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= = = = Massive stars = = = = |
During their helium @-@ burning phase , stars of more than nine solar masses expand to form red supergiants . When this fuel is exhausted at the core , they continue to fuse elements heavier than helium . |
The core contracts and the temperature and pressure rises enough to fuse carbon ( see Carbon burning process ) . This process continues , with the successive stages being fueled by neon ( see neon burning process ) , oxygen ( see oxygen burning process ) , and silicon ( see silicon burning process ) . Near the end of the star 's life , fusion continues along a series of onion @-@ layer shells within a massive star . Each shell fuses a different element , with the outermost shell fusing hydrogen ; the next shell fusing helium , and so forth . |
The final stage occurs when a massive star begins producing iron . Since iron nuclei are more tightly bound than any heavier nuclei , any fusion beyond iron does not produce a net release of energy . To a very limited degree such a process proceeds , but it consumes energy . Likewise , since they are more tightly bound than all lighter nuclei , such energy cannot be released by fission . In relatively old , very massive stars , a large core of inert iron will accumulate in the center of the star . The heavier elements in these stars can work their way to the surface , forming evolved objects known as Wolf @-@ Rayet stars that have a dense stellar wind which sheds the outer atmosphere . |
= = = = Collapse = = = = |
As a star 's core shrinks , the intensity of radiation from that surface increases , creating such radiation pressure on the outer shell of gas that it will push those layers away , forming a planetary nebula . If what remains after the outer atmosphere has been shed is less than 1 @.@ 4 M β , it shrinks to a relatively tiny object about the size of Earth , known as a white dwarf . White dwarfs lack the mass for further gravitational compression to take place . The electron @-@ degenerate matter inside a white dwarf is no longer a plasma , even though stars are generally referred to as being spheres of plasma . Eventually , white dwarfs fade into black dwarfs over a very long period of time . |
In larger stars , fusion continues until the iron core has grown so large ( more than 1 @.@ 4 M β ) that it can no longer support its own mass . This core will suddenly collapse as its electrons are driven into its protons , forming neutrons , neutrinos , and gamma rays in a burst of electron capture and inverse beta decay . The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova . Supernovae become so bright that they may briefly outshine the star 's entire home galaxy . When they occur within the Milky Way , supernovae have historically been observed by naked @-@ eye observers as " new stars " where none seemingly existed before . |
A supernova explosion blows away the star 's outer layers , leaving a remnant such as the Crab Nebula . The core is compressed into a neutron star , which sometimes manifests itself as a pulsar or X @-@ ray burster . In the case of the largest stars , the remnant is a black hole greater than 4 M β ) s . In a neutron star the matter is in a state known as neutron @-@ degenerate matter , with a more exotic form of degenerate matter , QCD matter , possibly present in the core . Within a black hole , the matter is in a state that is not currently understood . |
The blown @-@ off outer layers of dying stars include heavy elements , which may be recycled during the formation of new stars . These heavy elements allow the formation of rocky planets . The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium . |
= = = = Binary stars = = = = |
The post β main @-@ sequence evolution of binary stars may be significantly different from the evolution of single stars of the same mass . If stars in a binary system are sufficiently close , when one of the stars expands to become a red giant it may overflow its Roche lobe , the region around a star where material is gravitationally bound to that star , leading to transfer of material to the other . When the Roche lobe is violated , a variety of phenomena can result , including contact binaries , common @-@ envelope binaries , cataclysmic variables , and type Ia supernovae . |
= = Distribution = = |
In addition to isolated stars , a multi @-@ star system can consist of two or more gravitationally bound stars that orbit each other . The simplest and most common multi @-@ star system is a binary star , but systems of three or more stars are also found . For reasons of orbital stability , such multi @-@ star systems are often organized into hierarchical sets of binary stars . Larger groups called star clusters also exist . These range from loose stellar associations with only a few stars , up to enormous globular clusters with hundreds of thousands of stars . Such systems orbit our Milky Way galaxy . |
