard to know which color they really are “at rest”. Partly due to these complications, astronomers did not discover that galaxy light could be shifted until they started analyzing their spectra around the turn of the twentieth century.
Redshift, symbolized by z, is defined as:
For example, the Balmer gamma line of Hydrogen emission is emitted at rest in the laboratory at a wavelength (?rest) of 4340.5 Angstroms. If you identify this emission line in a galaxy spectrum, and find that it has been shifted to an observed wavelength (?observed) of 4780 Angstroms, you can simply calculate the redshift:
Note that if the observed wavelength were less than the rest wavelength, the value of z would be negative – that would tell us that we have a blueshift, and the galaxy is approaching us. It turns out that almost every galaxy in the sky has a redshift in its spectrum.
Using Spectra to Measure Redshifts
A spectrum (the plural of which 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 – Spectra for more information about how to read a spectrum.
The spectra of stars and galaxies almost always show a series of peaks and dips called “spectral lines.” These lines make good markers for redshift or blueshift. If astronomers look at a galaxy and see a pattern of spectral lines at longer wavelengths than they would be at rest (e.g., in a laboratory), they know that the galaxy is redshifted and is moving away from us. Similarly, if they identify lines a pattern of lines and find they are at shorter wavelengths than they would be at rest, the galaxy is described as blueshifted, and moving toward us.
The Sloan Digital Sky Survey has measured spectra for more than one million galaxies. Each spectrum is put into a computer program that automatically determines its redshift. The program outputs a picture like the one shown above, 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.