Stewwar evowution

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Representative wifetimes of stars as a function of deir masses
The change in size wif time of a Sun-wike star
Artist's depiction of de wife cycwe of a Sun-wike star, starting as a main-seqwence star at wower weft den expanding drough de subgiant and giant phases, untiw its outer envewope is expewwed to form a pwanetary nebuwa at upper right
Chart of stewwar evowution

Stewwar evowution is de process by which a star changes over de course of time. Depending on de mass of de star, its wifetime can range from a few miwwion years for de most massive to triwwions of years for de weast massive, which is considerabwy wonger dan de age of de universe. The tabwe shows de wifetimes of stars as a function of deir masses.[1] Aww stars are born from cowwapsing cwouds of gas and dust, often cawwed nebuwae or mowecuwar cwouds. Over de course of miwwions of years, dese protostars settwe down into a state of eqwiwibrium, becoming what is known as a main-seqwence star.

Nucwear fusion powers a star for most of its wife. Initiawwy de energy is generated by de fusion of hydrogen atoms at de core of de main-seqwence star. Later, as de preponderance of atoms at de core becomes hewium, stars wike de Sun begin to fuse hydrogen awong a sphericaw sheww surrounding de core. This process causes de star to graduawwy grow in size, passing drough de subgiant stage untiw it reaches de red giant phase. Stars wif at weast hawf de mass of de Sun can awso begin to generate energy drough de fusion of hewium at deir core, whereas more-massive stars can fuse heavier ewements awong a series of concentric shewws. Once a star wike de Sun has exhausted its nucwear fuew, its core cowwapses into a dense white dwarf and de outer wayers are expewwed as a pwanetary nebuwa. Stars wif around ten or more times de mass of de Sun can expwode in a supernova as deir inert iron cores cowwapse into an extremewy dense neutron star or bwack howe. Awdough de universe is not owd enough for any of de smawwest red dwarfs to have reached de end of deir wives, stewwar modews suggest dey wiww swowwy become brighter and hotter before running out of hydrogen fuew and becoming wow-mass white dwarfs.[2]

Stewwar evowution is not studied by observing de wife of a singwe star, as most stewwar changes occur too swowwy to be detected, even over many centuries. Instead, astrophysicists come to understand how stars evowve by observing numerous stars at various points in deir wifetime, and by simuwating stewwar structure using computer modews.

Birf of a star[edit]

Simpwistic representation of de stages of stewwar evowution
Schematic of stewwar evowution, uh-hah-hah-hah.


Stewwar evowution starts wif de gravitationaw cowwapse of a giant mowecuwar cwoud. Typicaw giant mowecuwar cwouds are roughwy 100 wight-years (9.5×1014 km) across and contain up to 6,000,000 sowar masses (1.2×1037 kg). As it cowwapses, a giant mowecuwar cwoud breaks into smawwer and smawwer pieces. In each of dese fragments, de cowwapsing gas reweases gravitationaw potentiaw energy as heat. As its temperature and pressure increase, a fragment condenses into a rotating sphere of superhot gas known as a protostar.[3]

A protostar continues to grow by accretion of gas and dust from de mowecuwar cwoud, becoming a pre-main-seqwence star as it reaches its finaw mass. Furder devewopment is determined by its mass. Mass is typicawwy compared to de mass of de Sun: 1.0 M (2.0×1030 kg) means 1 sowar mass.

Protostars are encompassed in dust, and are dus more readiwy visibwe at infrared wavewengds. Observations from de Wide-fiewd Infrared Survey Expworer (WISE) have been especiawwy important for unveiwing numerous Gawactic protostars and deir parent star cwusters.[4][5]

Brown dwarfs and sub-stewwar objects[edit]

Protostars wif masses wess dan roughwy 0.08 M (1.6×1029 kg) never reach temperatures high enough for nucwear fusion of hydrogen to begin, uh-hah-hah-hah. These are known as brown dwarfs. The Internationaw Astronomicaw Union defines brown dwarfs as stars massive enough to fuse deuterium at some point in deir wives (13 Jupiter masses (MJ), 2.5 × 1028 kg, or 0.0125 M). Objects smawwer dan 13 MJ are cwassified as sub-brown dwarfs (but if dey orbit around anoder stewwar object dey are cwassified as pwanets).[6] Bof types, deuterium-burning and not, shine dimwy and die away swowwy, coowing graduawwy over hundreds of miwwions of years.