It has been a long @-@ held assumption that the majority of stars occur in gravitationally bound , multiple @-@ star systems . This is particularly true for very massive O and B class stars , where 80 % of the stars are believed to be part of multiple @-@ star systems . The proportion of single star systems increases with decreasing star mass , so that only 25 % of red dwarfs are known to have stellar companions . As 85 % of all stars are red dwarfs , most stars in the Milky Way are likely single from birth . |
Stars are not spread uniformly across the universe , but are normally grouped into galaxies along with interstellar gas and dust . A typical galaxy contains hundreds of billions of stars , and there are more than 100 billion ( 1011 ) galaxies in the observable universe . In 2010 , one estimate of the number of stars in the observable universe was 300 sextillion ( 3 Γ 1023 ) . While it is often believed that stars only exist within galaxies , intergalactic stars have been discovered . |
The nearest star to the Earth , apart from the Sun , is Proxima Centauri , which is 39 @.@ 9 trillion kilometres , or 4 @.@ 2 light @-@ years . Travelling at the orbital speed of the Space Shuttle ( 8 kilometres per second β almost 30 @,@ 000 kilometres per hour ) , it would take about 150 @,@ 000 years to arrive . This it typical of stellar separations in galactic discs . Stars can be much closer to each other in the centres of galaxies and in globular clusters , or much farther apart in galactic halos . |
Due to the relatively vast distances between stars outside the galactic nucleus , collisions between stars are thought to be rare . In denser regions such as the core of globular clusters or the galactic center , collisions can be more common . Such collisions can produce what are known as blue stragglers . These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity of the cluster to which it belongs . |
= = Characteristics = = |
Almost everything about a star is determined by its initial mass , including such characteristics as luminosity , size , evolution , lifespan , and its eventual fate . |
= = = Age = = = |
Most stars are between 1 billion and 10 billion years old . Some stars may even be close to 13 @.@ 8 billion years old β the observed age of the universe . The oldest star yet discovered , HD 140283 , nicknamed Methuselah star , is an estimated 14 @.@ 46 Β± 0 @.@ 8 billion years old . ( Due to the uncertainty in the value , this age for the star does not conflict with the age of the Universe , determined by the Planck satellite as 13 @.@ 799 Β± 0 @.@ 021 ) . |
The more massive the star , the shorter its lifespan , primarily because massive stars have greater pressure on their cores , causing them to burn hydrogen more rapidly . The most massive stars last an average of a few million years , while stars of minimum mass ( red dwarfs ) burn their fuel very slowly and can last tens to hundreds of billions of years . |
= = = Chemical composition = = = |
When stars form in the present Milky Way galaxy they are composed of about 71 % hydrogen and 27 % helium , as measured by mass , with a small fraction of heavier elements . Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere , as iron is a common element and its absorption lines are relatively easy to measure . The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system . |
The star with the lowest iron content ever measured is the dwarf HE1327 @-@ 2326 , with only 1 / 200,000th the iron content of the Sun . By contrast , the super @-@ metal @-@ rich star ΞΌ Leonis has nearly double the abundance of iron as the Sun , while the planet @-@ bearing star 14 Herculis has nearly triple the iron . There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum ; especially chromium and rare earth elements . Stars with cooler outer atmospheres , including the Sun , can form various diatomic and polyatomic molecules . |
= = = Diameter = = = |
Due to their great distance from the Earth , all stars except the Sun appear to the unaided eye as shining points in the night sky that twinkle because of the effect of the Earth 's atmosphere . The Sun is also a star , but it is close enough to the Earth to appear as a disk instead , and to provide daylight . Other than the Sun , the star with the largest apparent size is R Doradus , with an angular diameter of only 0 @.@ 057 arcseconds . |
The disks of most stars are much too small in angular size to be observed with current ground @-@ based optical telescopes , and so interferometer telescopes are required to produce images of these objects . Another technique for measuring the angular size of stars is through occultation . By precisely measuring the drop in brightness of a star as it is occulted by the Moon ( or the rise in brightness when it reappears ) , the star 's angular diameter can be computed . |
Stars range in size from neutron stars , which vary anywhere from 20 to 40 km ( 25 mi ) in diameter , to supergiants like Betelgeuse in the Orion constellation , which has a diameter approximately 1 @,@ 070 times that of the Sun β about 1 @,@ 490 @,@ 171 @,@ 880 km ( 925 @,@ 949 @,@ 878 mi ) . Betelgeuse , however , has a much lower density than the Sun . |
