Editing Subgiant (section)

==Yerkes luminosity class IV==
The term subgiant was first used in 1930 for class G and early K stars with [[absolute magnitude]]s 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.<ref name=sandage>{{cite journal |last1=Sandage |first1=Allan |last2=Lubin |first2=Lori M. |last3=Vandenberg |first3=Don A. |year=2003 |title=The age of the oldest stars in the local galactic disk from ''Hipparcos'' ''Parallaxes'' of G and K subgiants |journal=Publications of the Astronomical Society of the Pacific |volume=115 |issue=812 |pages=1187–1206 |arxiv = astro-ph/0307128 |doi=10.1086/378243 |bibcode = 2003PASP..115.1187S |s2cid=7159325}}</ref>

The [[Yerkes spectral classification]] system is a two-dimensional scheme that uses a letter and number combination to denote the 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&nbsp;IV stars are the subgiants, located between main-sequence stars (luminosity class&nbsp;V) and [[red giant]]s (luminosity class&nbsp;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:<ref name=mk/><ref name=gray>{{cite book |last1=Gray |first1=Richard O. |last2=Corbally |first2=Christopher |year=2009 |title=Stellar Spectral Classification |publisher=Princeton University Press |bibcode=2009ssc..book.....G}}</ref>
* O: relative strength of [[Nitrogen|N]]{{sup| {{smallcaps|iii}} }} emission and [[Helium|He]]{{sup| {{smallcaps|ii}} }} absorption, strong emission is more luminous
* B: [[Balmer line]] [[line profile|profiles]], and strength of [[Oxygen|O]]{{sup| {{smallcaps|ii}} }} lines
* A: [[Balmer line|Balmer]] [[line profile]]s, broader wings means less luminous
* F: line strengths of [[Iron|Fe]], [[Titanium|Ti]], and [[Strontium|Sr]]
* G: [[Strontium|Sr]] and [[Iron|Fe]] line strengths, and wing widths in the [[Calcium|Ca]], [[H and K lines]]
* K: [[Calcium|Ca]], [[H and K lines|H, and K line profiles]], [[Strontium|Sr]]/[[Iron|Fe]] line ratios, and [[Magnesium monohydride|MgH]] and [[Titanium oxide|TiO]] line strengths
* M: strength of the 422.6&nbsp;nm Ca line and [[Titanium oxide|TiO]] bands

Morgan and Keenan listed examples of stars in luminosity class IV when they established the two-dimensional classification scheme:<ref name=mk>{{cite book |last1=Morgan |first1=William Wilson |last2=Keenan |first2=Philip Childs |last3=Kellman |first3=Edith |year=1943 |title=An Atlas of Stellar Spectra, with an Outline of Spectral Classification |place=Chicago, IL |publisher=University of Chicago Press |lccn=43-2093 |bibcode=1943assw.book.....M}}</ref>
{{div col}}
* B0: [[Gamma Cassiopeiae|γ Cassiopeiae]], [[Delta Scorpii|δ Scorpii]]
* B0.5: [[Beta Scorpii|β Scorpii]]
* B1: [[Omicron Persei|ο Persei]], [[Beta Cephei|β Cephei]]
* B2: [[Gamma Orionis|γ Orionis]], [[Pi Scorpii|π Scorpii]], [[Theta Ophiuchi|θ Ophiuchi]], [[Lambda Scorpii|λ Scorpii]]
* B2.5: [[Gamma Pegasi|γ Pegasi]], [[Zeta Cassiopeiae|ζ Cassiopeiae]]
* B3: [[Iota Herculis|ι Herculis]]
* B5: [[Tau Herculis|τ Herculis]]
* A2: [[Beta Aurigae|β Aurigae]], [[Lambda Ursae Majoris|λ Ursae Majoris]], [[Beta Serpentis|β Serpentis]]
* A3: [[Delta Herculis|δ Herculis]]
* F2: [[Delta Geminorum|δ Geminorum]], [[Zeta Serpentis|ζ Serpentis]]
* F5: [[Procyon]], [[110 Herculis]]
* F6: [[Tau Boötis|τ Boötis]], [[Theta Boötis|θ Boötis]], [[Gamma Serpentis|γ Serpentis]]
* F8: [[50 Andromedae]], [[Theta Draconis|θ Draconis]]
* G0: [[Eta Boötis|η Boötis]], [[Zeta Herculis|ζ Herculis]]
* G2: [[Mu2 Cancri|μ<sup>2</sup> Cancri]]
* G5: [[Mu Herculis|μ Herculis]]
* G8: [[Beta Aquilae|β Aquilae]]
* K0: [[Eta Cephei|η Cephei]]
* K1: [[Gamma Cephei|γ Cephei]]
{{div col end}}
Later 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.<ref name=garcia>{{cite journal |last1=Garcia |first1=B. |year=1989 |title=A list of MK standard stars |journal=Bulletin d'Information du Centre de Données Stellaires |volume=36 |pages=27 |bibcode=1989BICDS..36...27G}}</ref>