Most stars are currently classified under the Morgan–Keenan system using the letters O, B, A, F, G, K, M, L, T and Y, a sequence from the hottest to the coolest. The types R and N are carbon-based stars, the type S is zirconium-monoxide-based stars. Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest.
In the MKK 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 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 VI for sub-dwarfs, class VII for white dwarfs, and class VIII for brown dwarfs. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a temperature around 5,800K.
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