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{{About|the type of star|the dance group|Subgiant (band)}}
{{short description|Type of star larger than main-sequence but smaller than a giant}}
{{Star nav}}
A '''subgiant''' is a [[star]] that is brighter than a normal [[main sequence|main-sequence]] star of the same [[stellar classification|spectral class]], but not as bright as [[giant star]]s. The term subgiant is applied both to a particular spectral [[luminosity class]] and to a stage in the [[stellar evolution|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 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|doi=10.1086/378243|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|year=2003|last1=Sandage|first1=Allan|last2=Lubin|first2=Lori M.|last3=Vandenberg|first3=Don A.|arxiv = astro-ph/0307128 |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 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 giant]]s (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:<ref name=mk/><ref name=gray>{{cite journal|bibcode=2009ssc..book.....G|title=Stellar Spectral Classification|journal=Stellar Spectral Classification by Richard O. Gray and Christopher J. Corbally. Princeton University Press|last1=Gray|first1=Richard O.|last2=Corbally|first2=Christopher|year=2009}}</ref>
* O: relative strength of N&nbsp;{{smallcaps|iii}} emission and He&nbsp;{{smallcaps|ii}} absorption, strong emission is more luminous
* B: [[Balmer line]] profiles and strength of O&nbsp;{{smallcaps|ii}} lines
* A: Balmer [[line profile]]s, broader wings means less luminous
* F: line strengths of [[Iron|Fe]], [[Titanium|Ti]], and [[Strontium|Sr]]
* G: Sr and Fe line strengths, and wing widths in the [[Calcium|Ca]] [[H and K lines]]
* K: Ca H&K line profiles, Sr/Fe line ratios, and [[Magnesium|Mg]][[Hydrogen|H]] and [[Titanium oxide|TiO]] line strengths
* M: strength of the 422.6&nbsp;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:<ref name=mk>{{cite journal|bibcode=1943assw.book.....M|title=An atlas of stellar spectra, with an outline of spectral classification|journal=Chicago|last1=Morgan|first1=William Wilson|last2=Keenan|first2=Philip Childs|last3=Kellman|first3=Edith|year=1943}}</ref>
* 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: [[Mu Cancri|μ Cancri]]
* G5: [[Mu Herculis|μ Herculis]]
* G8: [[Beta Aquilae|β Aquilae]]
* K0: [[Eta Cephei|η Cephei]]
* K1: [[Gamma Cephei|γ Cephei]]
 
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|bibcode=1989BICDS..36...27G|title=A list of MK standard stars|journal=Bulletin d'Information du Centre de Données Stellaires|volume=36|pages=27|last1=Garcia|first1=B.|year=1989}}</ref>
 
==Subgiant branch==
[[File:Zams and tracks.png|thumb|left|upright=1.4|Stellar evolutionary tracks:{{unordered list|the {{solar mass|5}} track shows a hook and a subgiant branch crossing the [[Hertzsprung gap]]|the {{solar mass|2}} track shows a hook and pronounced subgiant branch|lower-mass tracks show very short long-lasting subgiant branches}}]]
The subgiant branch is a stage in the evolution of low to intermediate mass stars. Stars with a subgiant spectral type are not always on the evolutionary subgiant branch, and vice versa. For example, the stars [[FK Comae Berenices|FK Com]] and [[31 Comae Berenices|31 Com]] both lie in the Hertzsprung Gap and are likely evolutionary subgiants, but both are often assigned giant luminosity classes. The spectral classification can be influenced by metallicity, rotation, unusual chemical peculiarities, etc. The initial stages of the subgiant branch in a star like the sun are prolonged with little external indication of the internal changes. One approach to identifying evolutionary subgiants include chemical abundances such as Lithium which is diluted in subgiants,<ref name=lebre>{{cite journal|bibcode=1999A&A...345..936L|title=Lithium and rotation on the subgiant branch. I. Observations and spectral analysis|journal=Astronomy and Astrophysics|volume=345|pages=936|last1=Lèbre|first1=A.|last2=De Laverny|first2=P.|last3=De Medeiros|first3=J. R.|last4=Charbonnel|first4=C.|last5=Da Silva|first5=L.|year=1999}}</ref> and coronal emission strength.<ref name=ayres1998>{{cite journal|bibcode=1998ApJ...496..428A|title=The Coronae of Moderate-Mass Giants in the Hertzsprung Gap and the Clump|journal=The Astrophysical Journal|volume=496|pages=428–448|last1=Ayres|first1=Thomas R.|last2=Simon|first2=Theodore|last3=Stern|first3=Robert A.|last4=Drake|first4=Stephen A.|last5=Wood|first5=Brian E.|last6=Brown|first6=Alexander|year=1998|issue=1|doi=10.1086/305347|doi-access=free}}</ref>
 
