Editing Stellar population

{{short description|Grouping of stars by similar metallicity}}
{{redirect|Star generation|the process by which molecular clouds collapse and form stars|Star formation}}
[[File:Artist's impression of the Milky Way (updated - annotated).jpg|right|thumb|upright=1.4|Artist's conception of the spiral structure of the Milky Way showing Baade's general population categories. The ''blue'' regions in the spiral arms are composed of the younger population&nbsp;I stars, while the ''yellow'' stars in the central bulge are the older population&nbsp;II stars. In reality, many population&nbsp;I stars are also found mixed in with the older population&nbsp;II stars.]]

In [[1944]], [[Walter Baade]] categorized groups of stars within the [[Milky Way]] into '''stellar populations'''.
In the abstract of the article by Baade, he recognizes that [[Jan Oort]] originally conceived this type of classification in [[1926]].<ref name=Baade-1944/>

Baade observed that bluer stars were strongly associated with the spiral arms, and yellow stars dominated near the central [[Spiral galaxy#Structure#Galactic bulge|galactic bulge]] and within [[globular cluster|globular star clusters]].<ref name=Shapley-1977/> Two main divisions were deemed '''''population{{nbsp}}I''''' and '''''population{{nbsp}}II''' stars'', with another newer, hypothetical division called '''''population{{nbsp}}III''''' added in 1978.

Among the population types, significant differences were found with their individual observed stellar spectra. These were later shown to be very important and were possibly related to star formation, observed [[kinematics]],<ref name=Gibson-etal-2013/> stellar age, and even [[Galaxy formation and evolution|galaxy evolution]] in both [[Spiral galaxy|spiral]] and [[Elliptical galaxy|elliptical]] galaxies. These three simple population classes usefully divided stars by their chemical composition, or ''[[metallicity]]''.<ref name=Kunth-Östlin-2000/><ref name=schonrich2009/><ref name=Gibson-etal-2013/> In [[astrophysics]] nomenclature ''metal'' refers to all elements heavier than [[helium]], including chemical [[non-metals]] such as oxygen.<ref name=metal/>

By definition, each population group shows the trend where lower metal content indicates higher age of stars. Hence, the first stars in the universe (very low metal content) were deemed ''population&nbsp;III'', old stars (low metallicity) as ''population&nbsp;II'', and recent stars (high metallicity) as ''population&nbsp;I''.<ref name=Byant-2005/> The [[Sun]] is considered population&nbsp;I, a recent star with a relatively high 1.4% metallicity.

==Stellar development==
Observation of [[Astronomical spectroscopy#Stars and their properties|stellar spectra]] has revealed that stars older than the Sun have fewer heavy elements compared with the Sun.<ref name=Gibson-etal-2013/> This immediately suggests that metallicity has evolved through the generations of stars by the process of [[stellar nucleosynthesis]].

===Formation of the first stars===
Under current cosmological models, all matter created in the [[Big Bang]] was mostly [[hydrogen]] (75%) and [[helium]] (25%), with only a very tiny fraction consisting of other light elements such as [[lithium]] and [[beryllium]].<ref name=Cyburt-etal-2016/> When the universe had cooled sufficiently, the first stars were born as population&nbsp;III stars, without any contaminating heavier metals. This is postulated to have affected their structure so that their stellar masses became hundreds of times more than that of the Sun. In turn, these massive stars also evolved very quickly, and their [[nucleosynthesis|nucleosynthetic]] processes created the first 26&nbsp;elements (up to [[iron]] in the [[periodic table]]).<ref name=Heger-Woosley-2002/>

Many theoretical stellar models show that most high-mass population&nbsp;III stars rapidly exhausted their fuel and likely exploded in extremely energetic [[pair-instability supernova]]e. Those explosions would have thoroughly dispersed their material, ejecting metals into the interstellar medium (ISM), to be incorporated into the later generations of stars. Their destruction suggests that no galactic high-mass population&nbsp;III stars should be observable.<ref name=Schlaufman-Thompson-Casey-2018/> However, some population&nbsp;III stars might be seen in high-[[redshift]] galaxies whose light originated during the earlier history of the universe.<ref name=hao2013/>  Scientists have found evidence of [[2MASS J18082002−5104378|an extremely small ultra metal-poor star]], slightly smaller than the Sun, found in a binary system of the spiral arms in the [[Milky Way]]. The discovery opens up the possibility of observing even older stars.<ref name=scinews/>

