Editing Subgiant (section)

==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>