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. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of a star. Yerkes luminosity class IV The term subgiant was first used in 1930 for class G and early K stars with absolute magnitudes between +2.5 and +4. 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 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. A5 or M1) and a Roman numeral to indicate the luminosity relative to other stars of the same temperature. Luminosity class IV stars are the subgiants, located between main-sequence stars (luminosity class V) and red giants (luminosity class III). Rather than defining absolute features, a typical approach to determining a spectral luminosity class is to compare similar spectra against standard stars. 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: O: relative strength of N emission and He absorption, strong emission is more luminous B: Balmer line profiles, and strength of O lines A: Balmer line profiles, broader wings means less luminous F: line strengths of Fe, Ti, and Sr G: Sr and Fe line strengths, and wing widths in the Ca, H and K lines K: Ca, H, and K line profiles, Sr/Fe line ratios, and MgH and TiO line strengths M: strength of the 422.6 nm Ca line and TiO bands Morgan and Keenan listed examples of stars in luminosity class IV when they established the two-dimensional classification scheme: B0: γ Cassiopeiae, δ Scorpii B0.5: β Scorpii B1: ο Persei, β Cephei B2: γ Orionis, π Scorpii, θ Ophiuchi, λ Scorpii B2.5: γ Pegasi, ζ Cassiopeiae B3: ι Herculis B5: τ Herculis A2: β Aurigae, λ Ursae Majoris, β Serpentis A3: δ Herculis F2: δ Geminorum, ζ Serpentis F5: Procyon, 110 Herculis F6: τ Boötis, θ Boötis, γ Serpentis F8: 50 Andromedae, θ Draconis G0: η Boötis, ζ Herculis G2: μ2 Cancri G5: μ