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| [[Image:A Swarm of Ancient Stars - GPN-2000-000930.jpg|thumb|300px|right|The globular cluster [[Messier 80|M80]]. Stars in globular clusters are mainly older metal-poor members of Population II.]]
| | Marvella is what you can call her but it's not the most feminine name out there. Puerto Rico is where he's been residing for many years and he will never transfer. To collect badges is what her family and her appreciate. Hiring has been my occupation for some time but I've already applied for an additional 1.<br><br>Here is my weblog: [http://ece.modares.ac.ir/mnl/?q=node/1088806 ece.modares.ac.ir] |
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| In [[astronomy]] and [[physical cosmology]], the '''metallicity''' (also designated '''''Z'''''<ref>{{cite web|url=http://ned.ipac.caltech.edu/level5/Kunth/Kunth1.html |title=The Most Metal-Poor Galaxies |publisher=Ned.ipac.caltech.edu |date= |accessdate=2012-05-22}}</ref>) of an object is the proportion of its matter made up of [[chemical element]]s other than [[hydrogen]] and [[helium]]. Because [[stars]], which comprise most of the visible matter in the [[universe]], are composed mostly of hydrogen and helium, [[astronomers]] use for convenience the blanket term "metal" to describe all other elements collectively.<ref name="Martin"/> Thus, a [[nebula]] rich in [[carbon]], [[nitrogen]], [[oxygen]], and [[neon]] would be "metal-rich" in astrophysical terms even though those elements are non-metals in chemistry. This term should not be confused with the usual definition of "[[metal]]"; [[metallic bond]]s are impossible within stars, and the very strongest chemical bonds are only possible in the outer layers of [[K star|cool K]] and [[M star#Class M|M stars]]. Earth-like chemistry therefore has little or no relevance in stellar interiors.
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| The metallicity of an astronomical object may provide an indication of its age. When the universe first formed, according to the [[Big Bang]] theory, it consisted almost entirely of hydrogen which, through [[primordial nucleosynthesis]], created a sizeable proportion of helium and only trace amounts of [[lithium]] and [[beryllium]] and no heavier elements. Therefore, older [[stars]] have lower metallicities than younger stars such as our [[Sun]].
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| ==Definition==
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| In most astronomical systems, hydrogen and helium are the dominant elements. The hydrogen mass fraction is generally expressed as <math>X\equiv \frac{m_\mathrm{H}}{M}</math> where <math>M</math> is the total mass of the system and <math>m_\mathrm{H}</math> the mass of the hydrogen it contains. Similarly, the helium mass fraction is denoted as <math>Y\equiv \frac{m_\mathrm{He}}{M}</math>. The remainder of the elements are collectively referred to as 'metals', and the metallicity—the mass fraction of elements heavier than helium—can be calculated as
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| :<math>Z = \sum_{i>\mathrm{He}} \frac{m_i}{M} = 1 - X - Y.</math>
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| For the [[Sun]], these parameters are often assumed to have the following approximate values,<ref>The New Cosmos, Unsöld & Baschek, Springer-Verlag, 1991, pag. 277</ref> although recent research shows that lower values for <math>Z_\mathrm{sun}</math> might be more appropriate:<ref>{{cite web|url=http://adsabs.harvard.edu/abs/2006CoAst.147...76A |title=The new solar abundances - Part I: the observations |publisher=Communications in Asteroseismology |date=January 2006 |accessdate=2013-06-25}}</ref><ref>{{cite web|url=http://adsabs.harvard.edu/abs/2007ApJ...670..872C |title=Solar Heavy-Element Abundance: Constraints from Frequency Separation Ratios of Low-Degree p-Modes |publisher=The Astrophysical Journal |date=November 2007 |accessdate=2013-06-30}}</ref>
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| :{| class="wikitable"
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| ! Description !! Solar value
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| | Hydrogen mass fraction || <math>X_\mathrm{sun} = 0.73</math>
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| | Helium mass fraction || <math>Y_\mathrm{sun} = 0.25 </math>
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| | Metallicity || <math>Z_\mathrm{sun} = 0.02</math>
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| |}
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| For many astronomical objects the metallicity cannot be measured directly. Instead, proxies are used to obtain an indirect estimate. For example, an observer might measure the iron content of a galaxy (for example using the brightness of an iron [[Emission spectrum|emission line]]) directly, then compare that value with models to estimate the total metallicity.
