Editing Star (section)

==Structure==
{{Main|Stellar structure}}
[[Image:Star types.svg|left|thumb|upright=1.4|Internal structures of main sequence stars with masses indicated in solar masses, convection zones with arrowed cycles, and radiative zones with red flashes. Left to right, a [[red dwarf]], a [[Yellow dwarf star|yellow dwarf]], and a [[blue-white main sequence star]]]]

The interior of a stable star is in a state of [[hydrostatic equilibrium]]: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure [[gradient]] within the star. The [[pressure gradient]] is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of {{val|e=7|u=K}}. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for [[nuclear fusion]] to occur and for sufficient energy to be produced to prevent further collapse of the star.<ref name="hansen">{{cite book |last1=Hansen |first1=Carl J. |last2=Kawaler |first2=Steven D. |last3=Trimble |first3=Virginia |pages=[https://archive.org/details/springer_10.1007-978-1-4419-9110-2/page/n44 32]–33 |title=Stellar Interiors |url=https://archive.org/details/springer_10.1007-978-1-4419-9110-2 |publisher=Springer |date=2004 |isbn=978-0-387-20089-7}}</ref><ref name="Schwarzschild">{{cite book|title=Structure and Evolution of the Stars|last=Schwarzschild|first=Martin|date=1958|publisher=Princeton University Press|isbn=978-0-691-08044-4}}<!-- Book republished by Dover as ISBN 0-486-61479-4, but ISBN in the cite book template is the one as published by Prin. Univ. Press--></ref>

As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant, and energy production ceases at the core. Instead, for stars of more than {{Solar mass|0.4}}, fusion occurs in a slowly expanding shell around the [[degenerate matter|degenerate]] helium core.<ref>{{cite web |url= http://aether.lbl.gov/www/tour/elements/stellar/stellar_a.html |title= Formation of the High Mass Elements |publisher= Smoot Group |access-date= 2006-07-11}}</ref>

In addition to hydrostatic equilibrium, the interior of a stable star will maintain an energy balance of [[thermal equilibrium]]. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.<ref name="HuangYu1998">{{cite book |last1=Huang |first1=R. Q. |url=https://books.google.com/books?id=TInvAAAAMAAJ |title=Stellar Astrophysics |last2=Yu |first2=K. N. |publisher=Springer |year=1998 |isbn=978-981-3083-36-3 |page=70}}</ref>

The [[radiation zone]] is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone. In this region the plasma will not be perturbed, and any mass motions will die out. Where this is not the case, then the plasma becomes unstable and convection will occur, forming a [[convection zone]]. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high [[opacity (optics)|opacity]] (making radiatative heat transfer inefficient) as in the outer envelope.<ref name="Schwarzschild" />

The occurrence of convection in the outer envelope of a main sequence star depends on the star's mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.<ref name="imagine">{{cite web |date= 2006-09-01 |url= http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html |title= What is a Star? |publisher= NASA |access-date= 2006-07-11}}</ref> Red dwarf stars with less than {{Solar mass|0.4}} are convective throughout, which prevents the accumulation of a helium core.<ref name="late stages" /> For most stars the convective zones will vary over time as the star ages and the constitution of the interior is modified.<ref name="Schwarzschild" />

[[File:Sun parts big.jpg|thumb|upright=1.6|A cross-section of the [[Sun]]]]
The photosphere is that portion of a star that is visible to an observer. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate into space. It is within the photosphere that [[sun spots]], regions of lower than average temperature, appear.<ref name="NewcombHolden1887">{{cite book|author1=Simon Newcomb|author2=Edward Singleton Holden|title=Astronomy for High Schools and Colleges|url=https://books.google.com/books?id=87YXAAAAIAAJ&pg=PA278|year=1887|publisher=H. Holt|pages=278}}</ref>

Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin [[chromosphere]] region, where [[Solar spicule|spicules]] appear and [[solar flare|stellar flares]] begin. Above this is the transition region, where the temperature rapidly increases within a distance of only {{convert|100|km|0|abbr=on}}. Beyond this is the [[stellar corona|corona]], a volume of super-heated plasma that can extend outward to several million kilometres.<ref>{{cite press release
 | publisher= ESO | date=2001-08-01
 | title= The Glory of a Nearby Star: Optical Light from a Hot Stellar Corona Detected with the VLT
 | url= http://www.eso.org/public/news/eso0127/
 | access-date= 2006-07-10}}</ref> The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.<ref name= "imagine" /> Despite its high temperature, the corona emits very little light, due to its low gas density.<ref>{{Cite web |title=What Is the Sun's Corona? {{!}} NASA Space Place – NASA Science for Kids |url=https://spaceplace.nasa.gov/sun-corona/en/#:~:text=Why?,the%20surface%20of%20the%20Sun. |access-date=2023-11-21 |website=spaceplace.nasa.gov}}</ref> The corona region of the Sun is normally only visible during a [[solar eclipse]].

From the corona, a stellar wind of plasma particles expands outward from the star, until it interacts with the interstellar medium. For the Sun, the influence of its [[solar wind]] extends throughout a bubble-shaped region called the [[heliosphere]].<ref>{{cite journal | display-authors= 1
 | last1= Burlaga | first1= L. F. | last2= Ness | first2= N. F. | last3= Acuña | first3= M. H. | last4= Lepping | first4= R. P. | last5= Connerney | first5= J. E. P. | last6= Stone | first6= E. C. | last7= McDonald | first7= F. B.
 | title= Crossing the Termination Shock into the Heliosheath: Magnetic Fields
 | journal= Science | date= 2005 | volume= 309
 | issue= 5743 | pages= 2027–2029 | doi= 10.1126/science.1117542
 | pmid= 16179471 | bibcode= 2005Sci...309.2027B| s2cid= 5998363 }}</ref>