As the fraction of hydrogen remaining in the core of a main sequence star decreases, the core [[virial theorem|temperature increases]] and so the rate of fusion increases. This causes stars to evolve slowly to higher luminosities as they age and broadens the main sequence band in the [[Hertzsprung–Russell diagram]].
 
Once a main sequence star ceases to fuse hydrogen in its core, the core begins to collapse under its own weight. This causes it to increase in temperature and hydrogen fuses in a shell outside the core, which provides more energy than core hydrogen burning. Low- and intermediate-mass stars expand and cool until at about 5,000 K they begin to increase in luminosity in a stage known as the [[red-giant branch]]. The transition from the main sequence to the red giant branch is known as the subgiant branch. The shape and duration of the subgiant branch varies for stars of different masses, due to differences in the internal configuration of the star.
 
===Very-low-mass stars===
[[Red dwarf|Stars less massive]] than about {{solar mass|0.4}} are convective throughout most of the star. These stars continue to fuse hydrogen in their cores until essentially the entire star has been converted to helium, and they do not develop into subgiants. Stars of this mass have main-sequence lifetimes many times longer than the current age of the Universe.<ref name=salaris2005/>
 
==={{Solar mass|0.4}} to {{solar mass|1}}===
[[File:M5 colour magnitude diagram.png|thumb|right|upright=1.2|H–R diagram for [[globular cluster]] [[Messier 5|M5]], showing a short but densely-populated subgiant branch of stars slightly less massive than the Sun]]
Stars less massive than the Sun have non-convective cores with a strong temperature gradient from the centre outwards. When they exhaust hydrogen at the centre of the star, a thick shell of hydrogen outside the central core continues to fuse without interruption. The star is considered to be a subgiant at this point although there is little change visible from the exterior.<ref name=pols/>
 
The helium core mass is below the [[Schönberg–Chandrasekhar limit]] and it remains in thermal equilibrium with the fusing hydrogen shell. Its mass continues to increase and the star very slowly expands as the hydrogen shell migrates outwards. Any increase in energy output from the shell goes into expanding the envelope of the star and the luminosity stays approximately constant. The subgiant branch for these stars is short, horizontal, and heavily populated, as visible in very old clusters.<ref name=pols/>
 
After several billion years, the helium core becomes too massive to support its own weight and becomes degenerate. Its temperature increases, the rate of fusion in the hydrogen shell increases, the outer layers become strongly convective, and the luminosity increases at approximately the same effective temperature. The star is now on the red giant branch.<ref name=salaris2005/>
 
===Mass above {{Solar mass|1}}===
Stars more massive than the Sun have a convective core on the main sequence. They develop a more massive helium core, taking up a larger fraction of the star, before they exhaust the hydrogen in the entire convective region. Fusion in the star ceases entirely and the core begins to contract and increase in temperature. The entire star contracts and increases in temperature, with the radiated luminosity actually increasing despite the lack of fusion. This continues for several million years before the core becomes hot enough to ignite hydrogen in a shell, which reverses the temperature and luminosity increase and the star starts to expand and cool. This ''hook'' is generally defined as the end of the main sequence and the start of the subgiant branch in these stars.<ref name=pols/>
 