Stars too massive to produce pair-instability supernovae would have likely collapsed into [[black hole]]s through a process known as [[photodisintegration]]. Here some matter may have escaped during this process in the form of [[relativistic jet]]s, and this could have distributed the first metals into the universe.<ref name=fryer2001/><ref name=heger2003/>{{efn|It has been proposed that recent supernovae [[SN&nbsp;2006gy]] and [[SN&nbsp;2007bi]] may have been [[pair-instability supernova]]e where such super-massive population&nbsp;III stars exploded. Clark (2010) speculates that these stars could have formed relatively recently in [[dwarf galaxies]], since they contain mainly primordial, metal-free [[interstellar matter]]. Past supernovae in these small galaxies could have ejected their metal-rich contents at speeds high enough for them to escape the galaxy, keeping the small galaxies' metal content very low.<ref name=Clark-2010/>}}

===Formation of the observed stars===

The oldest stars observed thus far,<ref name=Schlaufman-Thompson-Casey-2018/> known as population&nbsp;II, have very low metallicities;<ref name=Salvaterra-Ferrara-Schneider-2004/><ref name=Byant-2005/> 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 of stars from population&nbsp;III.

As those population&nbsp;II stars died, they returned metal-enriched material to the [[interstellar medium]] via [[planetary nebula]]e and supernovae, enriching further the nebulae, out of which the newer stars formed. These youngest stars, including the [[Sun]], therefore have the highest metal content, and are known as population&nbsp;I stars.

==Chemical classification by Walter Baade==

===Population I stars===
{{anchor|Population I stars}}

[[File:Treasures3.jpg|thumb|right|Population I star [[Rigel]] with [[reflection nebula]] [[IC&nbsp;2118]]]]
Population&nbsp;I stars are young stars with the highest metallicity out of all three populations and are more commonly found in the [[spiral arm]]s of the [[Milky Way]] galaxy. The [[Sun]] is considered as an intermediate population&nbsp;I star, while the sun-like [[Mu Arae|{{mvar|μ}} Arae]] is much richer in metals.<ref name=Soriano-Vauclair-2009/> (The term ''metal-rich'' is used to describe stars with a significantly higher metallicity than the Sun; higher than can be explained by measurement error.)

Population&nbsp;I stars usually have regular [[elliptical orbit]]s of the [[Galactic Center]], with a low [[relative velocity]]. It was earlier hypothesized that the high metallicity of population&nbsp;I stars makes them more likely to possess [[planetary system]]s than the other two populations, because [[planet]]s, particularly [[terrestrial planet]]s, are thought to be formed by the [[accretion (astrophysics)|accretion]] of metals.<ref name=Lineweaver-2000/> However, observations of the [[Kepler Space Telescope]] data have found smaller planets around stars with a range of metallicities, while only larger, potential gas giant planets are concentrated around stars with relatively higher metallicity – a finding that has implications for theories of gas-giant formation.<ref name=Buchhave-etal-2012/> Between the intermediate population&nbsp;I and the population&nbsp;II stars comes the intermediate disc population.

===Population II stars===
{{anchor|Population II stars}}

[[File:Milky way profile.svg|right|thumb|upright=1.2|The Milky Way. Population&nbsp;II stars are in the galactic bulge and globular clusters.]]
[[File:Massive,_Population_III_Star_in_the_Early_Universe.jpg|right|thumb|upright=1.2|Artist’s impression of a field of population&nbsp;III stars 100&nbsp;million years after the [[Big Bang]].]]

Population&nbsp;II, or metal-poor, stars are those with relatively little of the elements heavier than helium. These objects were formed during an earlier time of the universe. Intermediate population&nbsp;II stars are common in the [[bulge (astronomy)|bulge]] near the centre of the [[Milky Way]], whereas population&nbsp;II stars found in the [[Galactic spheroid#Galactic spheroid|galactic halo]] are older and thus more metal-deficient. [[Globular clusters]] also contain high numbers of population&nbsp;II stars.<ref name=va1973/>

A characteristic of population&nbsp;II stars is that despite their lower overall metallicity, they often have a higher ratio of ''[[alpha elements]]'' (elements produced by the [[alpha process]], like [[oxygen]] and [[neon]]) relative to [[iron]] (Fe) as compared with population&nbsp;I stars; current theory suggests that this is the result of [[type&nbsp;II supernova]]s being more important contributors to the [[interstellar medium]] at the time of their formation, whereas [[type&nbsp;Ia supernova]] metal-enrichment came at a later stage in the universe's development.<ref name=Wolfe-etal-2005/>

