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58.6644287109375 57 WikiText2
23400 = = = = Binary stars = = = =
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23401 The post – 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 .
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23402 = = Distribution = =
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23403 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 .
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23404 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 .
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23405 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 .
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23406 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 .
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23407 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 .
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23408 = = Characteristics = =
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23409 Almost everything about a star is determined by its initial mass , including such characteristics as luminosity , size , evolution , lifespan , and its eventual fate .
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23410 = = = Age = = =
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23411 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 ) .
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23412 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 .
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23413 = = = Chemical composition = = =
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23414 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 .
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23415 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 .
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23416 = = = Diameter = = =
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23417 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 .
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23418 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 .
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23419 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 .
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23420 = = = Kinematics = = =
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23421 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 .
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23422 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 .
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23423 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 .
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23424 = = = Magnetic field = = =
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23425 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 .
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23426 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 .
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23427 = = = Mass = = =
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23428 One of the most massive stars known is Eta Carinae , which , with 100 – 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 .
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23429 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 .
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23430 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 .
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23431 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 .
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23432 = = = Rotation = = =
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23433 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 – 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 .
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23434 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 .
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23435 = = = Temperature = = =
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23436 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 .
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23437 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 ) .
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23438 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 .
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23439 = = Radiation = =
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23440 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 .
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23441 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 .
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23442 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 .
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23443 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 .
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23444 = = = Luminosity = = =
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23445 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 .
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23446 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 .
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23447 = = = Magnitude = = =
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23448 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 ) .
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23449 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 .