Editing Main sequence (section)

== Evolutionary tracks ==
{{Main|Stellar evolution}}
[[File:Evolutionary track 1m.svg|thumb|left|Evolutionary track of a star like the sun]]

When a main-sequence star has consumed the hydrogen at its core, the loss of energy generation causes its gravitational collapse to resume and the star evolves off the main sequence.  The path which the star follows across the HR diagram is called an evolutionary track.<ref name="Iben2012">{{cite book |author=Icko Iben |title=Stellar Evolution Physics |url=https://books.google.com/books?id=IU357EiecWwC&pg=PA1481 |date=29 November 2012 |publisher=Cambridge University Press |isbn=978-1-107-01657-6 |pages=1481–}}</ref>

[[File: Open cluster HR diagram ages.gif|right|thumb|upright=1.2|[[Hertzsprung–Russell diagram|H–R diagram]] for two open clusters: [[NGC 188]] (blue) is older and shows a lower turn off from the main sequence than [[Messier 67|M67]] (yellow). The dots outside the two sequences are mostly foreground and background stars with no relation to the clusters.]]

Stars with less than {{solar mass|0.23}}<ref name=romp69>{{cite journal |author1=Adams, Fred C. |author2=Laughlin, Gregory |title=A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects |journal=Reviews of Modern Physics |date=April 1997 |volume=69 |issue=2 |pages=337–372 |doi=10.1103/RevModPhys.69.337 |bibcode=1997RvMP...69..337A |arxiv=astro-ph/9701131 |s2cid=12173790}}</ref> are predicted to directly become [[white dwarf]]s when energy generation by nuclear fusion of hydrogen at their core comes to a halt, but stars in this mass range have main-sequence lifetimes longer than the current age of the universe, so no stars are old enough for this to have occurred.

In stars more massive than {{solar mass|0.23}}, the hydrogen surrounding the helium core reaches sufficient temperature and pressure to undergo fusion, forming a hydrogen-burning shell and causing the outer layers of the star to expand and cool. The stage as these stars move away from the main sequence is known as the [[subgiant branch]]; it is relatively brief and appears as a [[Hertzsprung gap|gap]] in the evolutionary track since few stars are observed at that point.

When the helium core of low-mass stars becomes degenerate, or the outer layers of intermediate-mass stars cool sufficiently to become opaque, their hydrogen shells increase in temperature and the stars start to become more luminous. This is known as the [[red-giant branch]]; it is a relatively long-lived stage and it appears prominently in H–R diagrams. These stars will eventually end their lives as white dwarfs.<ref name=pmss_atoe>{{cite web |author=Staff |date=12 October 2006 |url=http://outreach.atnf.csiro.au/education/senior/astrophysics/stellarevolution_postmain.html |title=Post-Main Sequence Stars |publisher=Australia Telescope Outreach and Education |access-date=2008-01-08 |archive-url=https://web.archive.org/web/20130120215215/http://outreach.atnf.csiro.au/education/senior/astrophysics/stellarevolution_postmain.html |archive-date=20 January 2013 }}</ref><ref name=aaas141>{{cite journal |author1=Girardi, L. |author2=Bressan, A. |author3=Bertelli, G. |author4=Chiosi, C. |title=Evolutionary tracks and isochrones for low- and intermediate-mass stars: From 0.15 to 7 M<sub>sun</sub>, and from Z=0.0004 to 0.03 |journal=Astronomy and Astrophysics Supplement |date=2000 |volume=141 |issue=3 |pages=371–383 |doi=10.1051/aas:2000126 |arxiv=astro-ph/9910164 |bibcode=2000A&AS..141..371G |s2cid=14566232}}</ref>

The most massive stars do not become red giants; instead, their cores quickly become hot enough to fuse helium and eventually heavier elements and they are known as [[supergiant]]s. They follow approximately horizontal evolutionary tracks from the main sequence across the top of the H–R diagram.  Supergiants are relatively rare and do not show prominently on most H–R diagrams. Their cores will eventually collapse, usually leading to a [[supernova]] and leaving behind either a [[neutron star]] or [[black hole]].<ref name=sitko00>{{cite web |last=Sitko |first=Michael L. |date=24 March 2000 |url=http://www.physics.uc.edu/~sitko/Spring00/4-Starevol/starevol.html |title=Stellar Structure and Evolution |publisher=University of Cincinnati |access-date=2007-12-05  |archive-url=https://web.archive.org/web/20050326090756/http://www.physics.uc.edu/~sitko/Spring00/4-Starevol/starevol.html |archive-date=26 March 2005}}</ref>

When a [[star cluster|cluster of stars]] is formed at about the same time, the main-sequence lifespan of these stars will depend on their individual masses.  The most massive stars will leave the main sequence first, followed in sequence by stars of ever lower masses. The position where stars in the cluster are leaving the main sequence is known as the [[turnoff point]]. By knowing the main-sequence lifespan of stars at this point, it becomes possible to estimate the age of the cluster.<ref name=science299_5603>{{cite journal |last=Krauss |first=Lawrence M. |author2=Chaboyer, Brian |title=Age Estimates of Globular Clusters in the Milky Way: Constraints on Cosmology |journal=Science |date=2003 |volume=299 |issue=5603 |pages=65–69 |doi=10.1126/science.1075631 |pmid=12511641 |bibcode=2003Sci...299...65K |s2cid=10814581 }}</ref>