text: star a star is a luminous sphere of plasma held together by its own gravity . the nearest star to earth is the sun . many other stars are visible to the naked eye from earth during the night , appearing as a multitude of fixed luminous points in the sky due to their immense distance from earth . historically , the most prominent stars were grouped into constellations and , the brightest of which gained proper names . astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations . however , most of the stars in the universe , including all stars outside our galaxy , the milky way , are invisible to the naked eye from earth . indeed , most are invisible from earth even through the most powerful telescopes . for at least a portion of its life , a star shines due to thermonuclear fusion of hydrogen into helium in its core , releasing energy that traverses the star 's interior and then radiates into outer space . almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star 's lifetime , and for some stars by supernova nucleosynthesis when it explodes . near the end of its life , a star can also contain degenerate matter . astronomers can determine the mass , age , metallicity ( chemical composition ) , and many other properties of a star by observing its motion through space , its luminosity , and spectrum respectively . the total mass of a star is the main factor that determines its evolution and eventual fate . other characteristics of a star , including diameter and temperature , change over its life , while the star 's environment affects its rotation and movement . a plot of the temperature of many stars against their luminosities produces a plot known as a hertzsprung – russell diagram ( h – r diagram ) . plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined . a star 's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen , along with helium and trace amounts of heavier elements . when the stellar core is sufficiently dense , hydrogen becomes steadily converted into helium through nuclear fusion , releasing energy in the process . the remainder of the star 's interior carries energy away from the core through a combination of radiative and convective heat transfer processes . the star 's internal pressure prevents it from collapsing further under its own gravity . when the hydrogen fuel at the core is exhausted , a star of mass 0 . 4 times greater than the sun 's will expand to become a red giant . in some cases , it will fuse heavier elements at the core or in shells around the core . as the star expands it throws a part of its mass , enriched with those heavier elements , into the interstellar environment , to be recycled later as new stars . meanwhile , the core becomes a stellar remnant : a white dwarf , a neutron star , or if it is sufficiently massive a black hole . binary and multi - star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits . when two such stars have a relatively close orbit , their gravitational interaction can have a significant impact on their evolution . stars can form part of a much larger gravitationally bound structure , such as a star cluster or a galaxy . observation history historically , stars have been important to civilizations throughout the world . they have been part of religious practices and used for celestial navigation and orientation . many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable . by convention , astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the sun . the motion of the sun against the background stars ( and the horizon ) was used to create calendars , which could be used to regulate agricultural practices . the gregorian calendar , currently used nearly everywhere in the world , is a solar calendar based on the angle of the earth 's rotational axis relative to its local star , the sun . the oldest accurately dated star chart was the result of ancient egyptian astronomy in 1534 bc . the earliest known star catalogues were compiled by the ancient babylonian astronomers of mesopotamia in the late 2nd millennium bc , during the kassite period ( ca . 1531 – 1155 bc ) . the first star catalogue in greek astronomy was created by in approximately 300 bc , with the help of . the star catalog of hipparchus ( 2nd century bc ) included 1020 stars , and was used to assemble ptolemy 's star catalogue . hipparchus is known for the discovery of the first recorded nova ( new star ) . many of the constellations and star names in use today derive from greek astronomy . in spite of the apparent immutability of the heavens , chinese astronomers were aware that new stars could appear . in 185 ad , they were the first to observe and write about a supernova , now known as the sn 185 . the brightest stellar event in recorded history was the sn 1006 supernova , which was observed in 1006 and written about by the egyptian astronomer ali ibn ridwan and several chinese astronomers . the sn 1054 supernova , which gave birth to the crab nebula , was also observed by chinese and islamic astronomers . medieval islamic astronomers gave arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars . they built the first large observatory research institutes , mainly for the purpose of producing zij star catalogues . among these , the book of fixed stars ( 964 ) was written by the persian astronomer