= = = Kinematics = = = |
The motion of a star relative to the Sun can provide useful information about the origin and age of a star , as well as the structure and evolution of the surrounding galaxy . The components of motion of a star consist of the radial velocity toward or away from the Sun , and the traverse angular movement , which is called its proper motion . |
Radial velocity is measured by the doppler shift of the star 's spectral lines , and is given in units of km / s . The proper motion of a star , its parallax , is determined by precise astrometric measurements in units of milli @-@ arc seconds ( mas ) per year . With knowledge of the star 's parallax and its distance , the proper motion velocity can be calculated . Together with the radial velocity , the total velocity can be calculated . Stars with high rates of proper motion are likely to be relatively close to the Sun , making them good candidates for parallax measurements . |
When both rates of movement are known , the space velocity of the star relative to the Sun or the galaxy can be computed . Among nearby stars , it has been found that younger population I stars have generally lower velocities than older , population II stars . The latter have elliptical orbits that are inclined to the plane of the galaxy . A comparison of the kinematics of nearby stars has allowed astronomers to trace their origin to common points in giant molecular clouds , and are referred to as stellar associations . |
= = = Magnetic field = = = |
The magnetic field of a star is generated within regions of the interior where convective circulation occurs . This movement of conductive plasma functions like a dynamo , wherein the movement of elecrical charges induce magnetic fields , as does a mechanical dynamo . Those magnetic fields have a great range that extend throughout and beyond the star . The strength of the magnetic field varies with the mass and composition of the star , and the amount of magnetic surface activity depends upon the star 's rate of rotation . This surface activity produces starspots , which are regions of strong magnetic fields and lower than normal surface temperatures . Coronal loops are arching magnetic field flux lines that rise from a star 's surface into the star 's outer atmosphere , its corona . The coronal loops can be seen due to the plasma they conduct along their length . Stellar flares are bursts of high @-@ energy particles that are emitted due to the same magnetic activity . |
Young , rapidly rotating stars tend to have high levels of surface activity because of their magnetic field . The magnetic field can act upon a star 's stellar wind , functioning as a brake to gradually slow the rate of rotation with time . Thus , older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity . The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time . During the Maunder minimum , for example , the Sun underwent a 70 @-@ year period with almost no sunspot activity . |
= = = Mass = = = |
One of the most massive stars known is Eta Carinae , which , with 100 β 150 times as much mass as the Sun , will have a lifespan of only several million years . Studies of the most massive open clusters suggests 150 M β as an upper limit for stars in the current era of the universe . This represents an empirical value for the theoretical limit on the mass of forming stars due to increasing radiation pressure on the accreting gas cloud . Several stars in the R136 cluster in the Large Magellanic Cloud have been measured with larger masses , but it has been determined that they could have been created through the collision and merger of massive stars in close binary systems , sidestepping the 150 M β limit on massive star formation . |
The first stars to form after the Big Bang may have been larger , up to 300 M β , due to the complete absence of elements heavier than lithium in their composition . This generation of supermassive population III stars is likely to have existed in the very early universe ( i.e. , they are observed to have a high redshift ) , and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life . In June 2015 , astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6 @.@ 60 . |
With a mass only 80 times that of Jupiter ( MJ ) , 2MASS J0523 @-@ 1403 is the smallest known star undergoing nuclear fusion in its core . For stars with metallicity similar to the Sun , the theoretical minimum mass the star can have and still undergo fusion at the core , is estimated to be about 75 MJ . When the metallicity is very low , however , the minimum star size seems to be about 8 @.@ 3 % of the solar mass , or about 87 MJ . Smaller bodies called brown dwarfs , occupy a poorly defined grey area between stars and gas giants . |
The combination of the radius and the mass of a star determines its surface gravity . Giant stars have a much lower surface gravity than do main sequence stars , while the opposite is the case for degenerate , compact stars such as white dwarfs . The surface gravity can influence the appearance of a star 's spectrum , with higher gravity causing a broadening of the absorption lines . |