The core of stars below about {{solar mass|2}} is still below the [[Schönberg–Chandrasekhar limit]], but hydrogen shell fusion quickly increases the mass of the core beyond that limit. More-massive stars already have cores above the Schönberg–Chandrasekhar mass when they leave the main sequence. The exact initial mass at which stars will show a hook and at which they will leave the main sequence with cores above the Schönberg–Chandrasekhar limit depend on the metallicity and the degree of [[convective overshoot|overshooting]] in the convective core. Low metallicity causes the central part of even low mass cores to be convectively unstable, and overshooting causes the core to be larger when hydrogen becomes exhausted.<ref name=salaris2005/>
 
Once the core exceeds the C-R limit, it can no longer remain in thermal equilibrium with the hydrogen shell. It contracts and the outer layers of the star expand and cool. The energy to expand the outer envelope causes the radiated luminosity to decrease. When the outer layers cool sufficiently, they become opaque and force convection to begin outside the fusing shell. The expansion stops and the radiated luminosity begins to increase, which is defined as the start of the red giant branch for these stars. Stars with an initial mass approximately {{solar mass|1-2}} can develop a degenerate helium core before this point and that will cause the star to enter the red giant branch as for lower mass stars.<ref name=salaris2005>{{cite journal|bibcode=2005essp.book.....S|title=Evolution of Stars and Stellar Populations|url=https://archive.org/details/evolutionofstars0000sala|url-access=registration|journal=Evolution of Stars and Stellar Populations|pages=400|last1=Salaris|first1=Maurizio|last2=Cassisi|first2=Santi|year=2005}}</ref>
 
The core contraction and envelope expansion is very rapid, taking only a few million years. In this time the temperature of the star will cool from its main sequence value of 6,000–30,000&nbsp;K to around 5,000&nbsp;K. Relatively few stars are seen in this stage of their evolution and there is an apparent lack in the H–R diagram known as the [[Hertzsprung gap]]. It is most obvious in clusters from a few hundred million to a few billion years old.<ref name=merlilliod>{{cite journal|bibcode=1981A&A....97..235M|title=Comparative studies of young open clusters. III – Empirical isochronous curves and the zero age main sequence|journal=Astronomy and Astrophysics|volume=97|pages=235|last1=Mermilliod|first1=J. C.|year=1981}}</ref>
 
===Massive stars===
Beyond about {{solar mass|8-12}}, depending on metallicity, stars have hot massive convective cores on the main sequence due to [[CNO cycle]] fusion. Hydrogen shell fusion and subsequent core helium fusion begin quickly following core hydrogen exhaustion, before the star could reach the red giant branch. Such stars, for example early B main sequence stars, experience a brief and shortened subgiant branch before becoming [[supergiant]]s. They may also be assigned a giant spectral luminosity class during this transition.<ref name=hurley>{{cite journal|bibcode=2000MNRAS.315..543H|title=Comprehensive analytic formulae for stellar evolution as a function of mass and metallicity|journal=Monthly Notices of the Royal Astronomical Society|volume=315|issue=3|pages=543|last1=Hurley|first1=Jarrod R.|last2=Pols|first2=Onno R.|last3=Tout|first3=Christopher A.|year=2000|doi=10.1046/j.1365-8711.2000.03426.x|arxiv = astro-ph/0001295 |s2cid=18523597}}</ref>
 
In very massive O-class main sequence stars, the transition from main sequence to giant to supergiant occurs over a very narrow range of temperature and luminosity, sometimes even before core hydrogen fusion has ended, and the subgiant class is rarely used. Values for the surface gravity, log(g), of O-class stars are around 3.6 cgs for giants and 3.9 for dwarfs.<ref name=martins>{{cite journal|bibcode= 2005A&A...436.1049M|doi=10.1051/0004-6361:20042386|title=A new calibration of stellar parameters of Galactic O stars|journal=Astronomy and Astrophysics|volume=436|issue=3|pages=1049–1065|year=2005|last1=Martins|first1=F.|last2=Schaerer|first2=D.|last3=Hillier|first3=D. J.|arxiv = astro-ph/0503346 |s2cid=39162419}}</ref> For comparison, typical log(g) values for K class stars are 1.59 ([[Aldebaran]]) and 4.37 ([[α Centauri B]]), leaving plenty of scope to classify subgiants such as [[Eta Cephei|η Cephei]] with log(g) of 3.47. Examples of massive subgiant stars include [[Theta2 Orionis|θ<sup>2</sup> Orionis A]] and the primary star of the [[Delta Circini|δ Circini system]], both class O stars with masses of over {{solar mass|20}}.
 
