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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 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 will be pushed together - the light's wavelength will be shorter, so the light will become bluer. If an object moves away from us, its light waves will be stretched out, and will become 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. But galaxies are moving fast enough with respect to us that we can see a noticeable shift.
Redshifts and Spectra
Astronomers can measure exactly how much redshift or blueshift a galaxy has by looking at its spectrum. A spectrum (the plural is "spectra") measures how much light an object gives off at different wavelengths.
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 wavelengths, so they make a good marker for redshift or blueshift. If astronomers look at a galaxy and see one spectral line at a longer wavelength than it would be on Earth, they would know that the galaxy was redshifted and was moving away from us. If they see the same line at a shorter wavelength, they would know that the galaxy was blueshifted and was moving toward us.
The Sloan Digital Sky Survey has measured spectra for more than 250,000 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 bottom of the spectrum (before the +/-) shows the redshift. Positive z values mean the galaxy has a redshift; negative z values mean the galaxy has a blueshift. (NOTE: this is not the same z as the z magnitude you looked at in Explore 4.)
Look at the spectrum above, but don't worry if you don't understand all the details. In the next exercise, you will look up the spectra of the six galaxies you examined in Explore 4.
Now that you have measured both distance and redshift for these six galaxies, you are ready to make a graph like the graph you made for the balloon - a Hubble diagram.