The stars bolometric magnitude is plotted against its mass, in units of the Sun's mass. This is determined through observation and then the mass of the star is read of the plot. Giants and main sequence stars tend to agree with this, but super giants do not and neither do white dwarfs. The Mass-Luminosity Relation is very useful because, due to the observation of binaries, particularly the visual binaries since the masses of many stars have been found this way, astronomers have gained insight into the evolution of stars, including how they are born. Generally speaking, there are three classes of binary systems. These can be determined by considering the colours of the two components. Systems consisting of a red or reddish primary star and a blueish secondary star, usually a magnitude or more fainter... Systems in which the differences in magnitude and colour are both small... Systems in which the fainter star is the redder of the two..." The luminosity of class 1. binaries is greater than that of class 3. binaries. There is a relationship between the colour difference of binaries and their reduced proper motions. In 1921, Frederick C. Leonard, at the Lick Observatory, wrote "1. The spectrum of the secondary component of a dwarf star is generally redder than that of the primary, whereas the spectrum of the fainter component of a giant star is usually bluer than that of the brighter one. In both cases, the absolute difference in spectral class seems ordinarily to be related to the disparity between the components...2. With some exceptions, the spectra of the components of double stars are so related to each other that they conform to the Hertzsprung-Russell configuration of the stars..." An interesting case for visual binaries occurs when one or both components are located above or below the Main-Sequence. If a star is more luminous than a Main-Sequence star, it is either very young, and therefore contracting due to gravity, or is at the post Main-Sequence stage of its evolution. The study of binaries is useful here because, unlike with single stars, it is possible to determine which reason is the case. If the primary is gravitationally contracting, then the companion will be further away from the Main-Sequence than the primary since the more massive star becomes a Main-Sequence star much faster than the less massive star.