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| ===Calculation===
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| The metallicity is often expressed as "[Fe/H]", which represents the [[logarithm]] of the ratio of a star's iron abundance compared to that of the Sun (iron is not the most abundant heavy element, but it is among the easiest to measure with spectral data in the visible spectrum). The formula for the logarithm is expressed thus:
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| <center><math> [\mathrm{Fe}/\mathrm{H}] = \log_{10}{\left(\frac{N_{\mathrm{Fe}}}{N_{\mathrm{H}}}\right)_\mathrm{star}} - \log_{10}{\left(\frac{N_{\mathrm{Fe}}}{N_{\mathrm{H}}}\right)_\mathrm{sun}} </math></center>
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| where <math>N_{\mathrm{Fe}}</math> and <math>N_{\mathrm{H}}</math> are the number of iron and hydrogen atoms per unit of volume respectively. The unit often used for metallicity is the "dex" which is a (now-deprecated) contraction of decimal exponent.<ref>[http://www.unc.edu/~rowlett/units/dictD.html A Dictionary of Units of Measurement]</ref> By this formulation, stars with a higher metallicity than the Sun have a positive logarithmic value, whereas those with a lower metallicity than the Sun have a negative value. The logarithm is based on [[powers of 10]]; stars with a value of +1 have ten times the metallicity of the Sun (10<sup>1</sup>). Conversely, those with a value of −1 have one tenth (10<sup>−1</sup>), whereas those with −2 have a hundredth (10<sup>−2</sup>), and so on.<ref name="Martin" /> Young Population I stars have significantly higher iron-to-hydrogen ratios than older Population II stars. Primordial Population III stars are estimated to have a metallicity of less than −6.0, that is, less than a millionth of the abundance of iron which is found in the Sun.{{citation needed|date=November 2012}}
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| This same sort of notation is used to express differences in the individual elements from the solar proportion. For example, the notation "[O/Fe]" represents the difference in the logarithm of the star's oxygen abundance compared to that of the Sun and the logarithm of the star's iron abundance compared to the Sun:
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| <center><math> [\mathrm{O}/\mathrm{Fe}] = \log_{10}{\left(\frac{N_{\mathrm{O}}}{N_{\mathrm{Fe}}}\right)_\mathrm{star}} - \log_{10}{\left(\frac{N_{\mathrm{O}}}{N_{\mathrm{Fe}}}\right)_\mathrm{sun}}
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| </math></center>
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| <center><math>
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| = \left[\log_{10}{\left(\frac{N_{\mathrm{O}}}{N_{\mathrm{H}}}\right)_\mathrm{star}} - \log_{10}{\left(\frac{N_{\mathrm{O}}}{N_{\mathrm{H}}}\right)_\mathrm{sun}}\right] -
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| \left[\log_{10}{\left(\frac{N_{\mathrm{Fe}}}{N_{\mathrm{H}}}\right)_\mathrm{star}} - \log_{10}{\left(\frac{N_{\mathrm{Fe}}}{N_{\mathrm{H}}}\right)_\mathrm{sun}}\right].
