Editing Stellar evolution (section)

===Mid-sized stars===
[[File:Evolutionary track 1m.svg|thumb|left|upright=1.35|The evolutionary track of a solar mass, solar metallicity, star from main sequence to post-AGB]]
Stars of roughly {{Solar mass|0.6–10}} become [[red giant]]s, which are large non-[[main sequence|main-sequence]] stars of [[stellar classification]] K or M. Red giants lie along the right edge of the Hertzsprung–Russell diagram due to their red color and large luminosity. Examples include [[Aldebaran]] in the constellation [[Taurus (constellation)|Taurus]] and [[Arcturus]] in the constellation of [[Boötes]].

Mid-sized stars are red giants during two different phases of their post-main-sequence evolution: red-giant-branch stars, with inert cores made of helium and hydrogen-burning shells, and asymptotic-giant-branch stars, with inert cores made of carbon and helium-burning shells inside the hydrogen-burning shells.<ref>{{harvtxt|Hansen|Kawaler|Trimble|2004|pages=55–56}}</ref>  Between these two phases, stars spend a period on the [[horizontal branch]] with a helium-fusing core.  Many of these helium-fusing stars cluster towards the cool end of the horizontal branch as K-type giants and are referred to as [[red clump]] giants.

====Subgiant phase====
{{Main|Subgiant}}
When a star exhausts the hydrogen in its core, it leaves the main sequence and begins to fuse hydrogen in a shell outside the core.  The core increases in mass as the shell produces more helium.  Depending on the mass of the helium core, this continues for several million to one or two billion years, with the star expanding and cooling at a similar or slightly lower luminosity to its main sequence state.  Eventually either the core becomes degenerate, in stars around the mass of the sun, or the outer layers cool sufficiently to become opaque, in more massive stars.  Either of these changes cause the hydrogen shell to increase in temperature and the [[luminosity]] of the star to increase, at which point the star expands onto the red-giant branch.<ref name="RyanNorton115">{{harvtxt|Ryan|Norton|2010|page=115}}</ref>

====Red-giant-branch phase====
[[Image:The life of Sun-like stars.jpg|thumb|upright=1.35|Artist's depiction of the life cycle of a Sun-like star, starting as a main-sequence star at lower left then expanding through the [[subgiant]] and [[giant star|giant]] phases, until its outer envelope is expelled to form a [[planetary nebula]] at upper right]]

{{Main|Red-giant branch}}
The expanding outer layers of the star are [[convection|convective]], with the material being mixed by turbulence from near the fusing regions up to the surface of the star.  For all but the lowest-mass stars, the fused material has remained deep in the stellar interior prior to this point, so the convecting envelope makes fusion products visible at the star's surface for the first time. At this stage of evolution, the results are subtle, with the largest effects, alterations to the [[isotopes]] of hydrogen and helium, being unobservable. The effects of the [[CNO cycle]] appear at the surface during the first [[dredge-up]], with lower <sup>12</sup>C/<sup>13</sup>C ratios and altered proportions of carbon and nitrogen. These are detectable with [[spectroscopy]] and have been measured for many evolved stars.

