<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> <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+ is used for hypergiants, class I 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 (or VI) for sub-dwarfs, and class D (or VII) 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> <P> The conventional color description takes into account only the peak of the stellar spectrum . In actuality, however, stars radiate in all parts of the spectrum . Because all spectral colors combined appear white, the actual apparent colors the human eye would observe are far lighter than the conventional color descriptions would suggest . This characteristic of' lightness' indicates that the simplified assignment of colors within the spectrum can be misleading . Excluding color - contrast illusions in dim light, there are no green, indigo, or violet stars . Red dwarfs are a deep shade of orange, and brown dwarfs do not literally appear brown, but hypothetically would appear dim grey to a nearby observer . </P>

The sun's spectral type is g2. what is the sun's luminosity class
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