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Spectral Class and Colour index As we have seen the colour of a star is related to its temperature as a consequence of Wien’s law. λmaxT = constant The spectral class (OBAFGKM) of a main sequence star is also a direct result of its temperature. One (relatively crude) way of determining the temperature (and spectral class) of a star is to measure the relative intensity of the light at two positions in the spectrum originally by using coloured filters. The wavelengths generally chosen are the blue (420nm) and the yellow (sometimes called the visual) 540nm A subtraction gives a B-V value. A crude measure of the energy distribution curve relative intensity λ Investigating The Nature of the Stars in a Cluster In the exercise below you are going to investigate the relationship between the spectral class of the stars, represented by the colour index and the apparent magnitude of the stars in the cluster. Making the assumption that all of the stars in the cluster are at the same distance from us, their apparent magnitude will be representative of their absolute magnitude.