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 class
D for
white dwarfs and class
C for
carbon stars.
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 5,800K.