The helium core continues to grow on the red-giant branch.  It is no longer in thermal equilibrium, either degenerate or above the [[Schönberg–Chandrasekhar limit]], so it increases in temperature which causes the rate of fusion in the hydrogen shell to increase.  The star increases in luminosity towards the [[tip of the red-giant branch]].  Red-giant-branch stars with a degenerate helium core all reach the tip with very similar core masses and very similar luminosities, although the more massive of the red giants become hot enough to ignite helium fusion before that point.
====Horizontal branch====
{{Main|Horizontal branch|Red clump}}
In the helium cores of stars in the 0.6 to 2.0 solar mass range, which are largely supported by [[electron degeneracy pressure]], helium fusion will ignite on a timescale of days in a [[helium flash]]. In the nondegenerate cores of more massive stars, the ignition of helium fusion occurs relatively slowly with no flash.<ref>{{harvtxt|Ryan|Norton|2010|page=125}}</ref> The nuclear power released during the helium flash is very large, on the order of 10<sup>8</sup> times the [[Solar luminosity|luminosity of the Sun]] for a few days<ref name="RyanNorton115" /> and 10<sup>11</sup> times the luminosity of the Sun (roughly the luminosity of the [[Milky Way Galaxy]]) for a few seconds.<ref name="Prialnik151">{{harvtxt|Prialnik|2000|page=151}}</ref> However, the energy is consumed by the thermal expansion of the initially degenerate core and thus cannot be seen from outside the star.<ref name="RyanNorton115" /><ref name="Prialnik151" /><ref name="Deupree1996">{{cite journal|last1= Deupree|first1=R. G.|title= A Reexamination of the Core Helium Flash|journal= The Astrophysical Journal|volume=471|issue= 1|date= 1996-11-01|pages= 377–384|doi= 10.1086/177976|bibcode= 1996ApJ...471..377D|citeseerx= 10.1.1.31.44|s2cid=15585754 }}</ref> Due to the expansion of the core, the hydrogen fusion in the overlying layers slows and total energy generation decreases. The star contracts, although not all the way to the main sequence, and it migrates to the [[horizontal branch]] on the Hertzsprung–Russell diagram, gradually shrinking in radius and increasing its surface temperature.
[[File:The life cycle of a Sun-like star (annotated).jpg|thumb|upright=1.35|left|The change in size with time of a Sun-like star]]

Core helium flash stars evolve to the red end of the horizontal branch but do not migrate to higher temperatures before they gain a degenerate carbon-oxygen core and start helium shell burning.  These stars are often observed as a [[red clump]] of stars in the colour-magnitude diagram of a cluster, hotter and less luminous than the red giants. Higher-mass stars with larger helium cores move along the horizontal branch to higher temperatures, some becoming unstable pulsating stars in the yellow [[instability strip]] ([[RR Lyrae variables]]), whereas some become even hotter and can form a blue tail or blue hook to the horizontal branch. The morphology of the horizontal branch depends on parameters such as metallicity, age, and helium content, but the exact details are still being modelled.<ref name=parameters>{{Cite journal | last1 = Gratton | first1 = R. G. | last2 = Carretta | first2 = E. | last3 = Bragaglia | first3 = A. | last4 = Lucatello | first4 = S. | last5 = d'Orazi | first5 = V. | title = The second and third parameters of the horizontal branch in globular clusters | doi = 10.1051/0004-6361/200912572 | journal = Astronomy and Astrophysics | volume = 517 | pages = A81 | year = 2010 |arxiv = 1004.3862 |bibcode = 2010A&A...517A..81G | s2cid = 55701280 }}</ref>

====Asymptotic-giant-branch phase====
{{Main|Asymptotic giant branch}}
After a star has consumed the helium at the core, hydrogen and helium fusion continues in shells around a hot core of [[carbon]] and [[oxygen]]. The star follows the [[asymptotic giant branch]] on the Hertzsprung–Russell diagram, paralleling the original red-giant evolution, but with even faster energy generation (which lasts for a shorter time).<ref>{{Cite journal | last1 = Sackmann | first1 = I. -J. | last2 = Boothroyd | first2 = A. I. | last3 = Kraemer | first3 = K. E. | title = Our Sun. III. Present and Future | doi = 10.1086/173407 | journal = The Astrophysical Journal | volume = 418 | pages = 457 | year = 1993 |bibcode = 1993ApJ...418..457S | doi-access = free }}</ref>  Although helium is being burnt in a shell, the majority of the energy is produced by hydrogen burning in a shell further from the core of the star.  Helium from these hydrogen burning shells drops towards the center of the star and periodically the energy output from the helium shell increases dramatically.  This is known as a [[thermal pulse]] and they occur towards the end of the asymptotic-giant-branch phase, sometimes even into the post-asymptotic-giant-branch phase. Depending on mass and composition, there may be several to hundreds of thermal pulses.

