<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> 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>
spectral classification of a star into the lettered categories o b a f g k and m is carried out by
spectral classification of a star into the lettered categories o b a f g k and m is carried out by
the Morgan -- Keenan ( MK ) system
the Morgan -- Keenan ( MK ) system