abd al - rahman al - sufi , who observed a number of stars , star clusters ( including the omicron velorum and 's clusters ) and galaxies ( including the andromeda galaxy ) . according to a. , in the 11th century , the persian polymath scholar abu biruni described the milky way galaxy as a multitude of fragments having the properties of nebulous stars , and also gave the latitudes of various stars during a lunar eclipse in 1019 . according to josep puig , the andalusian astronomer ibn bajjah proposed that the milky way was made up of many stars that almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material , citing his observation of the conjunction of jupiter and mars on 500 ah ( 1106 / 1107 ad ) as evidence . early european astronomers such as tycho brahe identified new stars in the night sky ( later termed novae ) , suggesting that the heavens were not immutable . in 1584 giordano bruno suggested that the stars were like the sun , and may have other planets , possibly even earth - like , in orbit around them , an idea that had been suggested earlier by the ancient greek philosophers , democritus and epicurus , and by medieval islamic cosmologists such as fakhr al - din al - razi . by the following century , the idea of the stars being the same as the sun was reaching a consensus among astronomers . to explain why these stars exerted no net gravitational pull on the solar system , isaac newton suggested that the stars were equally distributed in every direction , an idea prompted by the theologian richard bentley . the italian astronomer montanari recorded observing variations in luminosity of the star algol in 1667 . edmond halley published the first measurements of the proper motion of a pair of nearby " fixed " stars , demonstrating that they had changed positions since the time of the ancient greek astronomers ptolemy and hipparchus . william herschel was the first astronomer to attempt to determine the distribution of stars in the sky . during the 1780s he established a series of gauges in 600 directions and counted the stars observed along each line of sight . from this he deduced that the number of stars steadily increased toward one side of the sky , in the direction of the milky way core . his son john herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction . in addition to his other accomplishments , william herschel is also noted for his discovery that some stars do not merely lie along the same line of sight , but are also physical companions that form binary star systems . the science of stellar spectroscopy was pioneered by joseph von fraunhofer and angelo secchi . by comparing the spectra of stars such as sirius to the sun , they found differences in the strength and number of their absorption lines — the dark lines in a stellar spectra caused by the atmosphere 's absorption of specific frequencies . in 1865 secchi began classifying stars into spectral types . however , the modern version of the stellar classification scheme was developed by annie j. cannon during the 1900s . the first direct measurement of the distance to a star ( 61 cygni at 11 . 4 light - years ) was made in 1838 by friedrich bessel using the parallax technique . parallax measurements demonstrated the vast separation of the stars in the heavens . observation of double stars gained increasing importance during the 19th century . in 1834 , friedrich bessel observed changes in the proper motion of the star sirius and inferred a hidden companion . edward pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star mizar in a 104 - day period . detailed observations of many binary star systems were collected by astronomers such as william struve and s. w. burnham , allowing the masses of stars to be determined from computation of orbital elements . the first solution to the problem of deriving an orbit of binary stars from telescope observations was made by felix savary in 1827 . the twentieth century saw increasingly rapid advances in the scientific study of stars . the photograph became a valuable astronomical tool . karl schwarzschild discovered that the color of a star and , hence , its temperature , could be determined by comparing the visual magnitude against the photographic magnitude . the development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals . in 1921 albert a. michelson made the first measurements of a stellar diameter using an interferometer on the hooker telescope at mount wilson observatory . important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century . in 1913 , the hertzsprung - russell diagram was developed , propelling the astrophysical study of stars . successful models were developed to explain the interiors of stars and stellar evolution . cecilia payne - first proposed that stars were made primarily of hydrogen and helium in her 1925 phd thesis . the spectra of stars were further understood through advances in quantum physics . this allowed the chemical composition of the stellar atmosphere to be determined . with the exception of supernovae , individual stars have primarily been observed in the local group , and especially in the visible part of the milky way ( as demonstrated by the detailed star catalogues available for our galaxy ) . but some stars have been observed in the m100 galaxy of the virgo cluster , about 