<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 absorption lines . Each line indicates an ion of a certain chemical element, with the line strength indicating the abundance of that ion . The relative abundance of the different ions varies with the temperature of the photosphere . The spectral class of a star is a short code summarizing the ionization state, giving an objective measure of the photosphere's temperature and density . </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, 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 class C for carbon stars . </P> <P> In the MK system, a luminosity class is added to the spectral class using Roman numerals . This is based on the width of certain absorption lines in the star's spectrum, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs . Luminosity class 0 or Ia+ stars for hypergiants, class I stars for supergiants, class II for bright giants, class III for regular giants, class IV for sub-giants, class V for main - sequence stars, class sd for sub-dwarfs, and class D for white dwarfs . The full spectral class for the Sun is then G2V, indicating a main - sequence star with a temperature around 5,800 K . </P>

Spectral classification of a star into the lettered categories o b a f g k and m is carried out by