= = = Rotation = = = |
The rotation rate of stars can be determined through spectroscopic measurement , or more exactly determined by tracking their starspots . Young stars can have a rotation greater than 100 km / s at the equator . The B @-@ class star Achernar , for example , has an equatorial velocity of about 225 km / s or greater , causing its equator to be slung outward and giving it an equatorial diameter that is more than 50 % greater than between the poles . This rate of rotation is just below the critical velocity of 300 km / s at which speed the star would break apart . By contrast , the Sun rotates once every 25 β 35 days , with an equatorial velocity of 1 @.@ 994 km / s . A main sequence star 's magnetic field and the stellar wind serve to slow its rotation by a significant amount as it evolves on the main sequence . |
Degenerate stars have contracted into a compact mass , resulting in a rapid rate of rotation . However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum β the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin . A large portion of the star 's angular momentum is dissipated as a result of mass loss through the stellar wind . In spite of this , the rate of rotation for a pulsar can be very rapid . The pulsar at the heart of the Crab nebula , for example , rotates 30 times per second . The rotation rate of the pulsar will gradually slow due to the emission of radiation . |
= = = Temperature = = = |
The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius , and is often estimated from the star 's color index . The temperature is normally given in terms of an effective temperature , which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star . Note that the effective temperature is only a representative of the surface , as the temperature increases toward the core . The temperature in the core region of a star is several million kelvins . |
The stellar temperature will determine the rate of ionization of various elements , resulting in characteristic absorption lines in the spectrum . The surface temperature of a star , along with its visual absolute magnitude and absorption features , is used to classify a star ( see classification below ) . |
Massive main sequence stars can have surface temperatures of 50 @,@ 000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3 @,@ 600 K ; but they also have a high luminosity due to their large exterior surface area . |
= = Radiation = = |
The energy produced by stars , a product of nuclear fusion , radiates to space as both electromagnetic radiation and particle radiation . The particle radiation emitted by a star is manifested as the stellar wind , which streams from the outer layers as electrically charged protons and alpha and beta particles . Although almost massless , there also exists a steady stream of neutrinos emanating from the star 's core . |
The production of energy at the core is the reason stars shine so brightly : every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element , gamma ray photons are released from the nuclear fusion product . This energy is converted to other forms of electromagnetic energy of lower frequency , such as visible light , by the time it reaches the star 's outer layers . |
The color of a star , as determined by the most intense frequency of the visible light , depends on the temperature of the star 's outer layers , including its photosphere . Besides visible light , stars also emit forms of electromagnetic radiation that are invisible to the human eye . In fact , stellar electromagnetic radiation spans the entire electromagnetic spectrum , from the longest wavelengths of radio waves through infrared , visible light , ultraviolet , to the shortest of X @-@ rays , and gamma rays . From the standpoint of total energy emitted by a star , not all components of stellar electromagnetic radiation are significant , but all frequencies provide insight into the star 's physics . |
Using the stellar spectrum , astronomers can also determine the surface temperature , surface gravity , metallicity and rotational velocity of a star . If the distance of the star is found , such as by measuring the parallax , then the luminosity of the star can be derived . The mass , radius , surface gravity , and rotation period can then be estimated based on stellar models . ( Mass can be calculated for stars in binary systems by measuring their orbital velocities and distances . Gravitational microlensing has been used to measure the mass of a single star . ) With these parameters , astronomers can also estimate the age of the star . |
= = = Luminosity = = = |
The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time . It has units of power . The luminosity of a star is determined by its radius and surface temperature . Many stars do not radiate uniformly across their entire surface . The rapidly rotating star Vega , for example , has a higher energy flux ( power per unit area ) at its poles than along its equator . |
