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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 objec... |
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 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 angul... |
= = = 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 i... |
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 yea... |
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 ma... |
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... |
= = = 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 ... |
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 h... |
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 massi... |
= = = = 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 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 boun... |
= = = = 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 relative... |
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 ... |
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 s... |
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 i... |
= = 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... |
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 ... |
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 i... |
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 . Th... |
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 abno... |
= = 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 140283 , nicknamed Methuselah star , is an estimated 14 @.@ 46 ± 0 @.@ 8 billion years old . ( Due to the uncertainty in the va... |
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 ... |
= = = 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 a... |
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 μ Leonis has nearly double the abundance of iron as the Sun , while the planet @-@ bearing star 14 Herculis has nearly triple the iron... |
= = = 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 t... |
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 d... |
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 , h... |
= = = 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 ca... |
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 ... |
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... |
= = = Magnetic field = = = |
The magnetic field of a star is generated within regions of the interior where convective circulation occurs . This movement of conductive plasma functions like a dynamo , wherein the movement of elecrical charges induce magnetic fields , as does a mechanical dynamo . Those magnetic fields have a great range that exte... |
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 l... |
= = = 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... |
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 redsh... |
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 meta... |
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 ,... |
= = = 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 , ca... |
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 sp... |
= = = 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... |
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 . Altho... |
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 o... |
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 electromagneti... |
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 r... |
= = = Luminosity = = = |
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