A dense starfiewd in Sagittarius

Main seqwence[edit]

For a more-massive protostar, de core temperature wiww eventuawwy reach 10 miwwion kewvin, initiating de proton–proton chain reaction and awwowing hydrogen to fuse, first to deuterium and den to hewium. In stars of swightwy over 1 M (2.0×1030 kg), de carbon–nitrogen–oxygen fusion reaction (CNO cycwe) contributes a warge portion of de energy generation, uh-hah-hah-hah. The onset of nucwear fusion weads rewativewy qwickwy to a hydrostatic eqwiwibrium in which energy reweased by de core maintains a high gas pressure, bawancing de weight of de star's matter and preventing furder gravitationaw cowwapse. The star dus evowves rapidwy to a stabwe state, beginning de main-seqwence phase of its evowution, uh-hah-hah-hah.

A new star wiww sit at a specific point on de main seqwence of de Hertzsprung–Russeww diagram, wif de main-seqwence spectraw type depending upon de mass of de star. Smaww, rewativewy cowd, wow-mass red dwarfs fuse hydrogen swowwy and wiww remain on de main seqwence for hundreds of biwwions of years or wonger, whereas massive, hot O-type stars wiww weave de main seqwence after just a few miwwion years. A mid-sized yewwow dwarf star, wike de Sun, wiww remain on de main seqwence for about 10 biwwion years. The Sun is dought to be in de middwe of its main seqwence wifespan, uh-hah-hah-hah.

The evowutionary tracks of stars wif different initiaw masses on de Hertzsprung–Russeww diagram. The tracks start once de star has evowved to de main seqwence and stop when fusion stops (for massive stars) and at de end of de red giant branch (for stars 1 M and wess).[7]
A yewwow track is shown for de Sun, which wiww become a red giant after its main-seqwence phase ends before expanding furder awong de asymptotic giant branch, which wiww be de wast phase in which de Sun undergoes fusion, uh-hah-hah-hah.

Mature stars[edit]

Eventuawwy de core exhausts its suppwy of hydrogen and de star begins to evowve off of de main seqwence. Widout de outward pressure generated by de fusion of hydrogen to counteract de force of gravity de core contracts untiw eider ewectron degeneracy pressure becomes sufficient to oppose gravity or de core becomes hot enough (around 100 MK) for hewium fusion to begin, uh-hah-hah-hah. Which of dese happens first depends upon de star's mass.

Low-mass stars[edit]

What happens after a wow-mass star ceases to produce energy drough fusion has not been directwy observed; de universe is around 13.8 biwwion years owd, which is wess time (by severaw orders of magnitude, in some cases) dan it takes for fusion to cease in such stars.

Recent astrophysicaw modews suggest dat red dwarfs of 0.1 M may stay on de main seqwence for some six to twewve triwwion years, graduawwy increasing in bof temperature and wuminosity, and take severaw hundred biwwion more to cowwapse, swowwy, into a white dwarf.[8][9] Such stars wiww not become red giants as de whowe star is a convection zone and it wiww not devewop a degenerate hewium core wif a sheww burning hydrogen, uh-hah-hah-hah. Instead, hydrogen fusion wiww proceed untiw awmost de whowe star is hewium.

Internaw structures of main-seqwence stars, convection zones wif arrowed cycwes and radiative zones wif red fwashes. To de weft a wow-mass red dwarf, in de center a mid-sized yewwow dwarf and at de right a massive bwue-white main-seqwence star.

Swightwy more massive stars do expand into red giants, but deir hewium cores are not massive enough to reach de temperatures reqwired for hewium fusion so dey never reach de tip of de red giant branch. When hydrogen sheww burning finishes, dese stars move directwy off de red giant branch wike a post-asymptotic-giant-branch (AGB) star, but at wower wuminosity, to become a white dwarf.[2] A star wif an initiaw mass about 0.6 M wiww be abwe to reach temperatures high enough to fuse hewium, and dese "mid-sized" stars go on to furder stages of evowution beyond de red giant branch.[10]

Mid-sized stars[edit]

The evowutionary track of a sowar mass, sowar metawwicity, star from main seqwence to post-AGB

Stars of roughwy 0.6–10 M become red giants, which are warge non-main-seqwence stars of stewwar cwassification K or M. Red giants wie awong de right edge of de Hertzsprung–Russeww diagram due to deir red cowor and warge wuminosity. Exampwes incwude Awdebaran in de constewwation Taurus and Arcturus in de constewwation of Boötes.

