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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}}.
 
===Properties특성===
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 | Mass질량<br/>({{solar mass}}) !! rowspan="2" | Example예시 !! rowspan="2" | MS (GYrs) !! rowspan=2 | Hook (MYrs) !! rowspan=2 | SB<br/>(MYrs) !! colspan=4 | Start !! colspan=4 | End
(10억 년)
! rowspan="2" | Hook
(100만 년)
! rowspan="2" | SB<br/>(100만 년) !! colspan="4" | 시작 !! colspan="4" | 끝
|-
! He Core ({{solar mass}}) !! T<sub>eff</sub> (K) !! Radius반지름 ({{solar radius}}) !! Luminosity광도 ({{solar luminosity}}) !! He Core ({{solar mass}}) !! T<sub>eff</sub> (K) !! Radius반지름 ({{solar radius}}) !! Luminosity광도 ({{solar luminosity}})
|- style="text-align:right;"
| 0.6 || [[Lacaille라카유 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]]태양 || 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]]시리우스 || 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]]알카이드 || 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/>
 
== HR 도표에서의 준거성 ==
==Subgiants in the H–R diagram==
[[File:HRDiagram.png|thumb|left|upright=1.2|H–R히파르코스 diagram목록 of전체 the항성들의 entireHR [[Hipparcos]] catalog도표]]
헤르츠스프룽-러셀(HR) 도표는 x축에는 표면온도 또는 분광형, y축에는 절대등급 또는 광도를 표시한 도표이다. 모든 항성을 표시한 HR 도표에는 항성들 중 대다수를 포함하는 명확한 대각선 모양의 주계열대(帶)와, 두드러지게 많은 수효의 적색 거성(및 백색 왜성, 다만 어두운 별들을 충분히 관측할 경우)이 나타난다. 도표 내 다른 부분에 위치한 별들의 수효는 상대적으로 적다.
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/>
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>
 
==Variability변광==
Several types of [[variable star]] include subgiants:
* [[Beta Cephei variable]]s, early B 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>
 
==Planets행성==
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>
 
==References각주==
{{reflist|colwidth=25em}}
 
==Bibliography참고 문헌==
* {{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}}
 
==External외부 links링크==
* [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]