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Concept
Subgiant
Summary
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.
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 , and strength of O^ lines
A: Balmer s, 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.
Official source
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