Mid-sized stars are red giants during two different phases of deir post-main-seqwence evowution: red-giant-branch stars, wif inert cores made of hewium and hydrogen-burning shewws, and asymptotic-giant-branch stars, wif inert cores made of carbon and hewium-burning shewws inside de hydrogen-burning shewws.[11] Between dese two phases, stars spend a period on de horizontaw branch wif a hewium-fusing core. Many of dese hewium-fusing stars cwuster towards de coow end of de horizontaw branch as K-type giants and are referred to as red cwump giants.

Subgiant phase[edit]

When a star exhausts de hydrogen in its core, it weaves de main seqwence and begins to fuse hydrogen in a sheww outside de core. The core increases in mass as de sheww produces more hewium. Depending on de mass of de hewium core, dis continues for severaw miwwion to one or two biwwion years, wif de star expanding and coowing at a simiwar or swightwy wower wuminosity to its main seqwence state. Eventuawwy eider de core becomes degenerate, in stars around de mass of de sun, or de outer wayers coow sufficientwy to become opaqwe, in more massive stars. Eider of dese changes cause de hydrogen sheww to increase in temperature and de wuminosity of de star to increase, at which point de star expands onto de red giant branch.[12]

Red-giant-branch phase[edit]

The expanding outer wayers of de star are convective, wif de materiaw being mixed by turbuwence from near de fusing regions up to de surface of de star. For aww but de wowest-mass stars, de fused materiaw has remained deep in de stewwar interior prior to dis point, so de convecting envewope makes fusion products visibwe at de star's surface for de first time. At dis stage of evowution, de resuwts are subtwe, wif de wargest effects, awterations to de isotopes of hydrogen and hewium, being unobservabwe. The effects of de CNO cycwe appear at de surface during de first dredge-up, wif wower 12C/13C ratios and awtered proportions of carbon and nitrogen, uh-hah-hah-hah. These are detectabwe wif spectroscopy and have been measured for many evowved stars.

The hewium core continues to grow on de red giant branch. It is no wonger in dermaw eqwiwibrium, eider degenerate or above de Schoenberg-Chandrasekhar wimit, so it increases in temperature which causes de rate of fusion in de hydrogen sheww to increase. The star increases in wuminosity towards de tip of de red-giant branch. Red giant branch stars wif a degenerate hewium core aww reach de tip wif very simiwar core masses and very simiwar wuminosities, awdough de more massive of de red giants become hot enough to ignite hewium fusion before dat point.

Horizontaw branch[edit]

In de hewium cores of stars in de 0.6 to 2.0 sowar mass range, which are wargewy supported by ewectron degeneracy pressure, hewium fusion wiww ignite on a timescawe of days in a hewium fwash. In de nondegenerate cores of more massive stars, de ignition of hewium fusion occurs rewativewy swowwy wif no fwash.[13] The nucwear power reweased during de hewium fwash is very warge, on de order of 108 times de wuminosity of de Sun for a few days[12] and 1011 times de wuminosity of de Sun (roughwy de wuminosity of de Miwky Way Gawaxy) for a few seconds.[14] However, de energy is consumed by de dermaw expansion of de initiawwy degenerate core and dus cannot be seen from outside de star.[12][14][15] Due to de expansion of de core, de hydrogen fusion in de overwying wayers swows and totaw energy generation decreases. The star contracts, awdough not aww de way to de main seqwence, and it migrates to de horizontaw branch on de Hertzsprung–Russeww diagram, graduawwy shrinking in radius and increasing its surface temperature.

Core hewium fwash stars evowve to de red end of de horizontaw branch but do not migrate to higher temperatures before dey gain a degenerate carbon-oxygen core and start hewium sheww burning. These stars are often observed as a red cwump of stars in de cowour-magnitude diagram of a cwuster, hotter and wess wuminous dan de red giants. Higher-mass stars wif warger hewium cores move awong de horizontaw branch to higher temperatures, some becoming unstabwe puwsating stars in de yewwow instabiwity strip (RR Lyrae variabwes), whereas some become even hotter and can form a bwue taiw or bwue hook to de horizontaw branch. The morphowogy of de horizontaw branch depends on parameters such as metawwicity, age, and hewium content, but de exact detaiws are stiww being modewwed.[16]

Asymptotic-giant-branch phase[edit]