100 million light years from the earth . in the local supercluster it is possible to see star clusters , and current telescopes could in principle observe faint individual stars in the local group ( see cepheids ) . however , outside the local supercluster of galaxies , neither individual stars nor clusters of stars have been observed . the only exception is a faint image of a large star cluster containing hundreds of thousands of stars located at a distance of one billion light years — ten times further than the most distant star cluster previously observed . designations the concept of a constellation was known to exist during the babylonian period . ancient sky watchers imagined that prominent arrangements of stars formed patterns , and they associated these with particular aspects of nature or their myths . twelve of these formations lay along the band of the ecliptic and these became the basis of astrology . many of the more prominent individual stars were also given names , particularly with arabic or latin designations . as well as certain constellations and the sun itself , individual stars have their own myths . to the ancient greeks , some " stars " , known as planets ( greek ( ) , meaning " wanderer " ) , represented various important deities , from which the names of the planets mercury , venus , mars , jupiter and saturn were taken . ( uranus and neptune were also greek and roman gods , but neither planet was known in antiquity because of their low brightness . their names were assigned by later astronomers . ) circa 1600 , the names of the constellations were used to name the stars in the corresponding regions of the sky . the german astronomer johann bayer created a series of star maps and applied greek letters as designations to the stars in each constellation . later a numbering system based on the star 's right ascension was invented and added to john flamsteed 's star catalogue in his book " historia coelestis britannica " ( the 1712 edition ) , whereby this numbering system came to be called flamsteed designation or flamsteed numbering . the only internationally recognized authority for naming celestial bodies is the international astronomical union ( iau ) . a number of private companies sell names of stars , which the british library calls an unregulated commercial enterprise . the iau has disassociated itself from this commercial practice , and these names are neither recognized by the iau nor used by them . one such star - naming company is the international star registry , which , during the 1980s , was accused of deceptive practice for making it appear that the assigned name was official . this now - discontinued isr practice was informally labeled a scam and a fraud , and the new york city department of consumer affairs issued a violation against isr for engaging in a deceptive trade practice . units of measurement although stellar parameters can be expressed in si units or cgs units , it is often most convenient to express mass , luminosity , and radii in solar units , based on the characteristics of the sun : large lengths , such as the radius of a giant star or the semi - major axis of a binary star system , are often expressed in terms of the astronomical unit — approximately equal to the mean distance between the earth and the sun ( 150 million km or 93 million miles ) . formation and evolution stars condense from regions of space of higher density , yet those regions are less dense than within a vacuum chamber . these regions - known as molecular clouds - consist mostly of hydrogen , with about 23 to 28 percent helium and a few percent heavier elements . one example of such a star - forming region is the orion nebula . most stars form in groups of dozens to hundreds of thousands of stars . massive stars in these groups may powerfully illuminate those clouds , ionizing the hydrogen , and creating h ii regions . such feedback effects , from star formation , may ultimately disrupt the cloud and prevent further star formation . all stars spend the majority of their existence as main sequence stars , fueled primarily by the nuclear fusion of hydrogen into helium within their cores . however , stars of different masses have markedly different properties at various stages of their development . the ultimate fate of more massive stars differs from that of less massive stars , as do their luminosities and the impact they have on their environment . accordingly , astronomers often group stars by their mass : very low mass stars , with masses below 0 . 5 m ☉ , are fully convective and distribute helium evenly throughout the whole star while on the main sequence . therefore , they never undergo shell burning , never become red giants , which cease fusing and become helium white dwarfs and slowly cool after exhausting their hydrogen . however , as the lifetime of 0 . 5 m ☉ stars is longer than the age of the universe , no such star has yet reached the white dwarf stage . low mass stars ( including the sun ) , with a mass between 0 . 5 m ☉ and 1 . 8 – 2 . 5 m ☉ depending on composition , do become red giants as their core hydrogen is depleted and they begin to burn helium in core in a helium flash ; they develop a degenerate carbon - oxygen core later on the asymptotic giant branch ; they finally blow off their outer shell as a planetary nebula and leave behind their core in the form of a white dwarf . intermediate - mass stars , between 1 . 