If the stars in distant galaxies have the same composition as those in our own galaxy, the light we receive from them should exhibit the characteristic signatures of hydrogen and helium. This is indeed observed. However, there is one crucial difference between galactic and stellar light, and this has profound consequences for our understanding of the universe. Although it is true that the relative separation of the dark lines observed in the galactic light corresponds to hydrogen and helium atoms, the lines do not appear quite in their expected positions. When we examine the light from galaxies, we find that the dark lines have all been shifted slightly along the photograph.

Figure 3.1 shows schematic pictures of the light emitted by a typical star in our galaxy and the light received from an average distant galaxy. Light of a longer wavelength is positioned towards the right of the diagram. If this were a colour photograph, the picture would appear redder on the right-hand side and bluer on the left. Notice how the absorption lines in the galactic light are all positioned at slightly longer wavelengths than in the stellar case.

The key to understanding what causes the dark lines to be moved towards longer wavelengths lies in the fact that light travels through space as a wave. Consider what happens when a wave is emitted by a source and is then picked up by a receiver positioned some distance away.