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Qator 821:
| publisher = White Dwarf Research Corporation
| accessdate = 2006-07-19 }}</ref>
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==Oʻzgaruvchi yulduzlar==
{{Main|Oʻzgaruvchi yulduz}}
[[File:Mira 1997.jpg|left|thumb|200px|The asymmetrical appearance of [[Mira]], an oscillating variable star. ''NASA [[Hubble Space Telescope|HST]] image'']]
Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.
During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes [[Cepheid variable|Cepheid and cepheid-like stars]], and long-period variables such as [[Mira variable|Mira]].<ref name="variables">{{cite web | url=http://www.aavso.org/types-variables | title=Types of Variable | date=May 11, 2010 | publisher=AAVSO | accessdate=2010-08-20 }}</ref>
Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.<ref name="variables" /> This group includes protostars, Wolf-Rayet stars, and [[Flare star]]s, as well as giant and supergiant stars.
Cataclysmic or explosive variables undergo a dramatic change in their properties. This group includes [[nova]]e and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.<ref name="iben" /> The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.<ref>{{cite web
| date =2004-11-01 | url = http://imagine.gsfc.nasa.gov/docs/science/know_l2/cataclysmic_variables.html
| title = Cataclysmic Variables
| publisher = NASA Goddard Space Flight Center
| accessdate = 2006-06-08 }}</ref> Some novae are also recurrent, having periodic outbursts of moderate amplitude.<ref name="variables" />
Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.<ref name="variables" /> A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.3 to 3.5 over a period of 2.87 days. -->
==Tuzilishi==
{{Main|Yulduz tuzilishi}}
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 10<sup>7</sup> [[kelvin|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=32–33 | title=Stellar Interiors | publisher=Springer | year=2004 | isbn=0-387-20089-4 }}</ref><ref name="Schwarzschild">{{cite book
| first=Martin | last=Schwarzschild | title=Structure and Evolution of the Stars | publisher=Princeton University Press | year=1958 | isbn=0-691-08044-5}}<!-- 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 ray]]s. 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 0.4 solar masses, 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 | accessdate = 2006-07-11 }}</ref>
In addition to hydrostatic equilibrium, the interior of a stable star will also 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.
[[File:Sun parts big.jpg|thumb|360px|left|This diagram shows a cross-section of a solar-type star. ''NASA image'']]
The [[radiation zone]] is the region within the stellar interior where radiative transfer is sufficiently efficient to maintain the flux of energy. In this region the plasma will not be perturbed and any mass motions will die out. If this is not the case, however, 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]] as in the outer envelope.<ref name="Schwarzschild" />
The occurrence of convection in the outer envelope of a main sequence star depends on the 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 | accessdate = 2006-07-11 }}</ref> Red dwarf stars with less than 0.4 solar masses are convective throughout, which prevents the accumulation of a helium core.<ref name="late stages" /> For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.<ref name="Schwarzschild" />
The portion of a star that is visible to an observer is called the [[photosphere]]. 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 out into space. It is within the photosphere that [[sun spots]], or regions of lower than average temperature, appear.
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 is the thin [[chromosphere]] region, where [[spicule (solar physics)|spicule]]s appear and [[Solar flare|stellar flares]] begin. This is surrounded by a transition region, where the temperature rapidly increases within a distance of only {{convert|100|km|0|abbr=on}}. Beyond this is the [[corona]], a volume of super-heated plasma that can extend outward to several million kilometres.<ref>{{cite press release
| publisher=ESO | date=August 1, 2001
| 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/
| accessdate=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. 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, propagating until it interacts with the [[interstellar medium]]. For the Sun, the influence of its [[solar wind]] extends throughout the bubble-shaped region of 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 | year=2005 | volume=309
| issue=5743 | pages=2027–2029 | doi= 10.1126/science.1117542
| pmid=16179471 | bibcode=2005Sci...309.2027B }}</ref>
==Manbalar==
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