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| </math></center>
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| The point of this notation is that if a mass of gas is diluted with pure hydrogen, then its <nowiki>[Fe/H]</nowiki> value will decrease (because there are fewer iron atoms per hydrogen atom after the dilution), but for all other elements ''X'', the <nowiki>[X/Fe]</nowiki> ratios will remain unchanged. By contrast, if a mass of gas is polluted with some amount of pure oxygen, then its <nowiki>[Fe/H]</nowiki> will remain unchanged but its <nowiki>[O/Fe]</nowiki> ratio will increase. In general, a given [[stellar nucleosynthesis|stellar nucleosynthetic]] process alters the proportions of only a few elements or isotopes, so a star or gas sample with nonzero <nowiki>[X/Fe]</nowiki> values may be showing the signature of particular nuclear processes.
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| ===Relation between Z and [Fe/H]===
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| These two ways of expressing the ''metallic'' content of a star are related through the equation:
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| <center><math>\log_{10}\left(\frac{Z/X}{Z_\mathrm{sun}/X_\mathrm{sun}}\right) = [\mathrm{M}/\mathrm{H}]</math></center>
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| where [M/H] is the star's total metal abundance (i.e.: all elements heavier than helium) defined as a more general expression than the one for [Fe/H]: | |
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| <center><math> [\mathrm{M}/\mathrm{H}] = \log_{10}{\left(\frac{N_{\mathrm{M}}}{N_{\mathrm{H}}}\right)_\mathrm{star}} - \log_{10}{\left(\frac{N_{\mathrm{M}}}{N_{\mathrm{H}}}\right)_\mathrm{sun}} .</math></center>
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| The iron abundance and the total metal abundance are often assumed to be related through a constant A as:
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| <center><math> [\mathrm{M}/\mathrm{H}] = A*[\mathrm{Fe}/\mathrm{H}]</math></center>
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| where A assumes values between 0.9 and 1. Using the formulas presented above, the relation between Z and [Fe/H] can finally be written as:
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| <center><math>\log_{10}\left(\frac{Z/X}{Z_\mathrm{sun}/X_\mathrm{sun}}\right) = A*[\mathrm{Fe}/\mathrm{H}].</math></center>
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| ==Stellar populations==
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| [[File:Mu Arae star.jpg|thumb|A rendering of [[Mu Arae]], a metal-rich population I star.]]
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| Stellar populations are categorized as I, II, and III, with each group having decreasing metal content and increasing age. The populations were named in the order they were discovered, which is the reverse of the order of their formation. Thus, the first stars in the universe (low metal content) were population III, and recent stars (high metallicity) are population I.
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| Although older stars do have fewer heavy elements, the fact that all stars observed have some heavier elements poses something of a puzzle, and the current explanation for this proposes the existence of hypothetical metal-free Population III stars in the early universe. Soon after the [[Big Bang]], without metals, it is believed that only stars with masses hundreds of times that of the Sun could be formed; near the end of their [[stellar evolution|lives]] these stars would have created the first 26 elements up to [[iron]] in the [[periodic table]] via [[nucleosynthesis]].<ref name="Heger, A.; Woosley, S. E." />
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| Because of their high mass, current stellar models show that Population III stars would have soon exhausted their fuel and exploded in extremely energetic [[pair-instability supernova]]e. Those explosions would have thoroughly dispersed their material, ejecting metals throughout the universe to be incorporated into the later generations of stars that are observed today. The high mass of the first stars is used to explain why, {{As of|2010|lc=on}}, no Population III stars have been observed. Because they were all destroyed in supernovae in the early universe, Population III stars should only be seen in faraway galaxies whose light originated much earlier in the history of the universe, and searching for these stars or establishing their nonexistence (thereby invalidating the current model) is an active area of research in astronomy. Stars too massive to produce pair-instability supernovae would have collapsed into [[black hole]]s through a process known as [[photodisintegration]], but some matter escapes during this process in the form of [[relativistic jet]]s, and this could have "sprayed" the first metals into the universe.<ref>{{cite journal|last1=Fryer|first1=C. L.|last2=Woosley|first2=S. E.|last3=Heger|first3=A.|title=Pair-Instability Supernovae, Gravity Waves, and Gamma-Ray Transients|journal=The Astrophysical Journal|volume=550|page=372|year=2001|doi= 10.1086/319719|bibcode=2001ApJ...550..372F|arxiv = astro-ph/0007176 }}</ref><ref>{{cite journal|bibcode=2003ApJ...591..288H|author1=Heger, A.|doi=10.1086/375341|author2=Fryer, C. L.|year=2003|page=288|volume=591|journal=The Astrophysical Journal|author3=Woosley, S. E.|author4=Langer, N.|author5=Hartmann, D. H.|title=How Massive Single Stars End Their Life|arxiv = astro-ph/0212469 }}</ref> Though Population III stars have been and remain the goal of a number of searches for such stars, none have been definitely identified.