Scientists have targeted these oldest stars in several different surveys, including the HK objective-prism survey of [[Timothy&nbsp;C. Beers]] ''et&nbsp;al''.<ref name=Beers92/> and the Hamburg-[[European Southern Observatory|ESO]] survey of Norbert Christlieb et&nbsp;al.,<ref name=Christlieb98/> originally started for faint [[quasars]]. Thus far, they have uncovered and studied in detail about ten ultra-metal-poor (UMP) stars (such as [[Sneden's Star]], [[Cayrel's Star]], [[BD +17° 3248]]) and three of the oldest stars known to date: [[HE&nbsp;0107-5240]], [[HE&nbsp;1327-2326]] and [[HE&nbsp;1523-0901]]. [[SDSS J102915+172927|Caffau's star]] was identified as the most metal-poor star yet when it was found in 2012 using [[Sloan Digital Sky Survey]] data. However, in February&nbsp;2014 the discovery of an even lower-metallicity star was announced, [[SMSS&nbsp;J031300.36-670839.3]] located with the aid of [[SkyMapper]] astronomical survey data. Less extreme in their metal deficiency, but nearer and brighter and hence longer known, are [[HD&nbsp;122563]] (a [[red giant]]) and [[HD&nbsp;140283]] (a [[subgiant]]).

===Population III stars===
{{anchor|Population III stars}}

[[File:Ssc2005-22a1.jpg|thumb|right|upright=1.4|Possible glow of population&nbsp;III stars imaged by [[NASA]]'s [[Spitzer Space Telescope]]]]
Population&nbsp;III stars<ref name=Tominga-etal-2007/> are a hypothetical population of extremely massive, luminous and hot stars with virtually no [[metallicity|"metals"]], except possibly for intermixing ejecta from other nearby, early population&nbsp;III supernovae. The term was first introduced by Neville J. Woolf in 1965.<ref>{{cite journal |last1=Green |first1=Louis |title=Observational Aspects of Cosmology |journal=Sky and Telescope |date=April 1966 |volume=31 |page=199 |bibcode=1966S&T....31..199G |url=https://ui.adsabs.harvard.edu/abs/1966S%26T....31..199G/abstract}}</ref><ref>{{cite journal |last1=Thornton |first1=Page |title=Observational Aspects of Cosmology |journal=Science |date=March 1966 |volume=151 |issue=3716 |pages=1411-1414,1416-1418 |doi=10.1126/science.151.3716.1411 |pmid=17817304 |bibcode=1966Sci...151.1411P |url=https://ui.adsabs.harvard.edu/abs/1966Sci...151.1411P/abstract}}</ref> Such stars are likely to have existed in the very early universe (i.e., at high redshift) and may have started the production of [[chemical element]]s heavier than [[hydrogen]], which are needed for the later formation of [[planet]]s and [[life]] as we know it.<ref name=Sobral-etal-2015-06/><ref name=NYT-2015-06-17/>

The existence of population&nbsp;III stars is inferred from [[physical 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 name=Fosbury-etal-2003/> Their existence may 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]].<ref name=Heger-Woosley-2002/> They are also thought to be components of [[faint blue galaxy|faint blue galaxies]]. These stars likely triggered the universe's period of [[reionization]], a major [[phase transition]] of the hydrogen gas composing most of the interstellar medium. Observations of the galaxy [[UDFy-38135539]] suggest that it may have played a role in this reionization process. The [[European Southern Observatory]] discovered a bright pocket of early population stars in the very bright galaxy [[Cosmos Redshift&nbsp;7]] from the reionization period around 800&nbsp;million years after the Big Bang, at {{math|''z'' {{=}} 6.60}}. The rest of the galaxy has some later redder population&nbsp;II stars.<ref name=Sobral-etal-2015-06/><ref name=Astronomy-2017-06-17-ESO/> Some theories hold that there were two generations of population&nbsp;III stars.<ref name=Bromm/>

[[File:NASA-WMAP-first-stars.jpg|thumb|right|upright=1.3|Artist's impression of the first stars, 400&nbsp;million years after the [[Big Bang]]]]
Current theory is divided on whether the first stars were very massive or not. One possibility is that these stars were much larger than current stars: several hundred [[solar mass]]es, and possibly up to 1,000&nbsp;solar masses. Such stars would be very short-lived and last only 2–5&nbsp;million years.<ref name=Ohkubo-etal-2009/> Such large stars may have been possible due to the lack of heavy elements and a much warmer [[interstellar medium]] from the Big Bang.{{Citation needed|date=June 2017}} Conversely, theories proposed in 2009 and 2011 suggest that the first star groups might have consisted of a massive star surrounded by several smaller stars.<ref name=Space_FS/><ref name=Space_MS/><ref name=Carr-essay/> The smaller stars, if they remained in the birth cluster, would accumulate more gas and could not survive to the present day, but a 2017 study concluded that if a star of 0.8&nbsp;solar masses ({{solar mass}}) or less was ejected from its birth cluster before it accumulated more mass, it could survive to the present day, possibly even in our Milky Way galaxy.<ref name=Dutta-etal-2017/>