===Properties===
This table shows the typical lifetimes on the main sequence (MS) and subgiant branch (SB), as well as any hook duration between core hydrogen exhaustion and the onset of shell burning, for stars with different initial masses, all at solar metallicity (Z = 0.02). Also shown are the helium core mass, surface effective temperature, radius, and luminosity at the start and end of the subgiant branch for each star. The end of the subgiant branch is defined to be when the core becomes degenerate or when the luminosity starts to increase.<ref name=pols>{{cite journal|bibcode=1998MNRAS.298..525P|title=Stellar evolution models for Z = 0.0001 to 0.03|journal=Monthly Notices of the Royal Astronomical Society|volume=298|issue=2|pages=525|last1=Pols|first1=Onno R.|last2=Schröder|first2=Klaus-Peter|last3=Hurley|first3=Jarrod R.|last4=Tout|first4=Christopher A.|last5=Eggleton|first5=Peter P.|year=1998|doi=10.1046/j.1365-8711.1998.01658.x|doi-access=free}}</ref>
{| class="wikitable"
|-
! rowspan=2 | Mass<br/>({{solar mass}}) !! rowspan=2 | Example !! rowspan=2 | MS (GYrs) !! rowspan=2 | Hook (MYrs) !! rowspan=2 | SB<br/>(MYrs) !! colspan=4 | Start !! colspan=4 | End
|-
! He Core ({{solar mass}}) !! T<sub>eff</sub> (K) !! Radius ({{solar radius}}) !! Luminosity ({{solar luminosity}}) !! He Core ({{solar mass}}) !! T<sub>eff</sub> (K) !! Radius ({{solar radius}}) !! Luminosity ({{solar luminosity}})
|- style="text-align:right;"
| 0.6 || [[Lacaille 8760|{{nowrap|Lacaille 8760}}]] || 58.8 || N/A || 5,100 || 0.047 || 4,763 || 0.9 || 0.9 || 0.10 || 4,634 || 1.2 || 0.6
|- style="text-align:right;"
| 1.0 || The [[Sun]] || 9.3 || N/A || 2,600 || 0.025 || 5,766 || 1.2 || 1.5 || 0.13 || 5,034 || 2.0 || 2.2
|- style="text-align:right;"
| 2.0 || [[Sirius]] || 1.2 || 10 || 22 || 0.240 || 7,490 || 3.6 || 36.6 || 0.25 || 5,220 || 5.4 || 19.6
|- style="text-align:right;"
| 5.0 || [[Alkaid]] || 0.1 || 0.4 || 15 || 0.806 || 14,544 || 6.3 || 1,571.4 || 0.83 || 4,737 || 43.8 || 866.0
|}
 
In general, stars with lower metallicity are smaller and hotter than stars with higher metallicity. For subgiants, this is complicated by different ages and core masses at the [[main sequence turnoff]]. Low metallicity stars develop a larger helium core before leaving the main sequence, hence lower mass stars show a hook at the start of the subgiant branch. The helium core mass of a Z=0.001 (extreme [[population II]]) {{solar mass|1}} star at the end of the main sequence is nearly double that of a Z=0.02 ([[population I]]) star. The low metallicity star is also over 1,000 K hotter and over twice as luminous at the start of the subgiant branch. The difference in temperature is less pronounced at the end of the subgiant branch, but the low metallicity star is larger and nearly four times as luminous. Similar differences exist in the evolution of stars with other masses, and key values such as the mass of a star that will become a supergiant instead of reaching the red giant branch are lower at low metallicity.<ref name=pols/>
 