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| It has been proposed that recent supernovae [[SN 2006gy]] and [[SN 2007bi]] may have been [[pair-instability supernova]]e in which such super-massive Population III stars exploded. It has been speculated that these stars could have formed relatively recently in [[dwarf galaxies]] containing primordial metal-free [[interstellar matter]]; past supernovae in these galaxies could have ejected their metal-rich contents at speeds high enough for them to escape the galaxy, keeping the metal content of the galaxy very low.<ref>[http://www.newscientist.com/article/mg20527470.900-primordial-giant-the-star-that-time-forgot.html New Scientist article on pair-instability supernovae, 13 February 2010]</ref>
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| The next generation of stars was born out of those materials left by the death of the first. The oldest observed stars, known as Population II, have very low metallicities;<ref>{{cite web | author=Lauren J. Bryant | title=What Makes Stars Tick | work=Indiana University Research & Creative Activity | url=http://www.indiana.edu/~rcapub/v27n1/tick.shtml|accessdate=September 7, 2005 }}</ref><ref name="Salvaterra, R.; Ferrara, A.; Schneider, R." /> as subsequent generations of stars were born they became more metal-enriched, as the [[gas]]eous clouds from which they formed received the metal-rich [[cosmic dust|dust]] manufactured by previous generations. As those stars died, they returned metal-enriched material to the [[interstellar medium]] via [[planetary nebula]]e and supernovae, enriching the nebulae out of which the newer stars formed ever further. These youngest stars, including the [[Sun]], therefore have the highest metal content, and are known as Population I stars.
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| Across the [[Milky Way]], metallicity is higher in the [[galactic center]] and decreases as one moves outwards. The gradient in metallicity is attributed to the density of stars in the galactic center: there are more stars in the centre of the galaxy and so, over time, more metals have been returned to the interstellar medium and incorporated into new stars. By a similar mechanism, larger galaxies tend to have a higher metallicity than their smaller counterparts. In the case of the [[Magellanic Clouds]], two small [[irregular galaxy|irregular galaxies]] [[orbit]]ing the Milky Way, the [[Large Magellanic Cloud]] has a metallicity about forty percent that of the Milky Way, whereas the [[Small Magellanic Cloud]] has a metallicity about ten per cent that of the Milky Way. Of the stars we see nearby in our Milky Way around us, Population II stars are rare, and Population I stars make up the vast majority of visible stars bright enough to see with the unaided eye. The [[globular cluster|globular star clusters]] in the Milky Way are the most prominent representatives of Population II.
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| ===Population I stars=== <!-- This section is linked from redirect "Population I stars" -->
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| [[Image:Starpop.svg|thumb|400px|right|Populations I and II]]
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| '''Population I''', or '''metal-rich stars''', are those young stars whose metallicity is highest. The [[Earth]]'s Sun is an example of a metal-rich star. These are common in the [[spiral arm]]s of the [[Milky Way]] galaxy. | |
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| Generally, the youngest stars, the extreme Population I, are found farther in and intermediate Population I stars are farther out, etc. The Sun is considered an intermediate Population I star. Population I stars have regular [[elliptical orbit]]s of the galactic centre, with a low [[relative velocity]]. The high metallicity of Population I stars makes them more likely to possess [[planetary system]]s than the other two populations, because [[planets]], particularly [[terrestrial planet]]s, are thought to be formed by the [[accretion (astrophysics)|accretion]] of metals.<ref>{{cite journal| title=An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect|author=Charles H. Lineweaver |year=2000| doi=10.1006/icar.2001.6607| journal=Icarus| volume=151| issue=2| pages=307–313|arxiv=astro-ph/0012399|bibcode = 2001Icar..151..307L }}</ref>
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| Between the intermediate populations I and II comes the intermediary disc population.
