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Stellar classification
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Light 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.
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.
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.
==Modern classification==
The modern classification system is known as the Morgan–Keenan (MK) classification. Each star is assigned a ''spectral class'' from the older Harvard spectral classification and a ''luminosity class'' using Roman numerals as explained below, forming the star's spectral type.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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