Analysis of data of extremely low-[[metallicity]] population&nbsp;II stars such as [[HE&nbsp;0107-5240]], which are thought to contain the metals produced by population&nbsp;III stars, suggest that these metal-free stars had masses of 20~130&nbsp;solar masses.<ref name=Umeda-Nomoto-2003/> On the other hand, analysis of [[globular cluster]]s associated with [[elliptical galaxies]] suggests [[pair-instability supernova]]e, which are typically associated with very massive stars, were responsible for their [[metallicity|metallic]] composition.<ref name=Puzia-etal-2006/> This also explains why there have been no low-mass stars with zero [[metallicity]] observed, despite models constructed for smaller population&nbsp;III stars.<ref name=Siess-Livio-Lattanzio-2002/><ref name=gibson2012/> Clusters containing zero-metallicity [[red dwarfs]] or [[brown dwarfs]] (possibly created by pair-instability supernovae<ref name=Salvaterra-Ferrara-Schneider-2004/>) have been proposed as [[dark matter]] candidates,<ref name=Kerins-1997/><ref name=Sanchez-1997/> but searches for these types of [[Massive compact halo object|MACHO]]s through [[gravitational microlensing]] have produced negative results.{{Citation needed|date=February 2019}}

Population&nbsp;III stars are considered seeds of black holes in the early universe. Unlike high-mass [[black hole]] seeds, such as [[Direct collapse black hole|direct collapse black holes]], they would have produced light ones. If they could have grown to larger than expected masses, then they could have been [[Quasi-star|quasi-stars]], other hypothetical seeds of heavy black holes which would have existed in the early development of the Universe before hydrogen and helium were contaminated by heavier elements.

Detection of population&nbsp;III stars is a goal of NASA's [[James Webb Space Telescope]].<ref name=JW_PIII/> 

On 8 December 2022, astronomers reported the possible detection of Population III stars, in a high-[[redshift]] galaxy called RX J2129–z8He II.<ref name="ARX-20221208">{{cite arXiv |author=Wang, Xin |display-authors=et al. |title=A strong He II λ1640 emitter with extremely blue UV spectral slope at z=8.16: presence of Pop III stars? |date=8 December 2022 |class=astro-ph.GA |eprint=2212.04476 }}</ref><ref name="QUANT-20230130">{{cite news |last=Callaghan |first=Jonathan |title=Astronomers Say They Have Spotted the Universe's First Stars - Theory has it that "Population III" stars brought light to the cosmos. The James Webb Space Telescope may have just glimpsed them. |url=https://www.quantamagazine.org/astronomers-say-they-have-spotted-the-universes-first-stars-20230130/ |date=30 January 2023 |work=[[Quanta Magazine]] |accessdate=31 January 2023 }}</ref>

== See also ==
{{Portal|Stars}}
* [[Lists of astronomical objects]]
* [[Lists of stars]]
* [[Peekaboo Galaxy]]

== Notes ==
{{notelist}}

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}} <!-- end of "refs=" / reflist -->

==Further reading==
* {{cite journal
 |last1=Gibson
 |first1=B.&nbsp;K.
 |display-authors=etal
 |title=Review: Galactic Chemical Evolution
 |year=2013
 |journal=Publications of the Astronomical Society of Australia
 |url=http://astro.wsu.edu/hclee/pasa_review_GCE.pdf
 |access-date=17 April 2018
 |archive-date=20 January 2021
 |archive-url=https://web.archive.org/web/20210120224726/http://astro.wsu.edu/hclee/pasa_review_GCE.pdf
 |url-status=dead
 }}
* {{cite book
 |last=Ferris  |first=Timothy 
 |year=1988
 |title=Coming of Age in the Milky Way
 |page=512
 |publisher=[[William Morrow & Co.]]
 |isbn=978-0-688-05889-0
}}
* {{cite book
 |first=Rudolf  |last=Kippenhahn
 |year=1993
 |title=100 Billion Suns: The birth, life, and death of the stars
 |publisher=Princeton University Press
 |isbn=978-0-691-08781-8
 |url=https://books.google.com/books?id=ISoLPPIORdQC
 |via=Google Books
}}

{{Star}}
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[[Category:Physical cosmological concepts]]
[[Category:Stellar astronomy|Population]]