After a star has consumed de hewium at de core, hydrogen and hewium fusion continues in shewws around a hot core of carbon and oxygen. The star fowwows de asymptotic giant branch on de Hertzsprung–Russeww diagram, parawwewing de originaw red giant evowution, but wif even faster energy generation (which wasts for a shorter time).[17] Awdough hewium is being burnt in a sheww, de majority of de energy is produced by hydrogen burning in a sheww furder from de core of de star. Hewium from dese hydrogen burning shewws drops towards de center of de star and periodicawwy de energy output from de hewium sheww increases dramaticawwy. This is known as a dermaw puwse and dey occur towards de end of de asymptotic-giant-branch phase, sometimes even into de post-asymptotic-giant-branch phase. Depending on mass and composition, dere may be severaw to hundreds of dermaw puwses.

There is a phase on de ascent of de asymptotic-giant-branch where a deep convective zone forms and can bring carbon from de core to de surface. This is known as de second dredge up, and in some stars dere may even be a dird dredge up. In dis way a carbon star is formed, very coow and strongwy reddened stars showing strong carbon wines in deir spectra. A process known as hot bottom burning may convert carbon into oxygen and nitrogen before it can be dredged to de surface, and de interaction between dese processes determines de observed wuminosities and spectra of carbon stars in particuwar cwusters.[18]

Anoder weww known cwass of asymptotic-giant-branch stars are de Mira variabwes, which puwsate wif weww-defined periods of tens to hundreds of days and warge ampwitudes up to about 10 magnitudes (in de visuaw, totaw wuminosity changes by a much smawwer amount). In more-massive stars de stars become more wuminous and de puwsation period is wonger, weading to enhanced mass woss, and de stars become heaviwy obscured at visuaw wavewengds. These stars can be observed as OH/IR stars, puwsating in de infra-red and showing OH maser activity. These stars are cwearwy oxygen rich, in contrast to de carbon stars, but bof must be produced by dredge ups.


The Cat's Eye Nebuwa, a pwanetary nebuwa formed by de deaf of a star wif about de same mass as de Sun

These mid-range stars uwtimatewy reach de tip of de asymptotic-giant-branch and run out of fuew for sheww burning. They are not sufficientwy massive to start fuww-scawe carbon fusion, so dey contract again, going drough a period of post-asymptotic-giant-branch superwind to produce a pwanetary nebuwa wif an extremewy hot centraw star. The centraw star den coows to a white dwarf. The expewwed gas is rewativewy rich in heavy ewements created widin de star and may be particuwarwy oxygen or carbon enriched, depending on de type of de star. The gas buiwds up in an expanding sheww cawwed a circumstewwar envewope and coows as it moves away from de star, awwowing dust particwes and mowecuwes to form. Wif de high infrared energy input from de centraw star, ideaw conditions are formed in dese circumstewwar envewopes for maser excitation, uh-hah-hah-hah.

It is possibwe for dermaw puwses to be produced once post-asymptotic-giant-branch evowution has begun, producing a variety of unusuaw and poorwy understood stars known as born-again asymptotic-giant-branch stars.[19] These may resuwt in extreme horizontaw-branch stars (subdwarf B stars), hydrogen deficient post-asymptotic-giant-branch stars, variabwe pwanetary nebuwa centraw stars, and R Coronae Boreawis variabwes.

Massive stars[edit]

Reconstructed image of Antares, a red supergiant

In massive stars, de core is awready warge enough at de onset of de hydrogen burning sheww dat hewium ignition wiww occur before ewectron degeneracy pressure has a chance to become prevawent. Thus, when dese stars expand and coow, dey do not brighten as dramaticawwy as wower-mass stars; however, dey were more wuminous on de main seqwence and dey evowve to highwy wuminous supergiants. Their cores become massive enough dat dey cannot support demsewves by ewectron degeneracy and wiww eventuawwy cowwapse to produce a neutron star or bwack howe.

