Spectral classification

In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics. Stellar temperatures can be classified by using Wien's displacement law, but this poses difficulties for distant stars. Stellar spectroscopy offers a way to classify stars according to their absorption lines; particular absorption lines can be observed only for a certain range of temperatures because only in that range are the involved atomic energy levels populated. An early scheme (from the 19th century) ranked stars from A to Q, which is the origin of the currently used spectral classes.
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A method of classifying stars which is based upon the appearance of the spectral absorption lines in their spectra. Originally stars were classified as A to Q but better spectroscopy has led to regrouping and reorganisation. The classes now run OBAFGKM . Finer divisions are provided by 10 subclasses within each spectral type denoted as 0 to 9. Spectral classification is cross-referenced by luminosity classes. There are also suffixes to identify a star's peculiar features. For example a suffix 'wk' would denote that the star's spectral absorption lines are weak. Spectral type is one of the quantities plotted on the Hertzsprung-Russell diagram .