The spectrum of the Sun, when spread out by a spectrograph, shows dark 'absorption lines'. These are the result of photons of specific energy - and therefore wavelength - being absorbed by elements in the Sun's atmosphere, so that they do not reach Earth. There are some 20,000 of these lines, but this photograph shows only the major ones. The faint line at the red end of the spectrum is one of the hydrogen lines. The line in the yellow part is due to a pair of sodium lines, so close together that they are indistinguishable. It was by analysis of the solar spectrum that the presen- ce of helium in the Sun was first discovered.