The light emitted by a star may be split to form a spectrum containing a series of dark lines. The patterns of lines indicate the presence of particular chemical elements, enabling astronomers to deduce the composition of the stars atmosphere. The magnitude and spectral type of stars may be plotted on a graph called a Hertzsprung – Russell diagram, which shows that stars tend to fall into several well – defined groups. The principal groups are main sequence stars (those which are fusing hydrogen to form helium), giants, supergiants, and white dwarfs.