When observing stellar light from various distant stars (decomposed using a prism) it was found that, just as for the sun, they presented lines. But, curiously, these lines corresponded to no known element! This may imply that each star carries a new set of elements, but the simplest hypothesis (which should be investigated first, see Sect. 1.2.5) is that the mismatch between the laboratory and stellar lines is due to the Doppler effect which will shift the lines towards the red or blue according to the motion of the star (which is the source in this case) with respect to Earth. One can then use the shift in the observed stellar lines to determine the velocity at which the star is moving (with respect to us) and also the elements in it. In one fell swoop we determine the constitution and the speed of the stars using only the light we get from it.