8 – 2 . 5 m ☉ and 5 – 10 m ☉ , pass through evolutionary stages similar to low mass stars , but after a relatively short period on the rgb they ignite helium without a flash and spend an extended period in the red clump before forming a degenerate carbon - oxygen core . massive stars generally have a minimum mass of 7 – 10 m ☉ ( possibly as low as 5 – 6 m ☉ ) . after exhausting the hydrogen at the core these stars become supergiants and go on to fuse elements heavier than helium . they end their lives when their cores collapse and they explode as supernovae . star formation the formation of a star begins with gravitational instability within a molecular cloud , caused by regions of higher density - often triggered by compression of clouds by radiation from massive stars , expanding bubbles in the interstellar medium , the collision of different molecular clouds , or the collision of galaxies ( as in a starburst galaxy ) . when a region reaches a sufficient density of matter to satisfy the criteria for jeans instability , it begins to collapse under its own gravitational force . as the cloud collapses , individual conglomerations of dense dust and gas form " bok globules " . as a globule collapses and the density increases , the gravitational energy converts into heat and the temperature rises . when the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium , a protostar forms at the core . these pre – main sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy . the period of gravitational contraction lasts about 10 to 15 million years . early stars of less than 2 m ☉ are called t tauri stars , while those with greater mass are herbig ae / be stars . these newly formed stars emit jets of gas along their axis of rotation , which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as herbig – haro objects . these jets , in combination with radiation from nearby massive stars , may help to drive away the surrounding cloud from which the star was formed . early in their development , t tauri stars follow the hayashi track — they contract and decrease in luminosity while remaining at roughly the same temperature . less massive t tauri stars follow this track to the main sequence , while more massive stars turn onto the track . most stars are observed to be members of binary star systems , and the properties of those binaries are the result of the conditions in which they formed . a gas cloud must lose its angular momentum in order to collapse and form a star . the fragmentation of the cloud into multiple stars distributes some of that angular momentum . the primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters . these interactions tend to split apart more widely separated ( soft ) binaries while causing hard binaries to become more tightly bound . this produces the separation of binaries into their two observed populations distributions . main sequence stars spend about 90 % of their existence fusing hydrogen into helium in high - temperature and high - pressure reactions near the core . such stars are said to be on the main sequence , and are called dwarf stars . starting at zero - age main sequence , the proportion of helium in a star 's core will steadily increase , the rate of nuclear fusion at the core will slowly increase , as will the star 's temperature and luminosity . the sun , for example , is estimated to have increased in luminosity by about 40 % since it reached the main sequence 4 . 6 billion ( 4 . 6 × 109 ) years ago . every star generates a stellar wind of particles that causes a continual outflow of gas into space . for most stars , the mass lost is negligible . the sun loses 10 − 14 m ☉ every year , or about 0 . 01 % of its total mass over its entire lifespan . however , very massive stars can lose 10 − 7 to 10 − 5 m ☉ each year , significantly affecting their evolution . stars that begin with more than 50 m ☉ can lose over half their total mass while on the main sequence . the time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it . the sun 's is expected to live 10 billion ( 1010 ) years . massive stars consume their fuel very rapidly and are short - lived . low mass stars consume their fuel very slowly . stars less massive than 0 . 25 m ☉ , called red dwarfs , are able to fuse nearly all of their mass while stars of about 1 m ☉ can only fuse about 10 % of their mass . the combination of their slow fuel - consumption and relatively large usable fuel supply allows low mass stars to last about one trillion ( 1012 ) years ; the most extreme of 0 . 08 m ☉ ) will last for about 12 trillion years . red dwarfs become hotter and more luminous as they accumulate helium . when they eventually run out of hydrogen , they contract into a white dwarf and decline in temperature . however , since the lifespan of such stars is greater than the current age of the universe ( 13 . 8 billion years ) , no stars under about 0 . 