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S P E C T R A

Classification of Stellar Spectra

The Harvard sequence covers the majority (>99%) of stars arranging their spectral types as W,O,B,A,F,G,K,M. The sequence reflects mainly a sequence of falling temperatures and hereby a sequence of colors from hot blue Wolf-Rayet and O-stars where temperatures above 30000K are found, to cool red (M) stars, where T does not reach 3000K.

The Harvard sequence is further subdivided into decimal subclasses. Numbers are placed behind the spectral type letters. So a B5 star is posed in the middle between B0 and A0. The Sun is class G2 in this scheme. Other features in the spectrum are designated by adding a small letter. M3e indicates the presence of emission lines, a small letter n stands for nebulous lines,

1O5V
2B57V
A5V
F5V
G5V
K5V
M5V

The spectral sequence ranging from 350nm to 700nm. All shown examples are synthetic spectra representing dwarf-type stars.

s for sharp and m for metallic lines, whereas v designates variability and p stands for peculiar features in the spectrum.

Further complications result from different luminosity classes of the stars. These range from brightest supergiants (class Ia,Ib), bright giants (II), giants (III), subdwarfs (IV), main sequence dwarfs (V) up to white dwarfs (VII). To account for these features a two dimensional classification was developed at Yerkes Observatory by W.W. Morgan and Ph.Ch. Keenan (MK-classification). Spectral lines of class Ia super giants are much narrower than those of a main sequence star of the same spectral type. Subclasses are obtained by comparing the varying ratios of characteristic lines in the spectra. The MK-type of our Sun is G2V.