<P> In astronomy, stellar classification is the classification of stars based on their spectral characteristics . Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines . Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element . The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences . The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature . </P> <P> Most stars are currently classified under the Morgan - Keenan (MK) system using the letters O, B, A, F, G, K, and M, a sequence from the hottest (O type) to the coolest (M type). Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. A8, A9, F0, and F1 form a sequence from hotter to cooler). The sequence has been expanded with classes for other stars and star - like objects that do not fit in the classical system, such as class D for white dwarfs and classes S and C for carbon stars . </P>

The differences between the different spectral types of stars are primarily due to