==Subgiants in the H–R diagram==
[[File:HRDiagram.png|thumb|left|upright=1.2|H–R diagram of the entire [[Hipparcos]] catalog]]
A Hertzsprung–Russell (H–R) diagram is a scatter plot of stars with temperature or spectral type on the x-axis and absolute magnitude or luminosity on the y-axis. H–R diagrams of all stars, show a clear diagonal main sequence band containing the majority of stars, a significant number of red giants (and white dwarfs if sufficiently faint stars are observed), with relatively few stars in other parts of the diagram.
 
Subgiants occupy a region above (i.e. more luminous than) the main sequence stars and below the giant stars. There are relatively few on most H–R diagrams because the time spent as a subgiant is much less than the time spent on the main sequence or as a giant star. Hot, class B, subgiants are barely distinguishable from the main sequence stars, while cooler subgiants fill a relatively large gap between cool main sequence stars and the red giants. Below approximately spectral type K3 the region between the main sequence and red giants is entirely empty, with no subgiants.<ref name=mk/>
 
[[File:Open cluster HR diagram ages.gif|thumb|right|upright=1.0|Old open clusters showing a subgiant branch between the [[main sequence turnoff]] and the red giant branch, with a hook at the younger M67 turnoff<ref name=WIYN>{{cite journal|bibcode=1999AJ....118.2321S|title=WIYN Open Cluster Study. III. The Observed Variation of the Red Clump Luminosity and Color with Metallicity and Age|journal=The Astronomical Journal|volume=118|issue=5|pages=2321–2326|last1=Sarajedini|first1=Ata|year=1999|doi=10.1086/301112|doi-access=free}}</ref>]]
Stellar evolutionary tracks can be plotted on an H–R diagram. For a particular mass, these trace the position of a star throughout its life, and show a track from the initial main sequence position, along the subgiant branch, to the giant branch. When an H–R diagram is plotted for a group of stars which all have the same age, such as a cluster, the subgiant branch may be visible as a band of stars between the main sequence turnoff point and the red giant branch. The subgiant branch is only visible if the cluster is sufficiently old that {{solar mass|1-8}} stars have evolved away from the main sequence, which requires several billion years. [[Globular cluster]]s such as [[Omega Centauri|ω Centauri]] and old [[open cluster]]s such as [[Messier 67|M67]] are sufficiently old that they show a pronounced subgiant branch in their [[color–magnitude diagram]]s. ω Centauri actually shows several separate subgiant branches for reasons that are still not fully understood, but appear to represent stellar populations of different ages within the cluster.<ref name=pancino>{{cite journal|doi=10.1051/0004-6361/201016024|title=The subgiant branch ofω Centauri seen through high-resolution spectroscopy|journal=Astronomy & Astrophysics|volume=527|pages=A18|year=2011|last1=Pancino|first1=E.|last2=Mucciarelli|first2=A.|last3=Sbordone|first3=L.|last4=Bellazzini|first4=M.|last5=Pasquini|first5=L.|last6=Monaco|first6=L.|last7=Ferraro|first7=F. R.|arxiv = 1012.4756 |bibcode = 2011A&A...527A..18P |s2cid=54951859}}</ref>
 
==Variability==
Several types of [[variable star]] include subgiants:
* [[Beta Cephei variable]]s, early B main sequence and subgiant stars
* [[Slowly pulsating B-type star]]s, mid to late B main sequence and subgiant stars
* [[Delta Scuti variable]]s, late A and early F main sequence and subgiant stars
 
