B: Balmer line profiles, and strength of O ii lines.O: relative strength of N iii emission and He ii absorption, strong emission is more luminous.Many line ratios and profiles are sensitive to gravity, and therefore make useful luminosity indicators, but some of the most useful spectral features for each spectral class are: Rather than defining absolute features, a typical approach to determining a spectral luminosity class is to compare similar spectra against standard stars. Luminosity class IV stars are the subgiants, located between main-sequence stars (luminosity class V) and red giants (luminosity class III). A5 or M1) and a Roman numeral to indicate the luminosity relative to other stars of the same temperature. The Yerkes spectral classification system is a two-dimensional scheme that uses a letter and number combination to denote that temperature of a star (e.g. These were noted as being part of a continuum of stars between obvious main-sequence stars such as the Sun and obvious giant stars such as Aldebaran, although less numerous than either the main sequence or the giant stars. The term subgiant was first used in 1930 for class G and early K stars with absolute magnitudes between +2.5 and +4. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of a star. A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars.
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