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| ===Population II stars=== <!-- This section is linked from redirect "Population II stars" -->
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| '''Population II''', or '''metal-poor stars''', are those with relatively little metal. The idea of ''a relatively small amount'' must be kept in perspective as even metal-rich astronomical objects contain low percentages of any element other than hydrogen or helium; metals constitute only a tiny percentage of the overall chemical makeup of the universe, even 13.8 billion years after the Big Bang. However, metal-poor objects are even more primitive. These objects formed during an earlier time of the universe. Intermediate Population II stars are common in the [[bulge (astronomy)|bulge]] near the centre of [[Milky Way|our galaxy]]; whereas Population II stars found in the [[Galactic spheroid#Galactic spheroid|galactic halo]] are older and thus more metal-poor. [[Globular clusters]] also contain high numbers of Population II stars.<ref>{{cite journal | author=T. S. van Albada, Norman Baker | title=On the Two Oosterhoff Groups of Globular Clusters | journal=Astrophysical Journal | volume=185 | year=1973 | pages=477–498 | doi=10.1086/152434 | bibcode=1973ApJ...185..477V}}</ref> It is believed that Population II stars created all the other [[chemical element|elements]] in the [[periodic table]], except the more unstable ones. An interesting characteristic of Population II stars is that despite their lower overall metallicity, they often have a higher ratio of [[alpha elements]] ([[Oxygen|O]], [[Silicon|Si]], [[Neon|Ne]], etc.) relative to [[Iron|Fe]] as compared to Population I stars; current theory suggests this is the result of Type II supernovae being more important contributors to the [[interstellar medium]] at the time of their formation, whereas Type Ia supernovae metal enrichment came later in the universe's evolution.<ref>Wolfe, Gawiser, Prochaska, "DAMPED Lyalpha SYSTEMS", Annu. Rev. Astron. Astrophys. 2005. 43: 861–918 http://ned.ipac.caltech.edu/level5/Sept05/Wolfe/Wolfe3.html</ref>
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| Scientists have targeted these oldest stars in several different surveys, including the HK objective-prism survey of [[Timothy C. Beers]] et al. and the Hamburg-[[European Southern Observatory|ESO]] survey of [[Norbert Christlieb]] et al., originally started for faint [[quasars]]. Thus far, they have uncovered and studied in detail about ten very metal-poor stars (as [[Sneden's Star]], [[Cayrel's Star]], [[BD +17° 3248]]) and two of the oldest stars known to date and also the oldest star: [[HE0107-5240]] and [[HE1327-2326]] and [[HE 1523-0901]]. [[SDSS J102915+172927|Caffau's star]] has been identified as the most metal-poor star found {{as of|2012|lc=y}}, initially highlighted in an automated search of [[Sloan Digital Sky Survey]] data. Less extreme in their metal deficiency, but nearer and brighter and hence longer known, are [[HD 122563]] (a [[red giant]]) and [[HD 140283]] (a [[subgiant]]).
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| ===Population III stars===
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| [[Image:Ssc2005-22a1.jpg|thumb|300px|Possible glow of Population III stars imaged by [[NASA]]'s [[Spitzer Space Telescope]].<br />Credit: NASA / [[JPL]]-[[Caltech]] / [[A. Kashlinsky]] ([[Goddard Space Flight Center|GSFC]])]]
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| [[Image:NASA-WMAP-first-stars.jpg|thumb|300px|Artist's impression of the first stars, 400 million years after the [[Big Bang]].]]