Subgiants more massive than the sun cross the [[Cepheid variable|Cepheid]] [[instability strip]], called the ''first crossing'' since they may cross the strip again later on a [[blue loop]]. In the {{solar mass|2 – 3}} range, this includes Delta Scuti variables such as [[Beta Cassiopeiae|β Cas]].<ref name=ayres1084>{{cite journal|bibcode=1984iue..prop.1747A|title=A Far-Ultraviolet Study of the Bright Delta Scuti Variable Beta Cassiopeia|journal=IUE Proposal ID #DSGTA|last1=Ayres|first1=Thomas R.|year=1984|page=1747}}</ref> At higher masses the stars would pulsate as [[Classical Cepheid variables]] while crossing the instability strip, but massive subgiant evolution is very rapid and it is difficult to detect examples. [[SV Vulpeculae]] has been proposed as a subgiant on its first crossing<ref name=luck>{{cite journal|bibcode=2001A&A...373..589L|title=SV Vulpeculae: A first crossing Cepheid?|journal=Astronomy and Astrophysics|volume=373|issue=2|pages=589|last1=Luck|first1=R. E.|last2=Kovtyukh|first2=V. V.|last3=Andrievsky|first3=S. M.|year=2001|doi=10.1051/0004-6361:20010615|doi-access=free}}</ref> but was subsequently determined to be on its second crossing <ref name=turner>{{cite journal|bibcode=2004A&A...423..335T|title=On the crossing mode of the long-period Cepheid SV Vulpeculae|journal=Astronomy and Astrophysics|volume=423|last1=Turner|first1=D. G.|last2=Berdnikov|first2=L. N.|year=2004|doi=10.1051/0004-6361:20040163|pages=335–340|doi-access=free}}</ref>
 
==Planets==
Planets in orbit around subgiant stars include [[Kappa Andromedae b]]<ref>Plait, Phil. [http://www.slate.com/blogs/bad_astronomy/2012/11/19/newly_discovered_planet_kappa_andromedae_b_seen_in_picture_of_nearby_star.html "Astronomers Take a Picture of a Planet Orbiting Another Star"]. Accessed 1 Feb. 2018</ref> and [[HD 224693 b]].<ref>[http://exoplanet.eu/catalog/hd_224693_b/ "Planet HD 224693 b"], ''Extrasolar Planet Encyclopaedia''. Accessed 1 Feb. 2018</ref>
 
==References==
{{reflist|colwidth=25em}}
 
==Bibliography==
* {{cite journal|bibcode=1993ApJ...413..641V|title=Evolution of low- and intermediate-mass stars to the end of the asymptotic giant branch with mass loss|journal=Astrophysical Journal|volume=413|pages=641|last1=Vassiliadis|first1=E.|last2=Wood|first2=P. R.|year=1993|doi=10.1086/173033}}
* {{cite journal|bibcode=1998MNRAS.298..525P|title=Stellar evolution models for Z = 0.0001 to 0.03|journal=Monthly Notices of the Royal Astronomical Society|volume=298|issue=2|pages=525|last1=Pols|first1=Onno R.|last2=Schröder|first2=Klaus-Peter|last3=Hurley|first3=Jarrod R.|last4=Tout|first4=Christopher A.|last5=Eggleton|first5=Peter P.|year=1998|doi=10.1046/j.1365-8711.1998.01658.x|doi-access=free}}
* {{cite journal|bibcode= 2000A&AS..141..371G|doi=10.1051/aas:2000126|title=Evolutionary tracks and isochrones for low- and intermediate-mass stars: From 0.15 to 7 M?, and from Z=0.0004 to 0.03|journal=Astronomy and Astrophysics Supplement Series|volume=141|issue=3|pages=371–383|year=2000|last1=Girardi|first1=L.|last2=Bressan|first2=A.|last3=Bertelli|first3=G.|last4=Chiosi|first4=C.|arxiv = astro-ph/9910164 |s2cid=14566232}}
 
==External links==
* [https://astro.uni-bonn.de/~nlanger/siu_web/ssescript/new/chapter9.pdf Post-main sequence evolution through helium burning]
* [http://othes.univie.ac.at/17128/ Long period variables - period luminosity relations and classification in the Gaia Mission]
 
{{Star}}
 
[[Category:Star types]]
[[Category:Subgiant stars| ]]
[[Category:Hertzsprung–Russell classifications]]