Patches of the star 's surface with a lower temperature and luminosity than average are known as starspots . Small , dwarf stars such as our Sun generally have essentially featureless disks with only small starspots . Giant stars have much larger , more obvious starspots , and they also exhibit strong stellar limb darkening . That is , the brightness decreases towards the edge of the stellar disk . Red dwarf flare stars such as UV Ceti may also possess prominent starspot features . |
= = = Magnitude = = = |
The apparent brightness of a star is expressed in terms of its apparent magnitude . It is a function of the star 's luminosity , its distance from Earth , and the altering of the star 's light as it passes through Earth 's atmosphere . Intrinsic or absolute magnitude is directly related to a star 's luminosity , and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs ( 32 @.@ 6 light @-@ years ) . |
Both the apparent and absolute magnitude scales are logarithmic units : one whole number difference in magnitude is equal to a brightness variation of about 2 @.@ 5 times ( the 5th root of 100 or approximately 2 @.@ 512 ) . This means that a first magnitude star ( + 1 @.@ 00 ) is about 2 @.@ 5 times brighter than a second magnitude ( + 2 @.@ 00 ) star , and about 100 times brighter than a sixth magnitude star ( + 6 @.@ 00 ) . The faintest stars visible to the naked eye under good seeing conditions are about magnitude + 6 . |
On both apparent and absolute magnitude scales , the smaller the magnitude number , the brighter the star ; the larger the magnitude number , the fainter the star . The brightest stars , on either scale , have negative magnitude numbers . The variation in brightness ( ΞL ) between two stars is calculated by subtracting the magnitude number of the brighter star ( mb ) from the magnitude number of the fainter star ( mf ) , then using the difference as an exponent for the base number 2 @.@ 512 ; that is to say : |
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Relative to both luminosity and distance from Earth , a star 's absolute magnitude ( M ) and apparent magnitude ( m ) are not equivalent ; for example , the bright star Sirius has an apparent magnitude of β 1 @.@ 44 , but it has an absolute magnitude of + 1 @.@ 41 . |
The Sun has an apparent magnitude of β 26 @.@ 7 , but its absolute magnitude is only + 4 @.@ 83 . Sirius , the brightest star in the night sky as seen from Earth , is approximately 23 times more luminous than the Sun , while Canopus , the second brightest star in the night sky with an absolute magnitude of β 5 @.@ 53 , is approximately 14 @,@ 000 times more luminous than the Sun . Despite Canopus being vastly more luminous than Sirius , however , Sirius appears brighter than Canopus . This is because Sirius is merely 8 @.@ 6 light @-@ years from the Earth , while Canopus is much farther away at a distance of 310 light @-@ years . |
As of 2006 , the star with the highest known absolute magnitude is LBV 1806 @-@ 20 , with a magnitude of β 14 @.@ 2 . This star is at least 5 @,@ 000 @,@ 000 times more luminous than the Sun . The least luminous stars that are currently known are located in the NGC 6397 cluster . The faintest red dwarfs in the cluster were magnitude 26 , while a 28th magnitude white dwarf was also discovered . These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth . |
= = Classification = = |
The current stellar classification system originated in the early 20th century , when stars were classified from A to Q based on the strength of the hydrogen line . It thought that the hydrogen line strength was a simple linear function of temperature . Rather , it was more complicated ; it strengthened with increasing temperature , it peaked near 9000 K , and then declined at greater temperatures . When the classifications were reordered by temperature , it more closely resembled the modern scheme . |
Stars are given a single @-@ letter classification according to their spectra , ranging from type O , which are very hot , to M , which are so cool that molecules may form in their atmospheres . The main classifications in order of decreasing surface temperature are : O , B , A , F , G , K , and M. A variety of rare spectral types are given special classifications . The most common of these are types L and T , which classify the coldest low @-@ mass stars and brown dwarfs . Each letter has 10 sub @-@ divisions , numbered from 0 to 9 , in order of decreasing temperature . However , this system breaks down at extreme high temperatures as classes O0 and O1 may not exist . |
In addition , stars may be classified by the luminosity effects found in their spectral lines , which correspond to their spatial size and is determined by their surface gravity . These range from 0 ( hypergiants ) through III ( giants ) to V ( main sequence dwarfs ) ; some authors add VII ( white dwarfs ) . Most stars belong to the main sequence , which consists of ordinary hydrogen @-@ burning stars . These fall along a narrow , diagonal band when graphed according to their absolute magnitude and spectral type . The Sun is a main sequence G2V yellow dwarf of intermediate temperature and ordinary size . |
