Editing Stellar population (section)

==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>