Subgiant followed by red giant branch
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 iii  emission and He ii  absorption, strong emission is more luminous
B: Balmer line profiles, and strength of O ii  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 bandsMorgan 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 Herculis
G8: β Aquilae
K0: η Cephei
K1: γ CepheiLater analysis showed that some of these were blended spectra from double stars and some were variable, and the standards have been expanded to many more stars, but many of the original stars are still considered standards of the subgiant luminosity class. O-class stars and stars cooler than K1 are rarely given subgiant luminosity classes.