Additional nomenclature , in the form of lower @-@ case letters added to the end of the spectral type to indicate peculiar features of the spectrum . For example , an " e " can indicate the presence of emission lines ; " m " represents unusually strong levels of metals , and " var " can mean variations in the spectral type . |
White dwarf stars have their own class that begins with the letter D. This is further sub @-@ divided into the classes DA , DB , DC , DO , DZ , and DQ , depending on the types of prominent lines found in the spectrum . This is followed by a numerical value that indicates the temperature . |
= = Variable stars = = |
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 and Cepheid @-@ like stars , and long @-@ period variables such as Mira . |
Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events . This group includes protostars , Wolf @-@ Rayet stars , and flare stars , as well as giant and supergiant stars . |
Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties . This group includes novae 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 . The explosion is created when the white dwarf accretes hydrogen from the companion star , building up mass until the hydrogen undergoes fusion . Some novae are also recurrent , having periodic outbursts of moderate amplitude . |
Stars can also vary in luminosity because of extrinsic factors , such as eclipsing binaries , as well as rotating stars that produce extreme starspots . 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 . |
= = Structure = = |
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 107 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 . |
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 0 @.@ 4 M β , fusion occurs in a slowly expanding shell around the degenerate helium core . |
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 . |
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 . 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 ( making radiatative heat transfer inefficient ) as in the outer envelope . |
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 . Red dwarf stars with less than 0 @.@ 4 M β are convective throughout , which prevents the accumulation of a helium core . For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified . |
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 . |
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 spicules appear and stellar flares begin . Above this is the transition region , where the temperature rapidly increases within a distance of only 100 km ( 62 mi ) . Beyond this is the corona , a volume of super @-@ heated plasma that can extend outward to several million kilometres . The existence of a corona appears to be dependent on a convective zone in the outer layers of the star . Despite its high temperature , and the corona emits very little light , due to its low gas density . 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 . |
= = Nuclear fusion reaction pathways = = |
A variety of nuclear fusion reactions take place in the cores of stars , that depend upon their mass and composition . When nuclei fuse , the mass of the fused product is less than the mass of the original parts . This lost mass is converted to electromagnetic energy , according to the mass @-@ energy equivalence relationship E = mc2 . |
The hydrogen fusion process is temperature @-@ sensitive , so a moderate increase in the core temperature will result in a significant increase in the fusion rate . As a result , the core temperature of main sequence stars only varies from 4 million kelvin for a small M @-@ class star to 40 million kelvin for a massive O @-@ class star . |
In the Sun , with a 10 @-@ million @-@ kelvin core , hydrogen fuses to form helium in the proton @-@ proton chain reaction : |
41H β 22H + 2e + + 2Ξ½e ( 2 x 0 @.@ 4 MeV ) |
2e + + 2e- β 2Ξ³ ( 2 x 1 @.@ 0 MeV ) |
21H + 22H β 23He + 2Ξ³ ( 2 x 5 @.@ 5 MeV ) |
23He β 4He + 21H ( 12 @.@ 9 MeV ) |
These reactions result in the overall reaction : |
41H β 4He + 2e + + 2Ξ³ + 2Ξ½e ( 26 @.@ 7 MeV ) |
where e + is a positron , Ξ³ is a gamma ray photon , Ξ½e is a neutrino , and H and He are isotopes of hydrogen and helium , respectively . The energy released by this reaction is in millions of electron volts , which is actually only a tiny amount of energy . However enormous numbers of these reactions occur constantly , producing all the energy necessary to sustain the star 's radiation output . In comparison , the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5 @.@ 7 eV . |
In more massive stars , helium is produced in a cycle of reactions catalyzed by carbon called the carbon @-@ nitrogen @-@ oxygen cycle . |
In evolved stars with cores at 100 million kelvin and masses between 0 @.@ 5 and 10 M β , helium can be transformed into carbon in the triple @-@ alpha process that uses the intermediate element beryllium : |
4He + 4He + 92 keV β 8 * Be |
Subsets and Splits