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The Italian astronomer Geminiano 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 โ€” 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 @-@ Gaposchkin 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 โ€” 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 ฯ€ฮปฮฑฮฝฮฎฯ„ฮทฯ‚ ( planฤ“tฤ“s ) , 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 โ€” 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 โ˜‰ , 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 โ˜‰ 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 โ˜‰ and 1 @.@ 8 โ€“ 2 @.@ 5 M โ˜‰ 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 โ€“ 2 @.@ 5 M โ˜‰ and 5 โ€“ 10 M โ˜‰ , 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 โ€“ 10 M โ˜‰ ( possibly as low as 5 โ€“ 6 M โ˜‰ ) . 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 โ€“ 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 โ˜‰ 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 โ€“ 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 โ€” 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 Henyey 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 ร— 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 โˆ’ 14 M โ˜‰ every year , or about 0 @.@ 01 % of its total mass over its entire lifespan . However , very massive stars can lose 10 โˆ’ 7 to 10 โˆ’ 5 M โ˜‰ each year , significantly affecting their evolution . Stars that begin with more than 50 M โ˜‰ 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 โ˜‰ , called red dwarfs , are able to fuse nearly all of their mass while stars of about 1 M โ˜‰ 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 โ˜‰ ) 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 โ˜‰ 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 โ€“ main sequence = = =
As stars of at least 0 @.@ 4 M โ˜‰ 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 โ˜‰ , 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 โ˜‰ , 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 โ˜‰ ) 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 โ˜‰ ) 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 โ€“ 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 ร— 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 โ€” 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 โ€” the observed age of the universe . The oldest star yet discovered , HD 140283 , nicknamed Methuselah star , is an estimated 14 @.@ 46 ยฑ 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 ยฑ 0 @.@ 021 ) .
The more massive the star , the shorter its lifespan , primarily because massive stars have greater pressure on their cores , causing them to burn hydrogen more rapidly . The most massive stars last an average of a few million years , while stars of minimum mass ( red dwarfs ) burn their fuel very slowly and can last tens to hundreds of billions of years .
= = = Chemical composition = = =
When stars form in the present Milky Way galaxy they are composed of about 71 % hydrogen and 27 % helium , as measured by mass , with a small fraction of heavier elements . Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere , as iron is a common element and its absorption lines are relatively easy to measure . The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system .
The star with the lowest iron content ever measured is the dwarf HE1327 @-@ 2326 , with only 1 / 200,000th the iron content of the Sun . By contrast , the super @-@ metal @-@ rich star ฮผ 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 โ€” 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 .