Supergiant evowution[edit]

Extremewy massive stars (more dan approximatewy 40 M), which are very wuminous and dus have very rapid stewwar winds, wose mass so rapidwy due to radiation pressure dat dey tend to strip off deir own envewopes before dey can expand to become red supergiants, and dus retain extremewy high surface temperatures (and bwue-white cowor) from deir main-seqwence time onwards. The wargest stars of de current generation are about 100-150 M because de outer wayers wouwd be expewwed by de extreme radiation, uh-hah-hah-hah. Awdough wower-mass stars normawwy do not burn off deir outer wayers so rapidwy, dey can wikewise avoid becoming red giants or red supergiants if dey are in binary systems cwose enough so dat de companion star strips off de envewope as it expands, or if dey rotate rapidwy enough so dat convection extends aww de way from de core to de surface, resuwting in de absence of a separate core and envewope due to dorough mixing.[20]

The core of a massive star, defined as de region depweted of hydrogen, grows hotter and more dense as it accretes materiaw from de fusion of hydrogen outside de core. In sufficientwy massive stars, de core reaches temperatures and densities high enough to fuse carbon and heavier ewements via de awpha process. At de end of hewium fusion, de core of a star consists primariwy of carbon and oxygen, uh-hah-hah-hah. In stars heavier dan about 8 M, de carbon ignites and fuses to form neon, sodium, and magnesium. Stars somewhat wess massive may partiawwy ignite carbon, but are unabwe to fuwwy fuse de carbon before ewectron degeneracy sets in, and dese stars wiww eventuawwy weave a oxygen-neon-magnesium white dwarf.[21][22]

The exact mass wimit for fuww carbon burning depends on severaw factors such as metawwicity and de detaiwed mass wost on de asymptotic giant branch, but is approximatewy 8-9 M.[21] After carbon burning is compwete, de core of dese stars reaches about 2.5 M and becomes hot enough for heavier ewements to fuse. Before oxygen starts to fuse, neon begins to capture ewectrons which triggers neon burning. For a range of stars of approximatewy 8-12 M, dis process is unstabwe and creates runaway fusion resuwting in an ewectron capture supernova.[23][22]

The onion-wike wayers of a massive, evowved star just before core cowwapse (not to scawe)

In more massive stars, de fusion of neon proceeds widout a runaway defwagration, uh-hah-hah-hah. This is fowwowed in turn by compwete oxygen burning and siwicon burning, producing a core consisting wargewy of iron-peak ewements. Surrounding de core are shewws of wighter ewements stiww undergoing fusion, uh-hah-hah-hah. The timescawe for compwete fusion of a carbon core to an iron core is so short, just a few hundred years, dat de outer wayers of de star are unabwe to react and de appearance of de star is wargewy unchanged. The iron core grows untiw it reaches an effective Chandrasekhar mass, higher dan de formaw Chandrasekhar mass due to various corrections for de rewativistic effects, entropy, charge, and de surrounding envewope. The effective Chandrasekhar mass for an iron core varies from about 1.34 M in de weast massive red supergiants to more dan 1.8 M or more in more massive stars. Once dis mass is reached, ewectrons begin to be captured into de iron-peak nucwei and de core becomes unabwe to support itsewf. The core cowwapses and de star is destroyed, eider in a supernova or direct cowwapse to a bwack howe.[22]


The Crab Nebuwa, de shattered remnants of a star which expwoded as a supernova visibwe in 1054 AD

When de core of a massive stars cowwapses, it wiww form a neutron star, or in de case of cores dat exceed de Towman-Oppenheimer-Vowkoff wimit, a bwack howe. Through a process dat is not compwetewy understood, some of de gravitationaw potentiaw energy reweased by dis core cowwapse is converted into a Type Ib, Type Ic, or Type II supernova. It is known dat de core cowwapse produces a massive surge of neutrinos, as observed wif supernova SN 1987A. The extremewy energetic neutrinos fragment some nucwei; some of deir energy is consumed in reweasing nucweons, incwuding neutrons, and some of deir energy is transformed into heat and kinetic energy, dus augmenting de shock wave started by rebound of some of de infawwing materiaw from de cowwapse of de core. Ewectron capture in very dense parts of de infawwing matter may produce additionaw neutrons. Because some of de rebounding matter is bombarded by de neutrons, some of its nucwei capture dem, creating a spectrum of heavier-dan-iron materiaw incwuding de radioactive ewements up to (and wikewy beyond) uranium.[24] Awdough non-expwoding red giants can produce significant qwantities of ewements heavier dan iron using neutrons reweased in side reactions of earwier nucwear reactions, de abundance of ewements heavier dan iron (and in particuwar, of certain isotopes of ewements dat have muwtipwe stabwe or wong-wived isotopes) produced in such reactions is qwite different from dat produced in a supernova. Neider abundance awone matches dat found in de Sowar System, so bof supernovae and ejection of ewements from red giants are reqwired to expwain de observed abundance of heavy ewements and isotopes dereof.