There is a phase on the ascent of the asymptotic-giant-branch where a deep convective zone forms and can bring carbon from the core to the surface.  This is known as the second dredge up, and in some stars there may even be a third dredge up.  In this way a [[carbon star]] is formed, very cool and strongly reddened stars showing strong carbon lines in their spectra.  A process known as hot bottom burning may convert carbon into oxygen and nitrogen before it can be dredged to the surface, and the interaction between these processes determines the observed luminosities and spectra of carbon stars in particular clusters.<ref name=hbb>{{cite journal|author1=van Loon |author2=Zijlstra |author3=Whitelock|author4=Peter te Lintel Hekkert|author5=Chapman|author6=Cecile Loup|author7=Groenewegen|author8=Waters|author9=Trams|title=Obscured Asymptotic Giant Branch stars in the Magellanic Clouds IV. Carbon stars and OH/IR stars|date=1998|volume=329|issue=1|pages=169–85|journal= Monthly Notices of the Royal Astronomical Society|arxiv=astro-ph/9709119v1 |citeseerx=10.1.1.389.3269 |bibcode = 1996MNRAS.279...32Z |doi=10.1093/mnras/279.1.32 |doi-access=free |url=https://pure.uva.nl/ws/files/978044/1833_20357y.pdf}}</ref>

Another well known class of asymptotic-giant-branch stars is the [[Mira variable]]s, which pulsate with well-defined periods of tens to hundreds of days and large amplitudes up to about 10 magnitudes (in the visual, total luminosity changes by a much smaller amount). In more-massive stars the stars become more luminous and the pulsation period is longer, leading to enhanced mass loss, and the stars become heavily obscured at visual wavelengths.  These stars can be observed as [[OH/IR star]]s, pulsating in the infrared and showing OH [[maser]] activity.  These stars are clearly oxygen rich, in contrast to the carbon stars, but both must be produced by dredge ups.

====Post-AGB====
{{Main|Post-AGB}}
[[Image:NGC6543.jpg|thumb|The [[Cat's Eye Nebula]], a [[planetary nebula]] formed by the death of a star with about the same mass as the Sun]]
These mid-range stars ultimately reach the tip of the asymptotic-giant-branch and run out of fuel for shell burning. They are not sufficiently massive to start full-scale carbon fusion, so they contract again, going through a period of post-asymptotic-giant-branch superwind to produce a planetary nebula with an extremely hot central star. The central star then cools to a white dwarf. The expelled gas is relatively rich in heavy elements created within the star and may be particularly [[oxygen]] or [[carbon]] enriched, depending on the type of the star. The gas builds up in an expanding shell called a [[circumstellar envelope]] and cools as it moves away from the star, allowing [[Circumstellar dust|dust particles]] and molecules to form. With the high infrared energy input from the central star, ideal conditions are formed in these circumstellar envelopes for [[Astrophysical maser|maser]] excitation.

It is possible for thermal pulses to be produced once post-asymptotic-giant-branch evolution has begun, producing a variety of unusual and poorly understood stars known as born-again asymptotic-giant-branch stars.<ref name=bornagain>{{cite journal|bibcode=1991IAUS..145..363H|title=Atmospheres and Abundances of Blue Horizontal Branch Stars and Related Objects|journal=Evolution of Stars: The Photospheric Abundance Connection: Proceedings of the 145th Symposium of the International Astronomical Union|volume=145|pages=363|last1=Heber|first1=U.|year=1991}}</ref> These may result in extreme [[horizontal-branch]] stars ([[subdwarf B star]]s), hydrogen deficient post-asymptotic-giant-branch stars, variable planetary nebula central stars, and [[R Coronae Borealis variable]]s.