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| '''Population III''', or '''metal-free stars''', are a hypothetical extinct population of extremely massive and hot stars with virtually no surface metals, except for a small quantity of metals formed in the [[Big Bang]], such as [[lithium-7]]. These stars are believed to have been formed in the early universe. Their existence is inferred from [[cosmology]], but they have not yet been observed directly. Indirect evidence for their existence has been found in a [[gravitationally lensed galaxy]] in a very distant part of the universe.<ref>{{cite journal | author=R. A. E. Fosbury et al. | title=Massive Star Formation in a Gravitationally Lensed H II Galaxy at z = 3.357 | journal=Astrophysical Journal | year=2003 | volume=596 | issue=1 | pages=797–809 | url=http://iopscience.iop.org/0004-637X/596/2/797/pdf/0004-637X_596_2_797.pdf | doi=10.1086/378228 | bibcode=2003ApJ...596..797F|arxiv = astro-ph/0307162 }}</ref> They are also thought to be components of [[faint blue galaxy|faint blue galaxies]]. Their existence is proposed to account for the fact that heavy elements, which could not have been created in the Big Bang, are observed in [[quasar]] [[emission spectrum|emission spectra]], as well as the existence of faint blue galaxies.<ref name="Heger, A.; Woosley, S. E." /> It is believed that these stars triggered a period of [[reionization]]. [[UDFy-38135539]], a galaxy recently discovered, is believed to have been a part of this process. Some theories hold that there were two generations of Population III stars.<ref>[http://arxiv.org/PS_cache/arxiv/pdf/0905/0905.0929v1.pdf Formation of the First Stars and Galaxies]</ref>
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| Current theory is divided on whether the first stars were very massive or not. One theory, which seems to be borne out by computer models of [[star formation]], is that with no heavy elements and a much warmer [[interstellar medium]] from the Big Bang, it was easy to form stars with much greater total mass than the ones visible today. Typical masses for Population III stars would be expected to be about several hundred [[solar mass]]es, which is much larger than the current stars. Analysis of data on extremely low-metallicity Population II stars such as [[HE0107-5240]], which are thought to contain the metals produced by Population III stars, suggest that these metal-free stars had masses of 20 to 130 solar masses instead.<ref>{{cite journal |bibcode=2003Natur.422..871U |doi=10.1038/nature01571 |arxiv=astro-ph/0301315 |title=First-generation black-hole-forming supernovae and the metal abundance pattern of a very iron-poor star |year=2003 |last1=Umeda |first1=Hideyuki |last2=Nomoto |first2=Ken'Ichi |journal=Nature |volume=422 |issue=6934 |pages=871–873 |pmid=12712199}}</ref> On the other hand, analysis of globular clusters associated with [[elliptical galaxies]] suggests [[pair-instability supernova]]e, which are typically associated with very massive stars, were responsible for their metallic composition.<ref>{{cite journal |doi=10.1086/505679 |bibcode=2006ApJ...648..383P |arxiv=astro-ph/0605210 |title=Extremely α‐Enriched Globular Clusters in Early‐Type Galaxies: A Step toward the Dawn of Stellar Populations? |year=2006 |last1=Puzia |first1=Thomas H. |last2=Kissler‐Patig |first2=Markus |last3=Goudfrooij |first3=Paul |journal=The Astrophysical Journal |volume=648 |pages=383–388}}</ref> This also explains why there have been no low-mass stars with zero metallicity observed, although models have been constructed for smaller Population III stars.<ref>{{cite journal |doi=10.1086/339733 |bibcode=2002ApJ...570..329S |arxiv=astro-ph/0201284 |title=Structure, Evolution, and Nucleosynthesis of Primordial Stars |year=2002 |last1=Siess |first1=Lionel |last2=Livio |first2=Mario |last3=Lattanzio |first3=John |journal=The Astrophysical Journal |volume=570 |pages=329–343}}</ref> Clusters containing zero-metallicity [[red dwarfs]] or [[brown dwarfs]] (possibly created by pair-instability supernovae<ref name="Salvaterra, R.; Ferrara, A.; Schneider, R." />) have been proposed as [[dark matter]] candidates,<ref>{{cite journal |arxiv=astro-ph/9610070 |bibcode=1997A&A...322..709K |title=Zero-metallicity very low mass stars as halo dark matter |author1=Kerins, E. J. |volume=322 |year=1997 |page=709 |journal=Astronomy and Astrophysics}}</ref><ref>{{cite journal|author1=Sanchez-Salcedo, F. J.|title=On the Stringent Constraint on Massive Dark Clusters in the Galactic Halo|journal=Astrophysical Journal Letters v.487|volume=487|pages=L61|year=1997|doi=10.1086/310873|bibcode=1997ApJ...487L..61S}}</ref> but searches for these and other [[MACHO]]'s through [[gravitational microlensing]] have produced negative results.