The energy transferred from cowwapse of de core to rebounding materiaw not onwy generates heavy ewements, but provides for deir acceweration weww beyond escape vewocity, dus causing a Type Ib, Type Ic, or Type II supernova. Current understanding of dis energy transfer is stiww not satisfactory; awdough current computer modews of Type Ib, Type Ic, and Type II supernovae account for part of de energy transfer, dey are not abwe to account for enough energy transfer to produce de observed ejection of materiaw.[25] However, neutrino osciwwations may pway an important rowe in de energy transfer probwem as dey not onwy affect de energy avaiwabwe in a particuwar fwavour of neutrinos but awso drough oder generaw-rewativistic effects on neutrinos.[26][27]

Some evidence gained from anawysis of de mass and orbitaw parameters of binary neutron stars (which reqwire two such supernovae) hints dat de cowwapse of an oxygen-neon-magnesium core may produce a supernova dat differs observabwy (in ways oder dan size) from a supernova produced by de cowwapse of an iron core.[28]

The most massive stars dat exist today may be compwetewy destroyed by a supernova wif an energy greatwy exceeding its gravitationaw binding energy. This rare event, caused by pair-instabiwity, weaves behind no bwack howe remnant.[29] In de past history of de universe, some stars were even warger dan de wargest dat exists today, and dey wouwd immediatewy cowwapse into a bwack howe at de end of deir wives, due to photodisintegration.

Stewwar evowution of wow-mass (weft cycwe) and high-mass (right cycwe) stars, wif exampwes in itawics

Stewwar remnants[edit]

After a star has burned out its fuew suppwy, its remnants can take one of dree forms, depending on de mass during its wifetime.

White and bwack dwarfs[edit]

For a star of 1 M, de resuwting white dwarf is of about 0.6 M, compressed into approximatewy de vowume of de Earf. White dwarfs are stabwe because de inward puww of gravity is bawanced by de degeneracy pressure of de star's ewectrons, a conseqwence of de Pauwi excwusion principwe. Ewectron degeneracy pressure provides a rader soft wimit against furder compression; derefore, for a given chemicaw composition, white dwarfs of higher mass have a smawwer vowume. Wif no fuew weft to burn, de star radiates its remaining heat into space for biwwions of years.

A white dwarf is very hot when it first forms, more dan 100,000 K at de surface and even hotter in its interior. It is so hot dat a wot of its energy is wost in de form of neutrinos for de first 10 miwwion years of its existence, but wiww have wost most of its energy after a biwwion years.[30]

The chemicaw composition of de white dwarf depends upon its mass. A star of a few sowar masses wiww ignite carbon fusion to form magnesium, neon, and smawwer amounts of oder ewements, resuwting in a white dwarf composed chiefwy of oxygen, neon, and magnesium, provided dat it can wose enough mass to get bewow de Chandrasekhar wimit (see bewow), and provided dat de ignition of carbon is not so viowent as to bwow de star apart in a supernova.[31] A star of mass on de order of magnitude of de Sun wiww be unabwe to ignite carbon fusion, and wiww produce a white dwarf composed chiefwy of carbon and oxygen, and of mass too wow to cowwapse unwess matter is added to it water (see bewow). A star of wess dan about hawf de mass of de Sun wiww be unabwe to ignite hewium fusion (as noted earwier), and wiww produce a white dwarf composed chiefwy of hewium.

In de end, aww dat remains is a cowd dark mass sometimes cawwed a bwack dwarf. However, de universe is not owd enough for any bwack dwarfs to exist yet.

If de white dwarf's mass increases above de Chandrasekhar wimit, which is 1.4 M for a white dwarf composed chiefwy of carbon, oxygen, neon, and/or magnesium, den ewectron degeneracy pressure faiws due to ewectron capture and de star cowwapses. Depending upon de chemicaw composition and pre-cowwapse temperature in de center, dis wiww wead eider to cowwapse into a neutron star or runaway ignition of carbon and oxygen, uh-hah-hah-hah. Heavier ewements favor continued core cowwapse, because dey reqwire a higher temperature to ignite, because ewectron capture onto dese ewements and deir fusion products is easier; higher core temperatures favor runaway nucwear reaction, which hawts core cowwapse and weads to a Type Ia supernova.[32] These supernovae may be many times brighter dan de Type II supernova marking de deaf of a massive star, even dough de watter has de greater totaw energy rewease. This instabiwity to cowwapse means dat no white dwarf more massive dan approximatewy 1.4 M can exist (wif a possibwe minor exception for very rapidwy spinning white dwarfs, whose centrifugaw force due to rotation partiawwy counteracts de weight of deir matter). Mass transfer in a binary system may cause an initiawwy stabwe white dwarf to surpass de Chandrasekhar wimit.

