When we look at a galaxy at visible wavelengths, we are primarily detecting
the light from the stars which the galaxy contains. Thus, when we take
a galaxy’s spectrum at visible wavelengths, it typically contains absorption
lines created in the stars’ relatively cool upper atmospheres. 3 Suppose we
consider a particular absorption line whose wavelength, as measured in a
laboratory here on Earth, is λ em . The wavelength we measure for the same
absorption line in a distant galaxy’s spectrum, λ ob , will not, in general, be
the same. We say that the galaxy has a redshift z, given by the formula