85 m ☉ are expected to have moved off the main sequence . besides mass , the elements heavier than helium can play a significant role in the evolution of stars . astronomers label all elements heavier than helium " metals " , and call the chemical concentration of these elements in a star , its metallicity . a star 's metallicity can influence the time the star takes to burn its fuel , and controls the formation of its magnetic fields , which affects the strength of its stellar wind . older , population ii stars have substantially less metallicity than the younger , population i stars due to the composition of the molecular clouds from which they formed . over time , such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres . post – main sequence as stars of at least 0 . 4 m ☉ exhaust their supply of hydrogen at their core , they start to fuse hydrogen in a shell outside the helium core . their outer layers expand and cool greatly as they form a red giant . in about 5 billion years , when the sun enters the helium burning phase , it will expand to a maximum radius of roughly 1 astronomical unit ( 150 million kilometres ) , 250 times its present size , and lose 30 % of its current mass . as the hydrogen shell burning produces more helium , the core increases in mass and temperature . in a red giant of up to 2 . 25 m ☉ , the mass of the helium core becomes degenerate prior to helium fusion . finally , when the temperature increases sufficiently , helium fusion begins explosively in what is called a helium flash , and the star rapidly shrinks in radius , increases its surface temperature , and moves to the horizontal branch of the hr diagram . for more massive stars , helium core fusion starts before the core becomes degenerate , and the star spends some time in the red clump , slowly burning helium , before the outer convective envelope collapses and the star then moves to the horizontal branch . after the star has fused the helium of its core , the carbon product fuses producing a hot core with an outer shell of fusing helium . the star then follows an evolutionary path called the asymptotic giant branch ( agb ) that parallels the other described red giant phase , but with a higher luminosity . the more massive agb stars may undergo a brief period of carbon fusion before the core becomes degenerate . 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 ☉ , 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 ☉ ) 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 ☉ ) 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 – 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 × 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 — 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 — the observed age of the universe . the oldest star yet discovered , hd , nicknamed methuselah star , is an estimated 14 . 46 ± 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 ± 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 - 2326 , with only 1 / the iron content of the sun . by contrast , the super - metal - rich star μ 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 — 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 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 – 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 ☉ 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 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 ☉ limit on massive star formation . the first stars to form after the big bang may have been larger , up to 300 m ☉ , 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 - 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 – 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 — 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 ( ) 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 : 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 − 1 . 44 , but it has an absolute magnitude of + 1 . 41 . the sun has an apparent magnitude of − 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 − 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 − 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 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 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 . 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 ☉ , 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 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 ☉ 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 : → 22h + 2e + + ( 2 x 0 . 4 mev ) 2e + + → 2γ ( 2 x 1 . 0 mev ) 21h + 22h → + 2γ ( 2 x 5 . 5 mev ) → 4he + 21h ( 12 . 9 mev ) these reactions result in the overall reaction : → 4he + 2e + + 2γ + ( 26 . 7 mev ) where e + is a positron , γ is a gamma ray photon , 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 ☉ , helium can be transformed into carbon in the triple - alpha process that uses the intermediate element beryllium : 4he + 4he + 92 kev → 8 * be 4he + 8 * be + 67 kev → 12 * c 12 * c → 12c + γ + 7 . 4 mev for an overall reaction of : → 12c + γ + 7 . 2 mev in massive stars , heavier elements can also be burned in a contracting core through the neon burning process and oxygen burning process . the final stage in the stellar nucleosynthesis process is the silicon burning process that results in the production of the stable isotope iron - 56 , an endothermic process that consumes energy , and so further energy can only be produced through gravitational collapse . the example below shows the amount of time required for a star of 20 m ☉ to consume all of its nuclear fuel . as an o - class main sequence star , it would be 8 times the solar radius and 62 , 000 times the sun 's luminosity .