If a white dwarf forms a cwose binary system wif anoder star, hydrogen from de warger companion may accrete around and onto a white dwarf untiw it gets hot enough to fuse in a runaway reaction at its surface, awdough de white dwarf remains bewow de Chandrasekhar wimit. Such an expwosion is termed a nova.

Neutron stars[edit]

Bubbwe-wike shock wave stiww expanding from a supernova expwosion 15,000 years ago.

Ordinariwy, atoms are mostwy ewectron cwouds by vowume, wif very compact nucwei at de center (proportionawwy, if atoms were de size of a footbaww stadium, deir nucwei wouwd be de size of dust mites). When a stewwar core cowwapses, de pressure causes ewectrons and protons to fuse by ewectron capture. Widout ewectrons, which keep nucwei apart, de neutrons cowwapse into a dense baww (in some ways wike a giant atomic nucweus), wif a din overwying wayer of degenerate matter (chiefwy iron unwess matter of different composition is added water). The neutrons resist furder compression by de Pauwi Excwusion Principwe, in a way anawogous to ewectron degeneracy pressure, but stronger.

These stars, known as neutron stars, are extremewy smaww—on de order of radius 10 km, no bigger dan de size of a warge city—and are phenomenawwy dense. Their period of rotation shortens dramaticawwy as de stars shrink (due to conservation of anguwar momentum); observed rotationaw periods of neutron stars range from about 1.5 miwwiseconds (over 600 revowutions per second) to severaw seconds.[33] When dese rapidwy rotating stars' magnetic powes are awigned wif de Earf, we detect a puwse of radiation each revowution, uh-hah-hah-hah. Such neutron stars are cawwed puwsars, and were de first neutron stars to be discovered. Though ewectromagnetic radiation detected from puwsars is most often in de form of radio waves, puwsars have awso been detected at visibwe, X-ray, and gamma ray wavewengds.[34]

Bwack howes[edit]

If de mass of de stewwar remnant is high enough, de neutron degeneracy pressure wiww be insufficient to prevent cowwapse bewow de Schwarzschiwd radius. The stewwar remnant dus becomes a bwack howe. The mass at which dis occurs is not known wif certainty, but is currentwy estimated at between 2 and 3 M.

Bwack howes are predicted by de deory of generaw rewativity. According to cwassicaw generaw rewativity, no matter or information can fwow from de interior of a bwack howe to an outside observer, awdough qwantum effects may awwow deviations from dis strict ruwe. The existence of bwack howes in de universe is weww supported, bof deoreticawwy and by astronomicaw observation, uh-hah-hah-hah.

Because de core-cowwapse mechanism of a supernova is, at present, onwy partiawwy understood, it is stiww not known wheder it is possibwe for a star to cowwapse directwy to a bwack howe widout producing a visibwe supernova, or wheder some supernovae initiawwy form unstabwe neutron stars which den cowwapse into bwack howes; de exact rewation between de initiaw mass of de star and de finaw remnant is awso not compwetewy certain, uh-hah-hah-hah. Resowution of dese uncertainties reqwires de anawysis of more supernovae and supernova remnants.


A stewwar evowutionary modew is a madematicaw modew dat can be used to compute de evowutionary phases of a star from its formation untiw it becomes a remnant. The mass and chemicaw composition of de star are used as de inputs, and de wuminosity and surface temperature are de onwy constraints. The modew formuwae are based upon de physicaw understanding of de star, usuawwy under de assumption of hydrostatic eqwiwibrium. Extensive computer cawcuwations are den run to determine de changing state of de star over time, yiewding a tabwe of data dat can be used to determine de evowutionary track of de star across de Hertzsprung–Russeww diagram, awong wif oder evowving properties.[35] Accurate modews can be used to estimate de current age of a star by comparing its physicaw properties wif dose of stars awong a matching evowutionary track.[36]

See awso[edit]


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Furder reading[edit]

Externaw winks[edit]

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