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| Detection of Population III stars is a goal of NASA's [[James Webb Space Telescope]]. New [[Spectroscopy|spectroscopic]] surveys, such as [[SEGUE]] or [[Sloan Digital Sky Survey#SDSS-II|SDSS-II]], may also locate Population III stars.
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| Recent theories suggest the first star groups may have consisted of a massive star surrounded by several smaller stars.<ref>[http://www.space.com/10801-stars-early-universe-loners.html The Universe's First Stars Weren't Loners After All]</ref><ref>[http://www.space.com/6328-massive-stars-form-simple-solution.html How Massive Stars Form: Simple Solution Found]</ref>
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| ==See also==
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| {{Portal|Astronomy|Star}}
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| * [[Abundance of the chemical elements]]
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| * [[Galaxy formation and evolution]]
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| * [[Metallicity distribution function]]
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| * [[GRB 090423]]
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| {{Clear}}
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| ==Sources==
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| *Page 593-In Quest of the Universe Fourth Edition Karl F. Kuhn Theo Koupelis. Jones and Bartlett Publishers Canada. 2004. ISBN 0-7637-0810-0
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| *{{cite journal |doi=10.1146/annurev.astro.42.053102.134034 |title=THE FIRST STARS |year=2004 |last1=Bromm |first1=Volker |last2=Larson |first2=Richard B. |journal=Annual Review of Astronomy and Astrophysics |volume=42 |pages=79–118|arxiv = astro-ph/0311019 |bibcode = 2004ARA&A..42...79B }}
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| {{reflist
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| |refs=
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| <ref name="Martin">{{cite web | author=John C. Martin | title=What we learn from a star's metal content | work=New Analysis RR Lyrae Kinematics in the Solar Neighborhood | url=https://edocs.uis.edu/jmart5/www/rrlyrae/metals.htm|accessdate=September 7, 2005 }}</ref>
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| <ref name="Heger, A.; Woosley, S. E.">{{cite journal | author=A. Heger, S. E. Woosley | journal=Astrophysical Journal | year=2002 | title=The Nucleosynthetic Signature of Population III | volume=567 | issue=1 | pages=532–543|bibcode=2002ApJ...567..532H | doi=10.1086/338487|arxiv = astro-ph/0107037 }}</ref>
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| <ref name="Salvaterra, R.; Ferrara, A.; Schneider, R.">{{cite journal|author1=Salvaterra, R.|author2=Ferrara, A.|author3=Schneider, R.|title=Induced formation of primordial low-mass stars|journal=New Astronomy|volume=10|issue=2|page=113|year=2004|doi=10.1016/j.newast.2004.06.003|bibcode=2004NewA...10..113S|arxiv = astro-ph/0304074 }}</ref>
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| }}
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| {{Star}}
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| [[Category:Astrophysics]]
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| [[Category:Physical cosmology]]
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| [[Category:Stellar astronomy]]
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